Development of an artificial aging process for automotive lubricants

Abstract

The issue of engine oil dilution and degradation, as well as the chemically aggressive nature of certain bio-derived fuels is a known and well researched topic. However, the friction and wear phenomena which occur during long term operation of an internal combustion engine of a hybrid powertrain with alternative and synthetic fuels need further attention. The behaviour of contacting surfaces in the system in connection with contaminated engine oil, oxidation processes and fuel related dilution are of particular interest. A preliminary study of friction and wear testing of artificially aged engine oil samples is presented in this study. The goal of this study is to establish a basic understanding of artificial aging and develop a procedure that is capable of producing aged samples comparable to field aged engine oil. The artificial aging is conducted on 6x 100 ml oil samples concurrently in a regulated environment without any contaminants. The process comprises of several tempering and cooling phases accompanied by constant air circulation through the sample. Aged oil samples are subjected to a test procedure utilizing a ball-on-disc system on a high frequency reciprocating rig. The tribological characteristics of the samples are described through the analysis of the coefficient of friction curve and the averaged wear scar diameter. The quantitative results are supplemented by the analysis of the wear scar. The first results show that the artificial aging setup needs further revisions in order to produce a stable outcome. A large variance in coefficient of friction and averaged wear scar diameter between identically aged oil samples was registered. The differences between batches is believed to be a consequence of temperature probe placement and air throughput affecting the oxidation of the oil.

Full text

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DEVELOPMENT OF AN ARTIFICIAL AGING PROCESS FOR
AUTOMOTIVE LUBRICANTS
András Lajos Nagy
Széchenyi István University, assistant lecturer, PhD student, nagy.andras1@sze.hu
Abstract
The issue of engine oil dilution and degradation, as well as the chemically aggressive nature of
certain bio-derived fuels is a known and well researched topic. However, the friction and wear
phenomena which occur during long term operation of an internal combustion engine of a
hybrid powertrain with alternative and synthetic fuels need further attention. The behaviour of
contacting surfaces in the system in connection with contaminated engine oil, oxidation
processes and fuel related dilution are of particular interest.
A preliminary study of friction and wear testing of artificially aged engine oil samples is
presented in this study. The goal of this study is to establish a basic understanding of artificial
aging and develop a procedure that is capable of producing aged samples comparable to field
aged engine oil. The artificial aging is conducted on 6x 100 ml oil samples concurrently in a
regulated environment without any contaminants. The process comprises of several tempering
and cooling phases accompanied by constant air circulation through the sample. Aged oil
samples are subjected to a test procedure utilizing a ball-on-disc system on a high frequency
reciprocating rig. The tribological characteristics of the samples are described through the
analysis of the coefficient of friction curve and the averaged wear scar diameter. The
quantitative results are supplemented by the analysis of the wear scar. The first results show
that the artificial aging setup needs further revisions in order to produce a stable outcome. A
large variance in coefficient of friction and averaged wear scar diameter between identically
aged oil samples was registered. The differences between batches is believed to be a
consequence of temperature probe placement and air throughput affecting the oxidation of the
oil.
Keywords: lubricant, oxidation, tribology, friction, wear
1. Introduction
Environmental protection is one of the main criteria of modern powertrain development. New
fuel concepts are being developed in order to reduce harmful emissions locally and globally.
This presents a new environment for the powertrain and its components. Automotive lubricants
are formulated to withstand high mechanical load, elevated temperatures, extreme pressure,
contamination and oxidation to a certain extent. Novel fuel formulations like bio-derived fuels
or oxygenated compounds could contribute to accelerated aging processes or chemical reactions
with certain materials inside the powertrain and the internal combustion engine in particular.
State of the art passenger car and commercial vehicle engines implement direct injection fuel
systems with high injection pressures and varied injection timing strategies. Injecting fuel
directly inside the combustion chamber offers a more precise control over mixture formation
and the combustion process which together with charging allows for higher specific power and
torque. The dilution of the lubricant with fuel is a possible consequence of fuel impