Study of the Relationship between the Level of Lubricating Oil Contamination with Distillation Fuel and the Risk of Explosion in the Crankcase of a Marine Trunk Type Engine

Abstract

Fuel contamination of engine lubricating oil has been previously determined to arise from two independent phenomena: the effect on oil flash point, and the effect of changing lubrication conditions on tribological pairs. This paper combines these effects and holistically analyzes the consequences of fuel in the lubricating oil of a trunk piston engine on the risk of crankcase explosion. The author hypothesized that diesel fuel as an oil contaminant increases the risk of an explosion in the crankcase of an engine due to the independent interaction of two factors: (1) changes in the oil’s combustible properties, and (2) deterioration of the lubrication conditions of the engine’s tribological nodes, such as main bearings, piston pins, or crank bearings. An experiment was performed to evaluate the rheological, ignition, and lubrication properties of two oils (SAE 30 and SAE 40) commonly used for the recirculation lubrication of marine trunk piston engines for different levels of diesel contamination. The hypothesis was partially confirmed, and the results show that contamination of the lubricating oil with diesel fuel in an amount of no more than 10% does not significantly affect the risk of explosion in the crankcase. However, diesel concentrations above 10% call for corrective action because the viscosity index, lubricity, coefficient of friction and oil film resistance change significantly. Deterioration of the tribological conditions of the engine bearings, as seen in the change in viscosity, viscosity index, and lubricity of the oil, causes an increase in bearing temperature and the possibility of hot spots leading to crankcase explosion.

Full text

Citation: Chybowski, L. Study of the
Relationship between the Level of
Lubricating Oil Contamination with
Distillation Fuel and the Risk of
Explosion in the Crankcase of a
Marine Trunk Type Engine. Energies
2023,16, 683. https://doi.org/
10.3390/en16020683
Academic Editors: Haifeng Liu and
Zongyu Yue
Received: 25 November 2022
Revised: 27 December 2022
Accepted: 4 January 2023
Published: 6 January 2023
Copyright: © 2023 by the author.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
energies
Article
Study of the Relationship between the Level of Lubricating Oil
Contamination with Distillation Fuel and the Risk of Explosion
in the Crankcase of a Marine Trunk Type Engine
Leszek Chybowski
Department of Machine Construction and Materials, Faculty of Marine Engineering, Maritime University of
Szczecin, ul. Willowa 2, 71-650 Szczecin, Poland; l.chybowski@pm.szczecin.pl; Tel.: +48-91-48-09-412
Abstract:
Fuel contamination of engine lubricating oil has been previously determined to arise from
two independent phenomena: the effect on oil flash point, and the effect of changing lubrication
conditions on tribological pairs. This paper combines these effects and holistically analyzes the
consequences of fuel in the lubricating oil of a trunk piston engine on the risk of crankcase explosion.
The author hypothesized that diesel fuel as an oil contaminant increases the risk of an explosion
in the crankcase of an engine due to the independent interaction of two factors: (1) changes in
the oil’s combustible properties, and (2) deterioration of the lubrication conditions of the engine’s
tribological nodes, such as main bearings, piston pins, or crank bearings. An experiment was
performed to evaluate the rheological, ignition, and lubrication properties of two oils (SAE 30 and
SAE 40) commonly used for the recirculation lubrication of marine trunk piston engines for different
levels of diesel contamination. The hypothesis was partially confirmed, and the results show that
contamination of the lubricating oil with diesel fuel in an amount of no more than 10% does not
significantly affect the risk of explosion in the crankcase. However, diesel concentrations above
10% call for corrective action because the viscosity index, lubricity, coefficient of friction and oil film
resistance change significantly. Deterioration of the tribological conditions of the engine bearings, as
seen in the change in viscosity, viscosity index, and lubricity of the oil, causes an increase in bearing
temperature and the possibility of hot spots leading to crankcase explosion.
Keywords:
trunk type engine; crankcase explosion; lubricating oil properties; oil dilution with
distillation fuel; lubricity; flash point temperature
1. Introduction
1.1. Genesis of Undertaking the Research Topic
There have been many research papers report analyses on the mechanism of det-
onation in vapor cloud explosions [
1
–
3
]. Crankcase explosions still occur across many
marine internal combustion engine types (two- and four-stroke; slow, medium, and fast
speed; single and dual fuel) [
4
–
6
]. Such explosions damage the engine and its immediate
surroundings [7,8], and often cause serious injury and death to crew members [4,6,9].
By virtue of their design, trunk piston engines are prone to hot gas and fuel blow-
throughs from the combustion chamber into the crankcase when piston rings, pistons, and
cylinder liners malfunction or are damaged [
10
]. The pistons and sealing rings separate the
crankcase from the combustion chamber [
11
,
12
]. Hot gas blow-through into the crankcase
can cause increased oil evaporation or ignition [13–15].
An explosion can also be caused by diesel oil entering the crankcase through a cracked
piston bottom (crown). The risk is exacerbated by the possibility of fuel entering the
crankcase, e.g., from a malfunctioning injection apparatus [
16
–
18
]. Explosion in the
crankcase of trunk piston engines is also associated with increased temperatures out-
side the engine [
19
], i.e., due to a nearby fire or maintenance work performed in violation
Energies 2023,16, 683. https://doi.org/10.3390/en16020683 https://www.mdpi.com/journal/energies

Energies 2023,16, 683 2 of 38
of health and safety regulations (e.g., welding or grinding) [
4
]. Figures 1and 2summarize
the most important locations responsible for crankcase explosions for in-line cylinder and
V-shaped cylinder arrangement engines, respectively [11].
Energies 2023, 16, x FOR PEER REVIEW 2 of 41
the engine [19], i.e., due to a nearby fire or maintenance work performed in violation of
health and safety regulations (e.g., welding or grinding) [4]. Figures 1 and 2 summarize
the most important locations responsible for crankcase explosions for in-line cylinder and
V-shaped cylinder arrangement engines, respectively [11].
Figure 1. Components of an inline trunk type engine and its surroundings that can cause an explo-
sion in the crankcase [11]: 1—cylinder liner, piston, piston rings; 2—gudgeon pin bearing; 3—crank
pin bearing; 4—main bearing; 5—engine environment.
Figure 2. Components of a V-shape trunk type engine and its surroundings that can cause an explo-
sion in the crankcase [11]: 1—cylinder liner, piston, piston rings; 2—gudgeon pin bearing; 3—crank
pin bearing; 4—main bearing; 5—engine environment.
Figure 1.
Components of an inline trunk type engine and its surroundings that can cause an explosion
in the crankcase [
11
]: 1—cylinder liner, piston, piston rings; 2—gudgeon pin bearing; 3—crank pin
bearing; 4—main bearing; 5—engine environment.
Energies 2023, 16, x FOR PEER REVIEW 2 of 41
the engine [19], i.e., due to a nearby fire or maintenance work performed in violation of
health and safety regulations (e.g., welding or grinding) [4]. Figures 1 and 2 summarize
the most important locations responsible for crankcase explosions for in-line cylinder and
V-shaped cylinder arrangement engines, respectively [11].
Figure 1. Components of an inline trunk type engine and its surroundings that can cause an explo-
sion in the crankcase [11]: 1—cylinder liner, piston, piston rings; 2—gudgeon pin bearing; 3—crank
pin bearing; 4—main bearing; 5—engine environment.
Figure 2. Components of a V-shape trunk type engine and its surroundings that can cause an explo-
sion in the crankcase [11]: 1—cylinder liner, piston, piston rings; 2—gudgeon pin bearing; 3—crank
pin bearing; 4—main bearing; 5—engine environment.
Figure 2.
Components of a V-shape trunk type engine and its surroundings that can cause an
explosion in the crankcase [
11
]: 1—cylinder liner, piston, piston rings; 2—gudgeon pin bearing;
3—crank pin bearing; 4—main bearing; 5—engine environment.

Energies 2023,16, 683 3 of 38
Regulations require that marine engines be equipped with appropriate instrumenta-
tion to monitor crankcase conditions (e.g., oil mist detectors and/or crank-piston cartridge
bearing thermometers) and/or gas sensors and explosion valves [
20
–
23
]. The conditions
for the formation of crankcase explosion (CCE) have been discussed in detail in a number
of publications [24–26].
Fuel has a lower flash point than lubricating oil (minimum 60
◦
C), and its presence
could theoretically contribute to a crankcase explosion that would not have occurred
otherwise. Other authors have pointed out that fuel in oil is among the factors increasing
the risk of crankcase explosion [
27
], or indicating that volatility or flash point (FP) of
lubricating oil differentiates the category of liquid-fuels two-phase explosions [
28
]. For
safety, the circulation oil is subjected to periodic laboratory measurement of its viscosity
and flash point [
11
]. In practice, a 2–5% fuel dilution is considered excessive and calls for
immediate maintenance [
29
]. The causes of lubricating oil contamination with fuel and
recommended corrective actions are shown in Appendix ATable A1 [30].
The present article is a critical response to the publication “Guideline on the rele-
vance of lubrication flash point in connection with crankcase explosions” published by
CIMAC (the International Council on Combustion Engines) Working Group 8 “Marine
Lubricants” [
31
]. Among its conclusions is the statement: “Flash point testing of lubricating
oils as an accurate or early indicator of the potential risk of a crankcase explosion has not
been proven and so it is no longer recommended for this purpose”. Since CIMAC is a
generally recognized organization with a high opinion-forming influence, the author of
this article undertook to verify the publication’s statements. The purpose of this investiga-
tion was to settle the dispute among experts regarding the effect of fuel contamination of
engine-circulating oil on the risk of crankcase explosion.
The CIMAC publication [31] presents the following information in a concise way:
•main conditions required for crankcase explosion;
•methods of crankcase explosion risk detection;
•methods of crankcase explosion prevention and minimizing the effects;
•list of rules, standards, and regulations;
•description of secondary explosion phenomena;
•analysis of lubricant flash point as a reliable indicator for the risk of crankcase explosion.
The CIMAC publication’s conclusions [
31
] were drawn based on investigations con-
ducted by its authors, but no source documents were identified. It states that “fuel contami-
nation of lubricating oil has not been detected or reported in any of the reported cases of
crankcase explosions occurring in recent years”. This observation is debatable. That the
authors did not record such a situation in their reports does not invalidate the possibility.
Often, the detailed and exact causes of explosions are not fully known or are not always
disseminated to the public.
The publication [
31
] further states: “the latest research results also demonstrate that
fuel-contaminated oil does not increase the risk of a crankcase explosion” and “as such
flash point measurement of a lubricating oil is not a reliable or an early indicator for
detecting the risk of a crankcase explosion, neither is it deemed to be a suitable test
whose results can be depended on for this purpose”. These conclusions were presented
in a declarative manner without citing sources. Such statements are debatable because
contamination of the circulating lubricating oil with fuel can change its ignition temperature
and ignition index [
31
–
33
]. Contamination also can change the oil’s rheological properties,
thus contributing to a deterioration of lubrication conditions for engine bearings, their
accelerated wear, and subsequent seizure (formation of hot spots).
1.2. Dilution of Lubricating Oil with Fuel and Explosion Hazard
Experience shows that dilution of lubricating oil with fuel is detrimental to the engine,
and its effects include deterioration of engine efficiency, shortening of oil life, and reduction
in engine reliability and safety [
29
,
30
,
34
]. Diesel fuel has a much lower viscosity than
oil; therefore, distillate fuel contamination reduces oil viscosity [
35
,
36
]. This viscosity

Energies 2023,16, 683 4 of 38
reduction reduces both the oil’s lubricating effectiveness and the strength of the oil film,
which increases the wear on the cylinder liner and bearings [
30
,
34
,
37
]. Progressive dilution
of oil with fuel can lead to significant wear and tear and ultimately to engine failure.
The literature describes other problems that result from lower oil viscosity (or de-
graded oil in general), including reducing the effectiveness of oil additives, increasing oil
volatility, and increasing the rate of oil oxidation [
28
,
30
,
38
,
39
]. Deterioration of lubricating
oil properties in turn forces more frequent oil changes and increases engine-operating
costs [40,41].
The main cause of fuel leakage from the combustion chambers into the crankcase is
blow-through, which is related to the deterioration of piston rings, pistons, and cylinder
liners [
34
]. Fuel that penetrates the cylinder liner into the crankcase intensifies wear of
the piston–ring–liner tribological node, which can lead to piston seizure. The temperature
increase resulting from this process can be sufficient to initiate an explosion (hot spot) [
37
].
Seizure between two components, e.g., piston and cylinder liner, is the result of insufficient
lubrication (oil contamination, lack of oil, inadequate oil lubricity) or exceeding permissible
loads (deformation of cooperating components, excessive pressure force) [42–44].
The subject of the effect of oil ignition temperature on the formation of crankcase
explosions was taken up as early as the middle of the last century, by Ferguson, in a publi-
cation summarizing his paper at the Oil and Gas Power Conference (held in Dallas, TX,
USA in 1951) [
45
]. The study was prompted by a preliminary analysis of 104 crankcase ex-
plosions in various engine types occurring in the 10 years preceding publication. Ferguson
summarized prior studies of the mechanism of explosion formation and investigated the
inflammation properties of various oils employed for diesel engine lubrication. Ferguson
stated that “no significant differences were found in the minimum ignition temperature of a
wide variety of lubricating oils, even where diluted with up to 20 per cent diesel fuel”. The
ignition temperature was determined using a test bench, the crankcase explosion apparatus
(CEA), which simulated oil mist conditions in the crankcase. These temperatures were sig-
nificantly higher than the autogenous ignition temperature determined according to ASTM
guidelines and the flash point temperature. The flash point ranged from
1400–1600 ◦F
(760–871
◦
C), depending on the measurement conditions. Ferguson compared the CEA
results obtained with autoignition temperature and flash point temperature and found
the latter to be significantly lower. Figure 3shows the autoignition temperatures as a
function of flash point temperature for oil diluted with diesel fuel. It shows that the Celsius
autoignition temperature determined under laboratory conditions is about half that of the
CEA ignition temperature. Ferguson explained this fact through the influence of ignitor
size, inflammable mixture inlet temperature, and air flow during the test. According to his
observations, the ignition temperature in the CEA decreased with:
•decreasing air velocity;
•increasing the temperature of the flammable mixture;
•increasing the size of the ignitor.
Ferguson argued that by intensifying these factors, the ignition temperature in CEA
would tend toward the value established under laboratory conditions. It is not clear how
the above statement provided him with the basis for his conclusion that “normal fuel
dilution will have no significant effect on crankcase explosions” [
31
]. Nevertheless, he
also pointed out that a detailed analysis of the effect of lubricating oil dilution with fuel
and details of the mechanism of explosion formation in the crankcase required further
research. The large difference between the resulting CEA and ASTM autogenous ignition
temperatures depends largely on the specifics of how the test was conducted, including a
realistic atomization of the oil (Ferguson used large droplets).
The inadequacy of Ferguson’s physical model can be demonstrated by analyzing
crankcase oil flammability results presented in Freestone et al. [
46
], which were published
five years later. They showed the occurrence of two areas of flammability in the range of
270–350
◦
C and above 400
◦
C. The authors of the paper [
46
] write: “It is significant that
with all surfaces ignition was possible at a temperature as low as 270 deg. C (518 deg. F)

Energies 2023,16, 683 5 of 38
and in one instance at 265 deg. C (509 deg. F). Therefore, to ensure complete safety, a
detecting system should give warning before a temperature of 265 deg. C (509 deg. F) is
reached by an overheated part.”
Energies 2023, 16, x FOR PEER REVIEW 5 of 41
Figure 3. Dependence of autoignition temperature on flash point temperature for engine lubricating
oils diluted with diesel fuel in the Ferguson experiment (own work prepared on the basis of [45]);
blue dotted line is the trend line described by the function y = f(x).
Ferguson argued that by intensifying these factors, the ignition temperature in CEA
would tend toward the value established under laboratory conditions. It is not clear how
the above statement provided him with the basis for his conclusion that “normal fuel di-
lution will have no significant effect on crankcase explosions” [31]. Nevertheless, he also
pointed out that a detailed analysis of the effect of lubricating oil dilution with fuel and
details of the mechanism of explosion formation in the crankcase required further re-
search. The large difference between the resulting CEA and ASTM autogenous ignition
temperatures depends largely on the specifics of how the test was conducted, including a
realistic atomization of the oil (Ferguson used large droplets).
The inadequacy of Ferguson’s physical model can be demonstrated by analyzing
crankcase oil flammability results presented in Freestone et al. [46], which were published
five years later. They showed the occurrence of two areas of flammability in the range of
270–350 °C and above 400 °C. The authors of the paper [46] write: “It is significant that
with all surfaces ignition was possible at a temperature as low as 270 deg. C (518 deg. F)
and in one instance at 265 deg. C (509 deg. F). Therefore, to ensure complete safety, a
detecting system should give warning before a temperature of 265 deg. C (509 deg. F) is
reached by an overheated part.”
Further insights into the possible causes and consequences of crankcase explosions
are pointed out in guides for marine engine operators, such as “Diesel Engine Mainte-
nance Training Manual” issued by the German Bureau of Ships, which states that one of
the root causes of crankcase explosions may be “overheating or dilution of lube oil” [47].
In the light of modern knowledge, crankcase explosions are the result of a multi-stage
process, which consists of the following steps [11]:
Figure 3.
Dependence of autoignition temperature on flash point temperature for engine lubricating
oils diluted with diesel fuel in the Ferguson experiment (own work prepared on the basis of [
45
]);
blue dotted line is the trend line described by the function y= f(x).
Further insights into the possible causes and consequences of crankcase explosions
are pointed out in guides for marine engine operators, such as “Diesel Engine Maintenance
Training Manual” issued by the German Bureau of Ships, which states that one of the root
causes of crankcase explosions may be “overheating or dilution of lube oil” [47].
In the light of modern knowledge, crankcase explosions are the result of a multi-stage
process, which consists of the following steps [11]:
•evaporation of oil in contact with a hot spot inside the crankcase;
•
condensation of oil vapors in contact with cooler areas in the crankcase, resulting in a
white oil mist with a droplet diameter of 5–10 µm;
•
gradual increase in the concentration of oil mist, which increases until the lower
explosive limit is reached (47 mg of oil per 1 L of air, though some studies say this
value 50 mg/L [
11
,
28
,
48
]), which corresponds to a weight concentration of oil mist in
the air equal to 13%;
•
ignition of the flammable mixture, which can occur at temperatures of 270–330
◦
C and
above 400
◦
C (according to other sources, 280–400
◦
C [
49
], and even 200–400
◦
C [
50
]).
Ferguson used much lower fuel/air ratios of 1.3–5.8%, and the process itself was
simplified relative to the sequence of events indicated above. In addition, he noted that
ignition did not occur in an atmosphere of milky haze, indicating significant differences
between the conditions of his experiment and actual crankcase explosions.
Further evidence can be found by studying the occurrence of secondary explosions,
which are the result of air from outside entering the crankcase after the primary explosion.
The vacuum created after the primary explosion causes a fresh load of air to be sucked

Energies 2023,16, 683 6 of 38
in from outside, triggering a secondary explosion, which is usually more serious in its
consequences than the primary. Figure 4shows the failure of the high-speed crankshaft of
the main engine No. 2, which was followed by an explosion that injured a mechanic officer
who was at the engine. This observation confirms the fact that explosions can occur in the
crankcase when oil mist contacts components at a much lower temperature than found
using CEA.
Energies 2023, 16, x FOR PEER REVIEW 6 of 41
• evaporation of oil in contact with a hot spot inside the crankcase;
• condensation of oil vapors in contact with cooler areas in the crankcase, resulting in
a white oil mist with a droplet diameter of 5–10 μm;
• gradual increase in the concentration of oil mist, which increases until the lower ex-
plosive limit is reached (47 mg of oil per 1 L of air, though some studies say this value
50 mg/L [11,28,48]), which corresponds to a weight concentration of oil mist in the air
equal to 13%;
• ignition of the flammable mixture, which can occur at temperatures of 270–330 °C
and above 400 °C (according to other sources, 280–400 °C [49], and even 200–400 °C
[50]).
Ferguson used much lower fuel/air ratios of 1.3–5.8%, and the process itself was sim-
plified relative to the sequence of events indicated above. In addition, he noted that igni-
tion did not occur in an atmosphere of milky haze, indicating significant differences be-
tween the conditions of his experiment and actual crankcase explosions.
Further evidence can be found by studying the occurrence of secondary explosions,
which are the result of air from outside entering the crankcase after the primary explosion.
The vacuum created after the primary explosion causes a fresh load of air to be sucked in
from outside, triggering a secondary explosion, which is usually more serious in its con-
sequences than the primary. Figure 4 shows the failure of the high-speed crankshaft of the
main engine No. 2, which was followed by an explosion that injured a mechanic officer
who was at the engine. This observation confirms the fact that explosions can occur in the
crankcase when oil mist contacts components at a much lower temperature than found
using CEA.
(a) (b)
Figure 4. CCTV footage showing the explosion of a high-speed trunk piston engine: (a) just before
the accident; (b) at the time of the accident (adapted from [51]).
According to an analysis of the actuality of the topic of explosions in crankcases of
marine main propulsion engines presented in the publication [4], as many as 61% (60 out
of 98 explosions analyzed) of the exploded crankcases were observed in trunk piston en-
gines out of all engines. The aforementioned analysis did not capture auxiliary engines
(virtually all of them are now trunk piston engines), and many explosions of the main
sinuses may not have been reported. On the other hand, according to Rattenbury, in the
years 1990–2001, the causes of crankcase explosions in four-stroke engines were damage
to main or crank bearings (39%), damage to pistons (47%), and other reasons (14%) [52]. Ex-
amples of crankcase explosion-related damage to unshielded engines are shown in Figure 5.
Figure 4.
CCTV footage showing the explosion of a high-speed trunk piston engine: (
a
) just before
the accident; (b) at the time of the accident (adapted from [51]).
According to an analysis of the actuality of the topic of explosions in crankcases of
marine main propulsion engines presented in the publication [
4
], as many as 61% (60 out of
98 explosions analyzed) of the exploded crankcases were observed in trunk piston engines
out of all engines. The aforementioned analysis did not capture auxiliary engines (virtually
all of them are now trunk piston engines), and many explosions of the main sinuses may
not have been reported. On the other hand, according to Rattenbury, in the years
1990–2001
,
the causes of crankcase explosions in four-stroke engines were damage to main or crank
bearings (39%), damage to pistons (47%), and other reasons (14%) [
52
]. Examples of crankcase
explosion-related damage to unshielded engines are shown in Figure 5.
Energies 2023, 16, x FOR PEER REVIEW 7 of 41
(a) (b)
Figure 5. Damage of trunk piston engine after explosion in crankcases of trunk piston engines: (a)
burned oil mist detector after failure of the engine lubrication system [53]; (b) conrod as found after
explosion caused by failure of piston guide part of Wärtsilä Vasa 32 engine [54].
Each of these cases could potentially be linked to fuel dilution of lubrication oil. Thus,
a quick diagnosis of oil contamination with fuel makes it possible to avoid damage, par-
ticularly in the cylinder/ring area, thereby preventing serious and costly engine failures,
and avoiding crankcase explosion [55]. The above fact is pointed out by ship mechanics’
handbooks as one measure to prevent crankcase explosions: “routine test on used L.O. for
viscosity, flash point and contamination” [49].
1.3. Methods for Detecting Lubricating Oil Contamination with Fuel
In addition to evaluating lubricating oil contamination with fuel using analysis of oil
properties such as viscosity and flash point, advanced diagnostic methods can be used. A
summary of the main research methods and their advantages and disadvantages is pre-
sented in Appendix A Table A2.
There is a number of methods for analysis of fuel dilution in lubricants, e.g., gas chro-
matography (GC) based on ASTM methods D3524, D3525, and D7593 [56], flame ioniza-
tion detector (FID), Fourier-transform infrared (FTIR), spectroscopy, and Spectro Q6000
fuel dilution meter (FDM) using surface acoustic wave (SAW) sensor [14,56].
In the present study, the flash point and viscosity measurements of the lubricating
oil were used. These tests are the simplest and cheapest. Thus, they are most commonly
used in operational practice for marine engine lubricating oils.
2. Materials and Methods
The research procedure related to the implementation of the study in question con-
sisted of a series of stages according to the scheme shown in Figure 6.
Figure 5.
Damage of trunk piston engine after explosion in crankcases of trunk piston engines:
(
a
) burned oil mist detector after failure of the engine lubrication system [
53
]; (
b
) conrod as found
after explosion caused by failure of piston guide part of Wärtsilä Vasa 32 engine [54].

Energies 2023,16, 683 7 of 38
Each of these cases could potentially be linked to fuel dilution of lubrication oil.
Thus, a quick diagnosis of oil contamination with fuel makes it possible to avoid damage,
particularly in the cylinder/ring area, thereby preventing serious and costly engine failures,
and avoiding crankcase explosion [
55
]. The above fact is pointed out by ship mechanics’
handbooks as one measure to prevent crankcase explosions: “routine test on used L.O. for
viscosity, flash point and contamination” [49].
1.3. Methods for Detecting Lubricating Oil Contamination with Fuel
In addition to evaluating lubricating oil contamination with fuel using analysis of oil
properties such as viscosity and flash point, advanced diagnostic methods can be used.
A summary of the main research methods and their advantages and disadvantages is
presented in Appendix ATable A2.
There is a number of methods for analysis of fuel dilution in lubricants, e.g., gas
chromatography (GC) based on ASTM methods D3524, D3525, and D7593 [
56
], flame
ionization detector (FID), Fourier-transform infrared (FTIR), spectroscopy, and Spectro
Q6000 fuel dilution meter (FDM) using surface acoustic wave (SAW) sensor [14,56].
In the present study, the flash point and viscosity measurements of the lubricating oil
were used. These tests are the simplest and cheapest. Thus, they are most commonly used
in operational practice for marine engine lubricating oils.
2. Materials and Methods
The research procedure related to the implementation of the study in question con-
sisted of a series of stages according to the scheme shown in Figure 6.
Based on the background provided in the introduction, and stemming from the
recommendations presented in the CIMAC publication [
31
], a hypothesis was established:
Diesel Fuel (DO) contamination of oil increases the risk of crankcase explosion for a
trunk piston engine due to independent influence of two factors:
(1)
change in the flammable properties of the oil;
(2)
deterioration of lubrication conditions of the engine’s tribological nodes.
To test the hypothesis, a detailed literature search was conducted and a set of pa-
rameters were selected that may provide information about the change in ignition and
lubricity properties of the lubricating oil. Then, detailed tests were carried out on lubri-
cating oil samples (SAE 30 and SAE 40 grades) diluted with diesel oil at selected present
mass concentrations (0, 1, 2, 5, 10, 20, 50 and 100%). Oils and oil blends of the SAE 30 and
SAE 40 classes were used in the experiment because these oils are used in the circulating
lubrication systems of marine trunk combustion engines of almost all engine manufacturers.
The oils were tested to determine their properties:
•
rheological properties (kinematic viscosity
ν
, density
ρ
, temperature coefficient of
density change ε, dynamic viscosity η, and viscosity index VI were determined);
•
ignition properties (flash point FP temperature, derived cetane number DCN, cal-
culated cetane index CCI, calculated carbon aromaticity index CCAI, and calculated
ignition index CII were determined);
•
lubricant properties (the average wear scar diameter WSD during the tribometer test,
the coefficient of friction
µ
under test conditions, and the parameter describing the
thickness of the oil film under test conditions FILM were determined).
The characteristics of the lubricating oils and diesel fuel used in the experiment, along
with the various measurements methods, are described in the following subsections.

Energies 2023,16, 683 8 of 38
Energies 2023, 16, x FOR PEER REVIEW 8 of 41
Figure 6. Research methodology adopted (description in text).
Based on the background provided in the introduction, and stemming from the rec-
ommendations presented in the CIMAC publication [31], a hypothesis was established:
Diesel Fuel (DO) contamination of oil increases the risk of crankcase explosion for a
trunk piston engine due to independent influence of two factors:
(1) change in the flammable properties of the oil;
Figure 6. Research methodology adopted (description in text).
2.1. Tested Diesel and Lubricating Oils
The dataset in [
57
] outlines the key requirements of ISO 8217:2017 [
58
] distillation
diesel for marine engines. The dataset shows the requirements for engine lubricating oils
according to the viscosity classification of oils as described in SAE J300–2021 [
59
]. The
experiment used diesel fuel that met the requirements of fuels belonging to the DMX
category, as well as some of the most commonly used industrial marine engine lubricating
oils of the SAE 30 and SAE 40 viscosity classes. Orlen Efecta Diesel Bio (designation
CN27102011D) was used in the study [
60
], whose nominal parameters are shown in
Appendix ATable A3 and in the dataset [
57
]. Meanwhile, the lubricating oils used in
the study were Agip Cladium 120 SAE 30 CD and Agip Cladium 120 SAE 40 CD [
61
–
63
],
the characteristics of which are shown in Appendix ATable A4 and in the dataset [57].
Agip Cladium SAE 30 CD and Agip Cladium SAE 40 CD oils are AGIP-ENI’s high
quality API CD (Series III) grade engine oils for the lubrication of naturally aspirated and
highly charged marine, traction and industrial compression–ignition engines. The additive

Energies 2023,16, 683 9 of 38
package allows engines to run on inferior fuels (marine and higher sulphur fuels) while
maintaining high engine performance [61,62].
2.2. Density, Viscosity and Viscosity Index of Lubricating Oil
Changing density and viscosity affects both the anti-seize and ignition properties of
lubricating oil. These features will be discussed later in the article.
Density
ρ
is the ratio of the mass mof a particular substance to the volume Voccupied
by that substance under vacuum conditions:
ρ=m
V. (1)
Under operating conditions, oil/fuel is lighter than the same oil/fuel due to the air
displacement acting on it under atmospheric conditions. Such a difference is assumed to be
0.0011 tons/m3for fuels with a density of 0.800–1.010 tons/m3.
The reference temperature for conventional lubricating oils and liquid fuels is 15
◦
C.
The relationship between the density under operating conditions
ρt
and the density under
reference conditions
ρ15
(the information about the value of this density comes from the
laboratory analysis performed) is described by the formula:
ρt≈ρ15 −ε·(t−15), (2)
where tis the measured temperature of the substance under operating conditions, and
ε
is
the coefficient of change of density of a substance when it is heated by 1 ◦C (1 K).
Viscosity is a property of liquids and plastic solids that characterizes their internal
friction resulting from the movement of fluid layers relative to each other during flow.
Dynamic viscosity ηexpresses the ratio of shear stress τFto shear rate .
γ:
η=τF
.
γ. (3)
Kinematic viscosity νis defined as the ratio of dynamic viscosity ηto fluid density ρ:
ν=η
ρ. (4)
Viscosity index is a unitless measure of a fluid’s change in viscosity relative to temper-
ature change. The viscosity index can be calculated using the following formula:
VI =


L−U
L−H·100, if VI ≤100
100 +e(log H−log U
log Y)−1
0.00715 , if VI >100
, (5)
where Uis the oil’s kinematic viscosity at 40
◦
C, Yis the oil’s kinematic viscosity at 100
◦
C,
and Land Hare the viscosities at 40
◦
C for two hypothetical oils of viscosity indices equal
to 0 and 100, respectively, with the same viscosity at 100
◦
C as the oil whose viscosity index
is determined. These Land Hvalues are provided in tables in ASTM D2270 standard.
In this experiment, in order to generally evaluate the properties of lubricating oils
contaminated with diesel fuel, density and viscosity tests of lubricating oils with different
diesel content (0%, 1%, 2%, 5%, 10%, 20%, 50% and 100%) were conducted at selected
temperatures (15, 20, 30, 40, 50, 60, 70, 80, 90, 100 ◦C).
The density of individual samples in this experiment was determined using a DMA
4500 density analyzer (Anton Paar GmbH, Graz, Austria) with an oscillating U-tube
(Figure 7)
performing measurements in accordance with PN-EN ISO 12185:2002. The
accuracy of the measurement temperature setting is 0.02
◦
C, while the accuracy of density
measurement is 5·10−5g/cm3.

Energies 2023,16, 683 10 of 38
Energies 2023, 16, x FOR PEER REVIEW 10 of 41
𝜌≈𝜌 −𝜀⋅󰇛𝑡−15󰇜, (2)
where t is the measured temperature of the substance under operating conditions, and ε
is the coefficient of change of density of a substance when it is heated by 1 °C (1 K).
Viscosity is a property of liquids and plastic solids that characterizes their internal
friction resulting from the movement of fluid layers relative to each other during flow.
Dynamic viscosity η expresses the ratio of shear stress 𝜏 to shear rate 𝛾󰇗:
𝜂=𝜏
𝛾󰇗. (3)
Kinematic viscosity ν is defined as the ratio of dynamic viscosity η to fluid density ρ:
𝜈=𝜂
𝜌. (4)
Viscosity index is a unitless measure of a fluid’s change in viscosity relative to tem-
perature change. The viscosity index can be calculated using the following formula:
𝑉𝐼=⎩
⎪
⎨
⎪
⎧
𝐿−𝑈
𝐿−𝐻∙ 100, i
f
𝑉𝐼≤100
100+𝑒
 −1
0.00715 ,i
f
𝑉𝐼>100, (5)
where U is the oil’s kinematic viscosity at 40 °C, Y is the oil’s kinematic viscosity at 100
°C, and L and H are the viscosities at 40 °C for two hypothetical oils of viscosity indices
equal to 0 and 100, respectively, with the same viscosity at 100 °C as the oil whose viscosity
index is determined. These L and H values are provided in tables in ASTM D2270 standard.
In this experiment, in order to generally evaluate the properties of lubricating oils
contaminated with diesel fuel, density and viscosity tests of lubricating oils with different
diesel content (0%, 1%, 2%, 5%, 10%, 20%, 50% and 100%) were conducted at selected
temperatures (15, 20, 30, 40, 50, 60, 70, 80, 90, 100 °C).
The density of individual samples in this experiment was determined using a DMA
4500 density analyzer (Anton Paar GmbH, Graz, Austria) with an oscillating U-tube (Fig-
ure 7) performing measurements in accordance with PN-EN ISO 12185:2002. The accuracy
of the measurement temperature setting is 0.02 °C, while the accuracy of density meas-
urement is 5∙10−5 g/cm3.
Figure 7. The Anton Paar DMA 4500 used in the study (photo. M. Szmukała).
The kinematic viscosity of the individual samples in this experiment was determined
using a Cannon-Fenske Opaque glass capillary viscometer (Paradise Scientific Company
Figure 7. The Anton Paar DMA 4500 used in the study (photo. M. Szmukała).
The kinematic viscosity of the individual samples in this experiment was determined
using a Cannon-Fenske Opaque glass capillary viscometer (Paradise Scientific Company
Ltd., Dhaka, Bangladesh) and a TV 2000 viscometric bath (Labovisco bv, Zoetermeer, the
Nederlands); see Figure 8.
Energies 2023, 16, x FOR PEER REVIEW 11 of 41
Ltd., Dhaka, Bangladesh) and a TV 2000 viscometric bath (Labovisco bv, Zoetermeer, the
Nederlands); see Figure 8.
Figure 8. TV 2000 viscometric bath used in the study (Photo. M. Szmukała).
The viscosity measurement kit allows measurements to be taken in accordance with
PN-EN ISO 3104:2004. The accuracy of the measurement temperature setting is 0.01 °C,
while the accuracy of the viscosity measurement is 0.1 mm2/s (data verified based on the
calibration reports of the device).
2.3. Anti-Seizure Properties of Lubricating Oil
Figure 8. TV 2000 viscometric bath used in the study (Photo. M. Szmukała).

Energies 2023,16, 683 11 of 38
The viscosity measurement kit allows measurements to be taken in accordance with
PN-EN ISO 3104:2004. The accuracy of the measurement temperature setting is 0.01
◦
C,
while the accuracy of the viscosity measurement is 0.1 mm
2
/s (data verified based on the
calibration reports of the device).
2.3. Anti-Seizure Properties of Lubricating Oil
Lubricity is the ability of oil to form a boundary layer through chemical and physical
adsorption on solids. The function of the boundary layer is to reduce friction resistance and
protect the cooperating surfaces of the tribological pair from excessive wear and galling.
Lubricity characterizes the behavior of a lubricant during boundary friction; therefore, it
is an ensemble characteristic, since the lubricating properties do not depend only on the
characteristics of the oil, but also on the cooperating components (properties of structural
materials, contact geometry and the type of movement performed) and their load [
64
]. The
lubricating and anti-wear properties of oils and lubricants are determined using tribometers,
selected kinds of which are characterized in Appendix ATable A5.
In this experiment, in order to evaluate the lubricating and anti-wear properties of
lubricating oils contaminated with diesel, lubricating oils with different diesel contents
(0%, 1%, 2%, 5%, 10%, 20%, 50% and 100%) were tested for lubricity. Measurements were
made with a high frequency reciprocating rig (HFFR) tribometer model HFFR V1.0.3 (PCS
Instruments, London, UK) performing measurements in accordance with ASTM D6079 and
PN-EN ISO 12156-1—Figure 9. The construction of the device is shown in [65].
Energies 2023, 16, x FOR PEER REVIEW 12 of 41
Lubricity is the ability of oil to form a boundary layer through chemical and physical
adsorption on solids. The function of the boundary layer is to reduce friction resistance
and protect the cooperating surfaces of the tribological pair from excessive wear and gall-
ing. Lubricity characterizes the behavior of a lubricant during boundary friction; there-
fore, it is an ensemble characteristic, since the lubricating properties do not depend only
on the characteristics of the oil, but also on the cooperating components (properties of
structural materials, contact geometry and the type of movement performed) and their
load [64]. The lubricating and anti-wear properties of oils and lubricants are determined
using tribometers, selected kinds of which are characterized in Appendix A Table A5.
In this experiment, in order to evaluate the lubricating and anti-wear properties of
lubricating oils contaminated with diesel, lubricating oils with different diesel contents
(0%, 1%, 2%, 5%, 10%, 20%, 50% and 100%) were tested for lubricity. Measurements were
made with a high frequency reciprocating rig (HFFR) tribometer model HFFR V1.0.3 (PCS
Instruments, London, UK) performing measurements in accordance with ASTM D6079
and PN-EN ISO 12156-1—Figure 9. The construction of the device is shown in [65].
Figure 9. The PCS HFFR V1.0.3 instrument used in the study (photo. M. Szmukała).
A sample of the test liquid is placed in the test reservoir at 60 °C, according to ISO
[65], a temperature determined by the bath. A fixed steel ball is held in a vertically
mounted chuck and forced with mass against a horizontally installed stationary steel plate
4 with an applied load. The test ball oscillates at a constant speed with a constant pitch,
while the interface with the plate is fully immersed in the fluid. Oscillation is provided by
an electromagnetic vibrator along with a counterweight providing a fixed motion of the
ball determined using appropriate elements. The sample load is monitored with an in-
verter. Test conditions such as metallurgical properties of the plate and ball, fluid temper-
ature, load, oscillation frequency, and stroke length and external conditions are specified
in the norm [65]. The diameter of wear scar WSD-generated on the test ball is taken as a
measure of anti-wear properties and fluid lubricity. Wear assessment is carried out either
visually or with a digital camera. In addition, the device used provides information on the
averaged value of the friction coefficient μ and the percentage reduction in oil film re-
sistance (FILM parameter).
The FILM parameter provides a rough assessment of oil film thickness and quality
by measuring the electrical contact potential (ECP), which is a measure of contact re-
sistance (oil film resistance). The contact resistance circuit applies a potential of 15 mV to
the sample contact resistor and a series stabilizing resistor. The stabilizing resistance set
by the control software, together with the contact resistor, forms a potential divider circuit.
The default value of the stabilizing resistor is 10 Ω. After the test, the HFFR apparatus
Figure 9. The PCS HFFR V1.0.3 instrument used in the study (photo. M. Szmukała).
A sample of the test liquid is placed in the test reservoir at 60
◦
C, according to ISO [
65
], a
temperature determined by the bath. A fixed steel ball is held in a vertically mounted chuck
and forced with mass against a horizontally installed stationary steel plate 4 with an applied
load. The test ball oscillates at a constant speed with a constant pitch, while the interface
with the plate is fully immersed in the fluid. Oscillation is provided by an electromagnetic
vibrator along with a counterweight providing a fixed motion of the ball determined using
appropriate elements. The sample load is monitored with an inverter. Test conditions such as
metallurgical properties of the plate and ball, fluid temperature, load, oscillation frequency,
and stroke length and external conditions are specified in the norm [
65
]. The diameter of wear
scar WSD-generated on the test ball is taken as a measure of anti-wear properties and fluid
lubricity. Wear assessment is carried out either visually or with a digital camera. In addition,
the device used provides information on the averaged value of the friction coefficient
µ
and
the percentage reduction in oil film resistance (FILM parameter).
The FILM parameter provides a rough assessment of oil film thickness and quality by
measuring the electrical contact potential (ECP), which is a measure of contact resistance

Energies 2023,16, 683 12 of 38
(oil film resistance). The contact resistance circuit applies a potential of 15 mV to the sample
contact resistor and a series stabilizing resistor. The stabilizing resistance set by the control
software, together with the contact resistor, forms a potential divider circuit. The default
value of the stabilizing resistor is 10
Ω
. After the test, the HFFR apparatus provides the
value of the percentage reduction in the oil film resistance value. Large values of the FILM
parameter indicate separation of the cooperating metal surfaces, while values close to or
equal to zero indicate the existence of metallic contact (oil film breakage) between the
mating surfaces.
2.4. Ignition Properties of Lubricating Oil
The essential indicators for determining the ignition characteristics of liquid fuels are
flash point, autoignition temperature, and autoignition (ignition) delay. The flashpoint of a
material is the lowest liquid temperature at which, under certain standardized conditions,
a liquid gives off vapors in a quantity capable of forming an ignitable vapor/air mixture
(standard EN IEC 60079-10-1). The autoignition temperature or kindling point of a substance
is the lowest temperature at which it spontaneously ignites in a normal atmosphere without
an external source of ignition, such as a flame or spark. The flash point in this experiment was
determined in a closed crucible using the Pensky–Martens method in accordance with EN
ISO 2719. To carry out the test, the flashpoint Pensky–Martens semi-automatic apparatus was
used (Walter Herzog GmbH, Lauda-Königshofen, Germany); see Figure 10.
Energies 2023, 16, x FOR PEER REVIEW 13 of 41
provides the value of the percentage reduction in the oil film resistance value. Large val-
ues of the FILM parameter indicate separation of the cooperating metal surfaces, while
values close to or equal to zero indicate the existence of metallic contact (oil film breakage)
between the mating surfaces.
2.4. Ignition Properties of Lubricating Oil
The essential indicators for determining the ignition characteristics of liquid fuels are
flash point, autoignition temperature, and autoignition (ignition) delay. The flashpoint of
a material is the lowest liquid temperature at which, under certain standardized condi-
tions, a liquid gives off vapors in a quantity capable of forming an ignitable vapor/air
mixture (standard EN IEC 60079-10-1). The autoignition temperature or kindling point of
a substance is the lowest temperature at which it spontaneously ignites in a normal at-
mosphere without an external source of ignition, such as a flame or spark. The flash point
in this experiment was determined in a closed crucible using the Pensky–Martens method
in accordance with EN ISO 2719. To carry out the test, the flashpoint Pensky–Martens
semi-automatic apparatus was used (Walter Herzog GmbH, Lauda-Königshofen, Ger-
many); see Figure 10.
Figure 10. Flashpoint Pensky–Martens semi-automatic apparatus from Walter Herzog GmbH used
in the study (photo. M. Szmukała).
Figure 10.
Flashpoint Pensky–Martens semi-automatic apparatus from Walter Herzog GmbH used
in the study (photo. M. Szmukała).

Energies 2023,16, 683 13 of 38
The autoignition delay is defined as the time between the atomization of a flammable
substance and the start of the combustion process after autoignition has occurred. This
indicator is usually used for evaluating the diesel engine fuels. There are several methods
for measuring the autoignition properties of fuel [
66
–
68
]. Table A6 in Appendix Ashows
a comparison of several selected methods for measuring the autoignition properties of
fuel. An indicator of the combustion speed of diesel fuel and compression needed for
ignition is the cetane number (CN; alias cetane rating). For marine fuels, CN values above
45 correspond to very good ignition properties, 40–45 from good to very good, 35–40 from
good to acceptable, 28–35 from bad to acceptable, 25–28 from very bad to bad, while below
25 very bad or unfit for use.
Cetane numbers are difficult to measure accurately, as the method requires a special
diesel engine called a cooperative fuel research (CFR) engine. The test is conducted
in accordance with the guidelines of ASTM D613 (ISO 5165). In order to facilitate the
measurements, substitute methods were introduced:
•
constant volume combustion chamber instrument (CVCC) analyzers such as the
ignition quality tester (IQT) according to ASTM D6890, cetane ignition delay (CID)
according to ASTM D7668, or fuel ignition tester (FIT) according to ASTM D7170 for
measuring the derived cetane number (DCN);
•
laboratory methods for determining flammability indices, cetane indices (CI and CCI),
and others based on the physical and chemical properties of a substance, e.g., “four
variable equations” (ASTM D4737) based on density, 10% 50% and 90% recovery
temperatures or “two variable methods” (ASTM D976), which use just density and
the 50% recovery temperature.
Apart from the indicators determined experimentally, calculated indicators are used
in operational practice. This applies to situations where the measurement of autoignition
delay is hampered by technical capabilities. This situation includes residual fuels such as
heavy fuel oils. Among such indices are the calculated carbon aromaticity index (CCAI)
and the calculated ignition index (CII).
CCAI is Shell’s calculation of the autoignition capability of residual fuels (heavy
fuel oils, HFO). It is calculated based on the measured viscosity
ν
(mm
2
/s) for a given
fuel determined at t(
◦
C) and the density at 15
◦
C
ρ15
(kg/m
3
). For residual fuels, CCAI
falling within the range of 790–830 corresponds to excellent to good combustion quality,
830–850 corresponds
to good to average combustion quality, 850–870 corresponds to aver-
age combustion quality, and 870–950 corresponds to poor or very poor combustion quality.
CCAI can be determined from one of the equivalent formulas:
CCAI =ρ15 −140.7·log[log(ν+0.85)] −80.6 −210·lnt+273
323 , (6)
CCAI =ρ15 −140.7·log[log(ν+0.85)] −80.6 −483.5·logt+273
323 . (7)
CII is BP’s calculation of the autoignition capacity of residual fuels (HFO). It is cal-
culated based on the measured viscosity
ν
(mm
2
/s) for a given fuel determined at t(
◦
C)
and the density at 15
◦
C
ρ15
(kg/m
3
). The values obtained from the calculation of CII for
residual fuels are interpreted analogously as CN for distillate fuels. CII is determined from
the formula:
CII =(270.795 +0.1038·t)−0.254565·ρ15 +23.708·log[log(ν+0.7)]. (8)
In this experiment, to evaluate the ignition properties of lubricating oils contaminated
with diesel fuel, flash point temperature and cetane indices of lubricating oils with different
diesel content (0%, 1%, 2%, 5%, 10%, 20%, 50% and 100%) were tested. Determination of
the derived cetane number (DCN) was not possible for lubricating oil–diesel blends; thus,
only the diesel DCN value was determined. This was done using the Herzog cetane ID

Energies 2023,16, 683 14 of 38
510 instrument (PAC L.P., Houston, TX, USA). The inability to measure DON for mixtures
of fuel and lubricating oil is due to the specifics of the measuring device. Calculation of
the cetane index was possible only for mixtures with diesel fuel content in lubricating oil
equal to or greater than 50% m/m. This limitation was due to the complex composition of
lubricating oils, which are not a pure mixture of hydrocarbons (they contain a number of
additives). Since the rheological properties of lubricating oils are similar to residual fuels,
the values of CCAI and CII indices were determined for the measured values of density and
kinematic viscosity of mixtures of the lubricating oils tested with diesel fuel. The calculated
values of CII and CCAI were determined for all concentrations planned in the experiment.
3. Results and Discussion
3.1. Rheological Properties
Figures 11 and 12 show the measured density values of the tested lubricating oils of
the SAE 30 and SAE 40 grades, respectively, with different degrees of contamination with
diesel fuel: 0, 1, 2, 5, 10, 20, and 50% m/m; compared with pure diesel (i.e., 100%).
Energies 2023, 16, x FOR PEER REVIEW 15 of 41
for mixtures of fuel and lubricating oil is due to the specifics of the measuring device.
Calculation of the cetane index was possible only for mixtures with diesel fuel content in
lubricating oil equal to or greater than 50% m/m. This limitation was due to the complex
composition of lubricating oils, which are not a pure mixture of hydrocarbons (they con-
tain a number of additives). Since the rheological properties of lubricating oils are similar
to residual fuels, the values of CCAI and CII indices were determined for the measured
values of density and kinematic viscosity of mixtures of the lubricating oils tested with
diesel fuel. The calculated values of CII and CCAI were determined for all concentrations
planned in the experiment.
3. Results and Discussion
3.1. Rheological Properties
Figures 11 and 12 show the measured density values of the tested lubricating oils of
the SAE 30 and SAE 40 grades, respectively, with different degrees of contamination with
diesel fuel: 0, 1, 2, 5, 10, 20, and 50% m/m; compared with pure diesel (i.e., 100%).
Figure 11. Measured values of SAE 30 class oil density in the temperature range of 15–100 °C at
different levels of lubricating oil dilution with diesel fuel.
Figure 11.
Measured values of SAE 30 class oil density in the temperature range of 15–100
◦
C at
different levels of lubricating oil dilution with diesel fuel.
The density values at a given temperature are similar for oils of both classes. Increased
concentration of diesel in the lubricating oil results in a significant reduction in density.
From these data, the temperature coefficients of change of substance density,
ε
,were
calculated using Equation (2). The coefficient values were determined for a temperature of
100 ◦C. Figure 13 shows a summary of the values for different levels of dilution.
The
ε
values for SAE 30 oil are higher than those for SAE 40 oil. Moreover, the value
of the
ε
coefficient increases with an increase in the content of diesel fuel in the lubricating
oil. This trend is due to the higher value of this index for diesel fuel than for lubricating oil.
For the oils tested, the difference between diesel and lubricating oil
ε
values for SAE 30 oil
was 0.084, while for SAE 40 oil, it was 0.089.

Energies 2023,16, 683 15 of 38
Energies 2023, 16, x FOR PEER REVIEW 16 of 41
Figure 12. Measured values of SAE 40 class oil density in the temperature range of 15–100 °C at
different levels of lubricating oil dilution with diesel fuel.
The density values at a given temperature are similar for oils of both classes. In-
creased concentration of diesel in the lubricating oil results in a significant reduction in
density. From these data, the temperature coefficients of change of substance density, ε,
were calculated using Equation (2). The coefficient values were determined for a temper-
ature of 100 °C. Figure 13 shows a summary of the values for different levels of dilution.
The ε values for SAE 30 oil are higher than those for SAE 40 oil. Moreover, the value
of the ε coefficient increases with an increase in the content of diesel fuel in the lubricating
oil. This trend is due to the higher value of this index for diesel fuel than for lubricating
oil. For the oils tested, the difference between diesel and lubricating oil ε values for SAE
30 oil was 0.084, while for SAE 40 oil, it was 0.089.
Figure 12.
Measured values of SAE 40 class oil density in the temperature range of 15–100
◦
C at
different levels of lubricating oil dilution with diesel fuel.
Energies 2023, 16, x FOR PEER REVIEW 17 of 41
Figure 13. Calculated values of the coefficient of temperature change in density of the lubricating
oils tested at different levels of lubricating oil dilution with diesel fuel.
The measured kinematic viscosity values of lubricating oil–diesel blends for different
concentrations in the temperature range of 40–100 °C for SAE 30 and SAE 40 grade oils
are shown in Figures 14 and 15, respectively. Their viscosity decreases with increasing
temperature. Increasing the diesel content in the lubricating oil mixture results in a de-
crease in kinematic viscosity at a given temperature.
Figure 13.
Calculated values of the coefficient of temperature change in density of the lubricating oils
tested at different levels of lubricating oil dilution with diesel fuel.
The measured kinematic viscosity values of lubricating oil–diesel blends for different
concentrations in the temperature range of 40–100
◦
C for SAE 30 and SAE 40 grade oils

Energies 2023,16, 683 16 of 38
are shown in Figures 14 and 15, respectively. Their viscosity decreases with increasing
temperature. Increasing the diesel content in the lubricating oil mixture results in a decrease
in kinematic viscosity at a given temperature.
Energies 2023, 16, x FOR PEER REVIEW 18 of 41
Figure 14. Measured kinematic viscosity values of SAE 30 grade oil in the temperature range of 40–
100 °C at different levels of lubricating oil dilution with diesel fuel.
Figure 15. Measured kinematic viscosity values of SAE 40 grade oil in the temperature range of 40–
100 °C at different levels of lubricating oil dilution with diesel fuel.
Figure 14.
Measured kinematic viscosity values of SAE 30 grade oil in the temperature range of
40–100 ◦C at different levels of lubricating oil dilution with diesel fuel.
Energies 2023, 16, x FOR PEER REVIEW 18 of 41
Figure 14. Measured kinematic viscosity values of SAE 30 grade oil in the temperature range of 40–
100 °C at different levels of lubricating oil dilution with diesel fuel.
Figure 15. Measured kinematic viscosity values of SAE 40 grade oil in the temperature range of 40–
100 °C at different levels of lubricating oil dilution with diesel fuel.
Figure 15.
Measured kinematic viscosity values of SAE 40 grade oil in the temperature range of
40–100 ◦C at different levels of lubricating oil dilution with diesel fuel.

Energies 2023,16, 683 17 of 38
Viscosity of oils belonging to each SAE class must fall within an allowed range at
100
◦
C. Experimental results show that contamination of the lubricating oil with diesel
fuel in an amount within the range of 5–10% m/m results in the oil not meeting SAE
viscosity requirements.
Based on the measured values of kinematic viscosity and density of the blends, dy-
namic viscosity values were calculated for the various lubricating oil–diesel blends. The
results of calculations for lubricating oils of SAE 30 and SAE 40 grades are shown in
Figures 16 and 17, respectively.
Energies 2023, 16, x FOR PEER REVIEW 19 of 41
Viscosity of oils belonging to each SAE class must fall within an allowed range at 100
°C. Experimental results show that contamination of the lubricating oil with diesel fuel in
an amount within the range of 5–10% m/m results in the oil not meeting SAE viscosity
requirements.
Based on the measured values of kinematic viscosity and density of the blends, dy-
namic viscosity values were calculated for the various lubricating oil–diesel blends. The
results of calculations for lubricating oils of SAE 30 and SAE 40 grades are shown in Fig-
ures 16 and 17, respectively.
Figure 16. Measured values of dynamic viscosity of SAE 30 class oil in the temperature range of 40–
100 °C at different levels of lubricating oil dilution with diesel fuel.
Figure 17. Measured values of dynamic viscosity of SAE 40 class oil in the temperature range of 40–
100 °C at different levels of lubricating oil dilution with diesel fuel.
Figure 16.
Measured values of dynamic viscosity of SAE 30 class oil in the temperature range of
40–100 ◦C at different levels of lubricating oil dilution with diesel fuel.
Energies 2023, 16, x FOR PEER REVIEW 19 of 41
Viscosity of oils belonging to each SAE class must fall within an allowed range at 100
°C. Experimental results show that contamination of the lubricating oil with diesel fuel in
an amount within the range of 5–10% m/m results in the oil not meeting SAE viscosity
requirements.
Based on the measured values of kinematic viscosity and density of the blends, dy-
namic viscosity values were calculated for the various lubricating oil–diesel blends. The
results of calculations for lubricating oils of SAE 30 and SAE 40 grades are shown in Fig-
ures 16 and 17, respectively.
Figure 16. Measured values of dynamic viscosity of SAE 30 class oil in the temperature range of 40–
100 °C at different levels of lubricating oil dilution with diesel fuel.
Figure 17. Measured values of dynamic viscosity of SAE 40 class oil in the temperature range of 40–
100 °C at different levels of lubricating oil dilution with diesel fuel.
Figure 17.
Measured values of dynamic viscosity of SAE 40 class oil in the temperature range of
40–100 ◦C at different levels of lubricating oil dilution with diesel fuel.

Energies 2023,16, 683 18 of 38
Since both density and viscosity decrease with increasing oil temperature, the dynamic
viscosity value resulting from their product will also decrease with increasing temperature.
At the same time, the value of dynamic viscosity at a given temperature and the temperature
drop in dynamic viscosity in a given temperature range decrease when the diesel content
of the lubricating oils tested increases.
Based on the viscosity of the tested oils at 40
◦
C and 100
◦
C, viscosity index VI
values were calculated. Calculators available on the websites of Olezol [
69
] and Anton
Paar [
70
] were used for the calculations. The change in VI values for different lubricating
oil contaminants is shown in Figure 18.
Energies 2023, 16, x FOR PEER REVIEW 20 of 41
Since both density and viscosity decrease with increasing oil temperature, the dy-
namic viscosity value resulting from their product will also decrease with increasing tem-
perature. At the same time, the value of dynamic viscosity at a given temperature and the
temperature drop in dynamic viscosity in a given temperature range decrease when the
diesel content of the lubricating oils tested increases.
Based on the viscosity of the tested oils at 40 °C and 100 °C, viscosity index VI values
were calculated. Calculators available on the websites of Olezol [69] and Anton Paar [70]
were used for the calculations. The change in VI values for different lubricating oil con-
taminants is shown in Figure 18.
Figure 18. Calculated viscosity index values of the tested lubricating oils at different levels of lubri-
cating oil dilution with diesel fuel.
Changes in VI for SAE 30 and SAE 40 oil are similar. VI in the diesel concentration
range of 0–10% m/m remains at a similar level in the range of 100–110, gradually increas-
ing the value of the increment as the diesel oil content in the mixture increases. This trend
shows that with increasing diesel content in the lubricating oil mixture, the viscosity of
the mixture shows less and less variation with temperature (the higher the VI value, the
smaller the decrease in viscosity at a given temperature increment).
3.2. Anti-Seizure Properties of Oil
Figure 19 shows the results of oil lubricity measurements. Here, lubricity is defined
as the wear scar diameter WSD (the average of the major and minor axis of the wear scar)
measured on the test ball as a result of the HFFR apparatus test for different diesel con-
tents in the lubricating oils tested. Microscopic images of the wear marks for each of the
tests performed are summarized in Appendix B.
Figure 18.
Calculated viscosity index values of the tested lubricating oils at different levels of
lubricating oil dilution with diesel fuel.
Changes in VI for SAE 30 and SAE 40 oil are similar. VI in the diesel concentration
range of 0–10% m/m remains at a similar level in the range of 100–110, gradually increasing
the value of the increment as the diesel oil content in the mixture increases. This trend
shows that with increasing diesel content in the lubricating oil mixture, the viscosity of
the mixture shows less and less variation with temperature (the higher the VI value, the
smaller the decrease in viscosity at a given temperature increment).
3.2. Anti-Seizure Properties of Oil
Figure 19 shows the results of oil lubricity measurements. Here, lubricity is defined
as the wear scar diameter WSD (the average of the major and minor axis of the wear scar)
measured on the test ball as a result of the HFFR apparatus test for different diesel contents
in the lubricating oils tested. Microscopic images of the wear marks for each of the tests
performed are summarized in Appendix B.

Energies 2023,16, 683 19 of 38
Energies 2023, 16, x FOR PEER REVIEW 21 of 41
Figure 19. Lubricity of tested lubricating oils at different levels of lubricating oil dilution with diesel
fuel.
Lubricity values measured for pure oils are in the range of 180–190 μm. The results
show slight fluctuations in the lubricity values in the low diesel concentration range (con-
tamination of the lubricating oil with diesel up to approx. 10% m/m diesel in the lubricat-
ing oil). For this range, the maximum lubricity variations for the two oils tested are be-
tween 141 μm (SAE 40, 10% m/m diesel oil) and 205 μm (SAE 30, 5% m/m diesel oil). At a
diesel concentration of ~20% m/m, the greatest decrease in lubricity values is observed
(~100–107 μm), followed by an increase in lubricity. From about 50% m/m diesel content
to a value of 100% (pure diesel oil), the lubricity value varies in the range of ~±20 μm
relative to the lubricity value for pure diesel oil of 170 μm.
The HFFR used in the experiment also provides information on oil film continuity
(defined as the percentage loss of oil film resistance during the test, so that 100% corre-
sponds to full oil film separation of mating components), along with averaged friction
coefficient values. These are only auxiliary indicators that can supplement the analysis
based on measured oil lubricity. Figures 20 and 21 show the measurements for these two
indicators.
Figure 19.
Lubricity of tested lubricating oils at different levels of lubricating oil dilution with diesel fuel.
Lubricity values measured for pure oils are in the range of 180–190
µ
m. The results
show slight fluctuations in the lubricity values in the low diesel concentration range
(contamination of the lubricating oil with diesel up to approx. 10% m/m diesel in the
lubricating oil). For this range, the maximum lubricity variations for the two oils tested are
between 141
µ
m (SAE 40, 10% m/m diesel oil) and 205
µ
m (SAE 30, 5% m/m diesel oil). At
a diesel concentration of ~20% m/m, the greatest decrease in lubricity values is observed
(~100–107
µ
m), followed by an increase in lubricity. From about 50% m/m diesel content to
a value of 100% (pure diesel oil), the lubricity value varies in the range of ~
±
20
µ
m relative
to the lubricity value for pure diesel oil of 170 µm.
The HFFR used in the experiment also provides information on oil film continuity
(defined as the percentage loss of oil film resistance during the test, so that 100% corresponds
to full oil film separation of mating components), along with averaged friction coefficient
values. These are only auxiliary indicators that can supplement the analysis based on
measured oil lubricity. Figures 20 and 21 show the measurements for these two indicators.
The conducted tests show that in the range of lubricating oil contamination with diesel
fuel in the concentration range of 0–10% m/m, the resistance of the oil film does not change.
Thus, the mating elements in the HFFR apparatus are separated by a layer of lubricant, and
there is no metallic contact between elements.
The averaged friction coefficient of mating elements in the HFFR apparatus changed
little for diesel fuel concentration 0–10%. An increase in the friction coefficient was observed
above 10% m/m, followed by a decrease in its value above 50% m/m.

Energies 2023,16, 683 20 of 38
Energies 2023, 16, x FOR PEER REVIEW 22 of 41
Figure 20. Percentage loss of oil film resistance of the lubricating oils tested at different levels of
lubricating oil dilution with diesel fuel.
Figure 21. Averaged coefficient of friction of mating elements in the HFFR apparatus separated by
a layer from the tested lubricating oils at different levels of lubricating oil dilution with diesel fuel.
Figure 20.
Percentage loss of oil film resistance of the lubricating oils tested at different levels of
lubricating oil dilution with diesel fuel.
Energies 2023, 16, x FOR PEER REVIEW 22 of 41
Figure 20. Percentage loss of oil film resistance of the lubricating oils tested at different levels of
lubricating oil dilution with diesel fuel.
Figure 21. Averaged coefficient of friction of mating elements in the HFFR apparatus separated by
a layer from the tested lubricating oils at different levels of lubricating oil dilution with diesel fuel.
Figure 21.
Averaged coefficient of friction of mating elements in the HFFR apparatus separated by a
layer from the tested lubricating oils at different levels of lubricating oil dilution with diesel fuel.

Energies 2023,16, 683 21 of 38
The observed changes in lubricity, coefficient of friction, and percentage loss of oil
film resistance with diesel testify to the complexity of the wear process and the difficulty
of objectively determining the lubricating properties of oils. The lubricity information
obtained can be used as a coarse indicator showing the effect of lubricating oil contamina-
tion. Simultaneous consideration of the aforementioned indices along with the viscosity
values of the tested oils shows that their lubricating (anti-wear) properties deteriorate
with increasing fuel contamination. A detailed determination of the relationship between
these properties and the lubricity value obtained by the HFFR method requires further
detailed research.
3.3. Ignition Properties of Oil
Flash point temperature is the primary indicator describing the ignition properties
of oils and fuels. Figure 22 shows the results of measuring the flash point of the tested
mixtures of oil with DO. As expected, the ignition temperature of the mixture decreases
with an increase in the diesel content of the lubricating oil.
Energies 2023, 16, x FOR PEER REVIEW 23 of 41
The conducted tests show that in the range of lubricating oil contamination with die-
sel fuel in the concentration range of 0–10% m/m, the resistance of the oil film does not
change. Thus, the mating elements in the HFFR apparatus are separated by a layer of lub-
ricant, and there is no metallic contact between elements.
The averaged friction coefficient of mating elements in the HFFR apparatus changed
little for diesel fuel concentration 0–10%. An increase in the friction coefficient was ob-
served above 10% m/m, followed by a decrease in its value above 50% m/m.
The observed changes in lubricity, coefficient of friction, and percentage loss of oil
film resistance with diesel testify to the complexity of the wear process and the difficulty
of objectively determining the lubricating properties of oils. The lubricity information ob-
tained can be used as a coarse indicator showing the effect of lubricating oil contamina-
tion. Simultaneous consideration of the aforementioned indices along with the viscosity
values of the tested oils shows that their lubricating (anti-wear) properties deteriorate
with increasing fuel contamination. A detailed determination of the relationship between
these properties and the lubricity value obtained by the HFFR method requires further
detailed research.
3.3. Ignition Properties of Oil
Flash point temperature is the primary indicator describing the ignition properties of
oils and fuels. Figure 22 shows the results of measuring the flash point of the tested mix-
tures of oil with DO. As expected, the ignition temperature of the mixture decreases with
an increase in the diesel content of the lubricating oil.
Figure 22. Flash point temperature determined in a closed crucible for the lubricating oils tested at
different levels of lubricating oil dilution with diesel fuel.
Measured ignition temperatures were significantly lower than those obtained by Fer-
guson. This difference is due to both the different composition of the oil (lubricating oil
and diesel oil) as well as different conditions under which the experiments were con-
ducted. In the Ferguson experiment, the ignition temperature of the diesel oil was 103 °C,
while in the present experiment, the pure diesel fuel tested had an ignition temperature
Figure 22.
Flash point temperature determined in a closed crucible for the lubricating oils tested at
different levels of lubricating oil dilution with diesel fuel.
Measured ignition temperatures were significantly lower than those obtained by
Ferguson. This difference is due to both the different composition of the oil (lubricating oil
and diesel oil) as well as different conditions under which the experiments were conducted.
In the Ferguson experiment, the ignition temperature of the diesel oil was 103
◦
C, while
in the present experiment, the pure diesel fuel tested had an ignition temperature of only
65
◦
C. The observations show that the literature results are not authoritative for modern
distillate fuels and lubricating oils. Importantly, a significant reduction in the ignition
temperature of the oil as a result of its contamination with diesel was confirmed.
Note that for a 5% m/m contamination of the engine’s circulating oil with diesel
fuel, which can occur in operating practice, the flash point decreased by as much as 46 ◦C
compared to that of pure lubricating oil. Such a large decrease indicates a significant

Energies 2023,16, 683 22 of 38
relationship between fuel contamination and the ignition properties of the oil, and thus the
risk of ignition in the crankcase.
The autoignition delay described by the cetane number and other indicators indicated
in Section 2characterizes an oil in terms of its ability to ignite (the shorter the ignition
delay, the better the autoignition properties). In the case in question of the analysis of the
autoignition capability of lubricating oils, the lower the cetane number, the better the oil is.
Figure 23 shows results of the self-ignition capability tests.
Energies 2023, 16, x FOR PEER REVIEW 24 of 41
of only 65 °C. The observations show that the literature results are not authoritative for
modern distillate fuels and lubricating oils. Importantly, a significant reduction in the ig-
nition temperature of the oil as a result of its contamination with diesel was confirmed.
Note that for a 5% m/m contamination of the engine’s circulating oil with diesel fuel,
which can occur in operating practice, the flash point decreased by as much as 46 °C com-
pared to that of pure lubricating oil. Such a large decrease indicates a significant relation-
ship between fuel contamination and the ignition properties of the oil, and thus the risk
of ignition in the crankcase.
The autoignition delay described by the cetane number and other indicators indi-
cated in the materials and methods section characterizes an oil in terms of its ability to
ignite (the shorter the ignition delay, the better the autoignition properties). In the case in
question of the analysis of the autoignition capability of lubricating oils, the lower the
cetane number, the better the oil is. Figure 23 shows results of the self-ignition capability
tests.
Figure 23. Ignition and cetane indices of tested lubricating oils at different levels of lubricating oil
dilution with diesel fuel.
Figure 24 shows ignition indices analogous to the cetane number: derived cetane
number, DCN, calculated cetane index, CCI, and calculated ignition index, CII. The DCN
value was found to be 52 for pure diesel fuel. The CCI values were determined due to
technical feasibility (difficulty in distilling the fractions) for the 50% m/m concentration of
diesel fuel in the lubricating oils tested and for pure diesel fuel. In the concentration range
of 50–100% m/m, the CCI value ranges from 52.1 to 53.2 for the tested oils. In contrast, the
calculated average CII values for the entire concentration range are between 60.2 and 46.6
for SAE 30 oil and between 61.1 and 46.6 for SAE 40 oil. With an increase in the diesel
content of the mixture in both cases, the CCI value decreases, which may indicate a slight
increase in the ignition delay and a deterioration in the mixture’s self-ignition capability.
Nevertheless, these values are high throughout the measurement range (>45), which, by
Figure 23.
Ignition and cetane indices of tested lubricating oils at different levels of lubricating oil
dilution with diesel fuel.
Figure 24 shows ignition indices analogous to the cetane number: derived cetane
number, DCN, calculated cetane index, CCI, and calculated ignition index, CII. The DCN
value was found to be 52 for pure diesel fuel. The CCI values were determined due to
technical feasibility (difficulty in distilling the fractions) for the 50% m/m concentration of
diesel fuel in the lubricating oils tested and for pure diesel fuel. In the concentration range
of 50–100% m/m, the CCI value ranges from 52.1 to 53.2 for the tested oils. In contrast,
the calculated average CII values for the entire concentration range are between 60.2 and
46.6 for SAE 30 oil and between 61.1 and 46.6 for SAE 40 oil. With an increase in the diesel
content of the mixture in both cases, the CCI value decreases, which may indicate a slight
increase in the ignition delay and a deterioration in the mixture’s self-ignition capability.
Nevertheless, these values are high throughout the measurement range (>45), which, by
analogy with fuels, can be interpreted as a very low autoignition delay, and therefore very
good ignition properties.

Energies 2023,16, 683 23 of 38
Energies 2023, 16, x FOR PEER REVIEW 25 of 41
analogy with fuels, can be interpreted as a very low autoignition delay, and therefore very
good ignition properties.
Figure 24. Calculated carbon aromaticity index of the tested lubricating oils at different levels of
lubricating oil dilution with diesel fuel.
Analogous to CII, the value of the calculated carbon aromaticity index CCAI was de-
termined for the tested mixtures. CCAI values increase with increasing diesel fuel contam-
ination. CCAI values vary between 781.4 and 795.5 for SAE 30 oil blends and between
775.6 and 795.5 for SAE 40 oil blends. Relating these values to the standards provided for
residual fuels, lubricating oil contamination with diesel fuel has no significant effect on
the autoignition delay, which corresponds to very good autoignition properties.
4. Final Conclusions
There is no doubt that measuring the ignition temperature of the oil makes it possible
to quickly diagnose a situation where fuel contamination has occurred in the oil. Because
laboratory tests of lubricating oil are carried out periodically at fixed intervals between
tests, they will not be a valuable measure in the situation of sudden and rapidly develop-
ing damage to piston rings, pistons, cylinder liners, and injectors. However, it cannot be
ruled out that in situations of long-developing engine deterioration, testing the oil for po-
tential fuel contamination may be a key element in increasing the safety of engine opera-
tion [71]. For the same reason, the flash point test has been adopted by all major used oil
laboratories as a service standard to vessel operators.
It should be emphasized that the authors of the CIMAC publication [31] do not rule
out the possibility of using an indicator such as flash point to diagnose whether or not oil
contamination with fuel (residual or distillate) has occurred. The publication states that
“For engines which alternate between heavy fuel and distillate fuel usage, flash point
measurements can be used to monitor the origin of possible fuel contamination (the
Figure 24.
Calculated carbon aromaticity index of the tested lubricating oils at different levels of
lubricating oil dilution with diesel fuel.
Analogous to CII, the value of the calculated carbon aromaticity index CCAI was
determined for the tested mixtures. CCAI values increase with increasing diesel fuel
contamination. CCAI values vary between 781.4 and 795.5 for SAE 30 oil blends and
between 775.6 and 795.5 for SAE 40 oil blends. Relating these values to the standards
provided for residual fuels, lubricating oil contamination with diesel fuel has no significant
effect on the autoignition delay, which corresponds to very good autoignition properties.
4. Final Conclusions
There is no doubt that measuring the ignition temperature of the oil makes it possible
to quickly diagnose a situation where fuel contamination has occurred in the oil. Because
laboratory tests of lubricating oil are carried out periodically at fixed intervals between
tests, they will not be a valuable measure in the situation of sudden and rapidly developing
damage to piston rings, pistons, cylinder liners, and injectors. However, it cannot be ruled
out that in situations of long-developing engine deterioration, testing the oil for potential
fuel contamination may be a key element in increasing the safety of engine operation [71].
For the same reason, the flash point test has been adopted by all major used oil laboratories
as a service standard to vessel operators.
It should be emphasized that the authors of the CIMAC publication [
31
] do not rule
out the possibility of using an indicator such as flash point to diagnose whether or not
oil contamination with fuel (residual or distillate) has occurred. The publication states
that “For engines which alternate between heavy fuel and distillate fuel usage, flash
point measurements can be used to monitor the origin of possible fuel contamination
(the contamination of heavy fuel and distillate fuel can compensate the resulting changes
in viscosity)”.
The main observations from the author’s own experiment are as follows:
1.
A decrease in the lubricating oil flash point is an indicator of oil contamination with fuel.

Energies 2023,16, 683 24 of 38
2.
A change in viscosity does not necessarily indicate contamination (among other
contaminations), since, depending on the type of fuel, oil viscosity can:
•not change (engines powered by different fuels);
•decrease (engines powered by distillation fuel);
•increase (engines fueled by residual fuel).
A change in viscosity itself is not necessarily caused by contamination of the oil with
fuel, but could be due to contamination with other oil (such as cylinder oil), contamination
of the oil with water, bacteria, fungi, and protozoa; oxidation of the oil, etc. [72–74].
3.
Changing the oil composition can change its viscosity, which alters the distribution of
the number and size of droplets in the oil mist. Occurrence of this mist, once set limits
are exceeded, promotes the initiation of explosions.
4.
A reduction in the ignition temperature of a mixture may be indirectly associated
with a reduction in the autoignition temperature (although not necessarily) [
14
,
75
]. If
this situation occurs, the risk of explosion increases in view of the higher volatility
of diesel fuel than lubricating oil. Contamination of lubricating oil with distillation
fuel does not significantly affect the autoignition delay, which, for clean and contami-
nated oil, corresponds to conditions that can be classified as very good autoignition
properties [76].
5.
Deterioration of the tribological conditions of the bearings, as seen in the change
in viscosity, viscosity index, and lubricity of the oil, causes an increase in bearing
temperature and the possibility of hot spots. Viscosity index, lubricity, coefficient of
friction and reduction in oil film resistance change significantly when the concentra-
tion of diesel fuel in the lubricating oil exceeds 10%. The observation is in line with
some previous works [
36
]. At such concentrations, increased friction in tribological
pairs lubricated with lubricating oil contaminated with diesel fuel can intensify the
wear of mating components, increase their temperature, and ultimately intensify the
formation of white oil mist in the crankcase.
Summing up, there are two extreme approaches in the literature to the subject of diesel
lubricating oil contamination and the effects of this contamination on engine operation. The
first, which was the inspiration for the article, is the approach presented by Ferguson and in
the paper published by CIMAC. This approach states that the effects of diesel contamination
in lubricating oil are negligible even at high diesel concentrations in the lubricating oil. The
second approach points to an increased risk of engine seizure and cites 2–5% diesel content
in lubricating oil as alarming levels. However, neither approach is well-argued, and a
holistic approach to the subject has not been presented. In this article, results show that
when the diesel oil content in the lubricating oil exceeds 10%, oil–film rupture occurs, and
this can result in increased wear of tribological couples, such as bearings in the crank–piston
system. At the same time, given the decrease in viscosity and thus the deterioration of
lubrication conditions, and the decrease in evaporation and ignition temperatures, one
can conclude that the synergistic interaction of these factors increases the risk of explosion
in the engine crankcase. Further conclusions could be provided as a result of an oil mist
morphology analysis, which is planned to be the next step in the future research.
Funding:
This research was funded by the Ministry of Science and Higher Education (MEiN) of
Poland, grant number 1/S/KPBMiM/22. The APC was funded by MDPI.
Data Availability Statement:
All data are available in the dataset: Chybowski, L. Lube oil—diesel
oil mixes-dataset; 2022, Ver. 3, DOI: 10.17632/scbx3h2bmf.3. Dataset is available at https://data.
mendeley.com/datasets/scbx3h2bmf/3 (accessed on 8 November 2022) [57].
Acknowledgments:
Laboratory tests were performed on behalf of the author at the Center for Testing
Fuels, Working Fluids, and Environmental Protection (CBPCRiOS) of the Maritime University of
Szczecin. The author would like to thank Magdalena Szmukała and Barbara ˙
Zura´nska for the
technical support.

Energies 2023,16, 683 25 of 38
Conflicts of Interest:
The author declares no conflict of interest. The funders had no role in the design
of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript, or
in the decision to publish the results.
Abbreviations
ASTM American Society for Testing and Materials
CCE crankcase explosion
CEA crankcase explosion apparatus
CFR Cooperative Fuel Research
CID cetane ignition delay
CIMAC International Council on Combustion Engines
CCAI calculated carbon aromaticity index
CCI calculated cetane index
CI cetane index
CII calculated ignition index
CN cetane number
CVCC constant volume combustion chamber
DCN derived cetane number
DO diesel oil
ECP electrical contact potential
FDM fuel dilution meter
FID flame ionization detector
FILM measure of oil film resistance
FIT fuel ignition tester
FP flash point temperature
FTIF Fourier-transform infrared
GC gas chromatography
HFO heavy fuel oil
HFRR high frequency reciprocating rig
IEC International Electrotechnical Commission
IQT ignition quality tester
ISO International Organization for Standardization
SAE Society of Automotive Engineers
SAE 30, SAE 40 viscosity grades of lubricating oils according to SAE J300-2021 standard
SAW surface acoustic wave
VI viscosity index
WSD wear scar diameter
Symbols
mmass of the substance
tmeasured temperature of the substance under operating conditions
Vvolume of the substance
.
γshear rate
εthe coefficient of change of density of a substance when it is heated by 1 ◦C
ηdynamic viscosity of the substance
λthermal conductivity coefficient
µcoefficient of friction
νkinematic viscosity of the substance
ρdensity of the substance
ρ15 density of the substance at 15 ◦C
ρtdensity of the substance at temperature t
τtime
τFshear stresses

Energies 2023,16, 683 26 of 38
Appendix A.
Table A1. Fuel inflow caused by mechanical effect (based on [30]).
Action Effect What to Do
Continued operation with
stops and start. The fuel does not burn off completely.
Reduce the mileage change interval to the
strictest change interval indicated by the
manufacturer.
Starting in the cold. The fuel does not burn off well because the
combustion temperature is low.
Wait for the engine to increase in temperature
before accelerating.
Problems in the
injection system.
The droplets of fuel being injected into the
chamber are big, leading to poor combustion.
Incomplete combustion is occurring; inspect
the injectors.
Poor combustion. The fuel is not burning off completely.
Incomplete combustion is occurring. Check that
the combustion chamber and the injection
system are working properly.
Worn-out engine parts: valve
guides, injectors, and wear.
Conditions change in the combustion
chamber, meaning it is no longer optimized. Inspect the engine and injectors.
Excessive acceleration. Excess inflow of fuel. Incomplete combustion is occurring; adjust
control system.
Mixture of rich fuels. Excess fuel. Incomplete combustion is occurring; inspect the
injection system.
Faulty injectors. Can produce excessive inflow of fuel or
inadequate fuel injection.
It does not burn fuel as well, resulting in
deposits. Inspection of the injection system.
Table A2.
Advantages and disadvantages of the main methods of detecting fuel in lubricating oil
(prepared on the basis of [56]).
Method Advantages Disadvantages
Gas Chromatography
Widely accepted industry standard
Highly precise
Suited for high volume labs
Can detect biodiesel and ethanol
Can only be carried out in a lab
Mandates costly equipment and gases
Takes much time to produce best results
Requires an expensive equipment and gases
Viscosity Analysis
The availability of portable instruments and
lab instruments
Accepted routine test for testing lubricant condition
Optimum screening test for probable fuel dilution
Ability to detect ethanol and biodiesel
Inability to definitively indicate fuel
dilution issue
Mandates a careful technician
Flash Point Testing
A pass/fail result is enough in the case of most
applications
Ability to detect ethanol
Very little sample required (1–2 mL)
Inability to detect biodiesel
Mandates a careful technician
Knowledge of the oil/fuel type mandatory for
quantitative measurement
Risks posed by heating fuel-laden samples
FTIR Spectroscopy Low cost per sample after initial equipment
purchase; test can be carried out quickly
Mandates the use of costly equipment;
calibrations are mostly specific to a narrow
sample type
Surface Acoustic Wave
Sensing
Easy to use
Portable
Requires only 0.5 mL of sample
Less expensive than gas chromatographs
Can complete the test quickly
Easily adaptable to different oil/fuel types
Inability to measure biodiesel
Mandates calibration with a reference fluid

Energies 2023,16, 683 27 of 38
Table A3. Properties of Orlen Efecta Diesel Bio as declared by the manufacturer [60].
Specification Parameter
Cetane index ≤51
Initial boiling point 75–180 ◦C
Boiling temperature range 95% vol. distils to 360 ◦C
Flash point (determined in a closed crucible) >56 ◦C
Autoignition temperature (according to DIN51794:2003-05) approx. 240 ◦C
Kinematic viscosity (according to EN ISO 3104) 1.5–4.5 mm2/s (2.549 mm2/s) at 40 ◦C
approx. 2.151 mm2/s at 50 ◦C
Density 820–845 kg/m3at 15 ◦C
Relative vapor density approx. 6 (air = 1)
Cloud point −7◦C
Cold filter plugging point −8◦C
Table A4.
Manufacturer-declared physicochemical properties of Agip Cladium 120 CD lubricating
oils used in tests [61–63].
Specification Parameter
Oil Agip Cladium 120
SAE 30 CD
Agip Cladium 120
SAE 40 CD
Kinematic viscosity (according to EN ISO 3104)
108 mm2/s at 40 ◦C
12.0 mm2/s at 100 ◦C
160 mm2/s at 40 ◦C
15.7 mm2/s at 100 ◦C
Viscosity index 100 100
Total base number 12 mg KOH/g 12 mg KOH/g
Flash point (marked in closed crucible) 225 235 ◦C
Pour point −18 ◦C−15 ◦C
Density 895 kg/m3at 15 ◦C 900 kg/m3at 15 ◦C
Table A5.
Basic characteristics of several machines to study the lubricating properties (compiled
from [65,77]).
Tribometer Friction Association Type of Contact Application of Tests
Four-ball machine
1
Tribometer Friction Association Type of Contact Application of Tests
Four-ball machine
Punctual
Testing the anti-wear properties of lubricating
oils, plastic lubricants and other operating
fluids.
Timken
Linear Testing the anti-wear properties of plastic
lubricants.
Falex
Linear
Testing the properties of the solid lubrication
film, anti-wear properties of lubricating oils and
the properties of plastic lubricants.
Almen-Eieland
Surface Testing the anti-wear properties and maximum
load of lubricating oils and plastic lubricants.
FZG
Linear Testing the anti-wear properties of lubricating
oils and plastic lubricants, especially gear oils.
Vickers
Punctual
Testing the anti-wear properties of thermo-
oxidizing fluids hydraulic fluids, turbine oils,
gear oils.
HFFR
Punctual Testing the anti-wear properties of diesel oils,
heating oils and lubricating oils.
Punctual Testing the anti-wear properties of lubricating oils, plastic lubricants
and other operating fluids.
Timken
1
Tribometer Friction Association Type of Contact Application of Tests
Four-ball machine
Punctual
Testing the anti-wear properties of lubricating
oils, plastic lubricants and other operating
fluids.
Timken
Linear Testing the anti-wear properties of plastic
lubricants.
Falex
Linear
Testing the properties of the solid lubrication
film, anti-wear properties of lubricating oils and
the properties of plastic lubricants.
Almen-Eieland
Surface Testing the anti-wear properties and maximum
load of lubricating oils and plastic lubricants.
FZG
Linear Testing the anti-wear properties of lubricating
oils and plastic lubricants, especially gear oils.
Vickers
Punctual
Testing the anti-wear properties of thermo-
oxidizing fluids hydraulic fluids, turbine oils,
gear oils.
HFFR
Punctual Testing the anti-wear properties of diesel oils,
heating oils and lubricating oils.
Linear Testing the anti-wear properties of plastic lubricants.
Falex
1
Tribometer Friction Association Type of Contact Application of Tests
Four-ball machine
Punctual
Testing the anti-wear properties of lubricating
oils, plastic lubricants and other operating
fluids.
Timken
Linear Testing the anti-wear properties of plastic
lubricants.
Falex
Linear
Testing the properties of the solid lubrication
film, anti-wear properties of lubricating oils and
the properties of plastic lubricants.
Almen-Eieland
Surface Testing the anti-wear properties and maximum
load of lubricating oils and plastic lubricants.
FZG
Linear Testing the anti-wear properties of lubricating
oils and plastic lubricants, especially gear oils.
Vickers
Punctual
Testing the anti-wear properties of thermo-
oxidizing fluids hydraulic fluids, turbine oils,
gear oils.
HFFR
Punctual Testing the anti-wear properties of diesel oils,
heating oils and lubricating oils.
Linear Testing the properties of the solid lubrication film, anti-wear
properties of lubricating oils and the properties of plastic lubricants.
Almen-Eieland
1
Tribometer Friction Association Type of Contact Application of Tests
Four-ball machine
Punctual
Testing the anti-wear properties of lubricating
oils, plastic lubricants and other operating
fluids.
Timken
Linear Testing the anti-wear properties of plastic
lubricants.
Falex
Linear
Testing the properties of the solid lubrication
film, anti-wear properties of lubricating oils and
the properties of plastic lubricants.
Almen-Eieland
Surface Testing the anti-wear properties and maximum
load of lubricating oils and plastic lubricants.
FZG
Linear Testing the anti-wear properties of lubricating
oils and plastic lubricants, especially gear oils.
Vickers
Punctual
Testing the anti-wear properties of thermo-
oxidizing fluids hydraulic fluids, turbine oils,
gear oils.
HFFR
Punctual Testing the anti-wear properties of diesel oils,
heating oils and lubricating oils.
Surface Testing the anti-wear properties and maximum load of lubricating
oils and plastic lubricants.
FZG
1
Tribometer Friction Association Type of Contact Application of Tests
Four-ball machine
Punctual
Testing the anti-wear properties of lubricating
oils, plastic lubricants and other operating
fluids.
Timken
Linear Testing the anti-wear properties of plastic
lubricants.
Falex
Linear
Testing the properties of the solid lubrication
film, anti-wear properties of lubricating oils and
the properties of plastic lubricants.
Almen-Eieland
Surface Testing the anti-wear properties and maximum
load of lubricating oils and plastic lubricants.
FZG
Linear Testing the anti-wear properties of lubricating
oils and plastic lubricants, especially gear oils.
Vickers
Punctual
Testing the anti-wear properties of thermo-
oxidizing fluids hydraulic fluids, turbine oils,
gear oils.
HFFR
Punctual Testing the anti-wear properties of diesel oils,
heating oils and lubricating oils.
Linear Testing the anti-wear properties of lubricating oils and plastic
lubricants, especially gear oils.
Vickers
1
Tribometer Friction Association Type of Contact Application of Tests
Four-ball machine
Punctual
Testing the anti-wear properties of lubricating
oils, plastic lubricants and other operating
fluids.
Timken
Linear Testing the anti-wear properties of plastic
lubricants.
Falex
Linear
Testing the properties of the solid lubrication
film, anti-wear properties of lubricating oils and
the properties of plastic lubricants.
Almen-Eieland
Surface Testing the anti-wear properties and maximum
load of lubricating oils and plastic lubricants.
FZG
Linear Testing the anti-wear properties of lubricating
oils and plastic lubricants, especially gear oils.
Vickers
Punctual
Testing the anti-wear properties of thermo-
oxidizing fluids hydraulic fluids, turbine oils,
gear oils.
HFFR
Punctual Testing the anti-wear properties of diesel oils,
heating oils and lubricating oils.
Punctual
Testing the anti-wear properties of thermo-oxidizing fluids hydraulic
fluids, turbine oils, gear oils.
HFFR
1
Tribometer Friction Association Type of Contact Application of Tests
Four-ball machine
Punctual
Testing the anti-wear properties of lubricating
oils, plastic lubricants and other operating
fluids.
Timken
Linear Testing the anti-wear properties of plastic
lubricants.
Falex
Linear
Testing the properties of the solid lubrication
film, anti-wear properties of lubricating oils and
the properties of plastic lubricants.
Almen-Eieland
Surface Testing the anti-wear properties and maximum
load of lubricating oils and plastic lubricants.
FZG
Linear Testing the anti-wear properties of lubricating
oils and plastic lubricants, especially gear oils.
Vickers
Punctual
Testing the anti-wear properties of thermo-
oxidizing fluids hydraulic fluids, turbine oils,
gear oils.
HFFR
Punctual Testing the anti-wear properties of diesel oils,
heating oils and lubricating oils.
Punctual Testing the anti-wear properties of diesel oils, heating oils and
lubricating oils.

Energies 2023,16, 683 28 of 38
Table A6. ASTM cetane rating Standards and Applicable Ranges (prepared on the basis of [66–68]).
ASTM Standard CN Applicable Range Range (mix.—min.) Instrument
D6890 (DCN)33–64 31.0 IQT (CVCC)
64–100 36.0
D613 (CN) 40–56 16.0 CFR engine
D7170 (DCN) 39.5–55.2 15.7 FIT (CVCC)
D7668 (DCN)
39.4–66.8 [66] 27.4 [66]
CID 510 (CVCC)
35.0–60.0 [67] 25.0 [67]
15.0–100.0 [68] 85.0 [68]
D976 (CI) 30–60 30.0 Correlation
D4737 (CCI) 32.5–56.5 24.0 Correlation
Appendix B. Photographs of Traces of Wear on a Moving Component after an HFRR
Lubricity Test
Energies 2023, 16, x FOR PEER REVIEW 31 of 41
Appendix B. Photographs of Traces of Wear on a Moving Component after an HFRR
Lubricity Test
Figure A1. SAE 30–100%; DO—0%.
Figure A2. SAE 30—99% m/m; DO—1% m/m.
Figure A1. SAE 30–100%; DO—0%.

Energies 2023,16, 683 29 of 38
Energies 2023, 16, x FOR PEER REVIEW 31 of 41
Appendix B. Photographs of Traces of Wear on a Moving Component after an HFRR
Lubricity Test
Figure A1. SAE 30–100%; DO—0%.
Figure A2. SAE 30—99% m/m; DO—1% m/m.
Figure A2. SAE 30—99% m/m; DO—1% m/m.
Energies 2023, 16, x FOR PEER REVIEW 32 of 41
Figure A3. SAE 30—98% m/m; DO—2% m/m.
Figure A4. SAE 30—95% m/m; DO—5% m/m.
Figure A3. SAE 30—98% m/m; DO—2% m/m.

Energies 2023,16, 683 30 of 38
Energies 2023, 16, x FOR PEER REVIEW 32 of 41
Figure A3. SAE 30—98% m/m; DO—2% m/m.
Figure A4. SAE 30—95% m/m; DO—5% m/m.
Figure A4. SAE 30—95% m/m; DO—5% m/m.
Energies 2023, 16, x FOR PEER REVIEW 33 of 41
Figure A5. SAE 30—90% m/m; DO—10% m/m.
Figure A6. SAE 30—80% m/m; DO—20% m/m.
Figure A5. SAE 30—90% m/m; DO—10% m/m.

Energies 2023,16, 683 31 of 38
Energies 2023, 16, x FOR PEER REVIEW 33 of 41
Figure A5. SAE 30—90% m/m; DO—10% m/m.
Figure A6. SAE 30—80% m/m; DO—20% m/m.
Figure A6. SAE 30—80% m/m; DO—20% m/m.
Energies 2023, 16, x FOR PEER REVIEW 34 of 41
Figure A7. SAE 30—50% m/m; DO—50% m/m.
Figure A8. SAE 40—100%; DO—0%.
Figure A7. SAE 30—50% m/m; DO—50% m/m.

Energies 2023,16, 683 32 of 38
Energies 2023, 16, x FOR PEER REVIEW 34 of 41
Figure A7. SAE 30—50% m/m; DO—50% m/m.
Figure A8. SAE 40—100%; DO—0%.
Figure A8. SAE 40—100%; DO—0%.
Energies 2023, 16, x FOR PEER REVIEW 35 of 41
Figure A9. SAE 40—99% m/m; DO—1% m/m.
Figure A10. SAE 40—98% m/m; DO—2% m/m.
Figure A9. SAE 40—99% m/m; DO—1% m/m.

Energies 2023,16, 683 33 of 38
Energies 2023, 16, x FOR PEER REVIEW 35 of 41
Figure A9. SAE 40—99% m/m; DO—1% m/m.
Figure A10. SAE 40—98% m/m; DO—2% m/m.
Figure A10. SAE 40—98% m/m; DO—2% m/m.
Energies 2023, 16, x FOR PEER REVIEW 36 of 41
Figure A11. SAE 40—95% m/m; DO—5% m/m.
Figure A12. SAE 40—90% m/m; DO—10% m/m.
Figure A11. SAE 40—95% m/m; DO—5% m/m.

Energies 2023,16, 683 34 of 38
Energies 2023, 16, x FOR PEER REVIEW 36 of 41
Figure A11. SAE 40—95% m/m; DO—5% m/m.
Figure A12. SAE 40—90% m/m; DO—10% m/m.
Figure A12. SAE 40—90% m/m; DO—10% m/m.
Energies 2023, 16, x FOR PEER REVIEW 37 of 41
Figure A13. SAE 40—80% m/m; DO—20% m/m.
Figure A14. SAE 40—50% m/m; DO—50% m/m.
Figure A13. SAE 40—80% m/m; DO—20% m/m.

Energies 2023,16, 683 35 of 38
Energies 2023, 16, x FOR PEER REVIEW 37 of 41
Figure A13. SAE 40—80% m/m; DO—20% m/m.
Figure A14. SAE 40—50% m/m; DO—50% m/m.
Figure A14. SAE 40—50% m/m; DO—50% m/m.
Energies 2023, 16, x FOR PEER REVIEW 38 of 41
Figure A15. SAE 30/SAE 40—0%; DO—100%.
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Figure A15. SAE 30/SAE 40—0%; DO—100%.

Energies 2023,16, 683 36 of 38
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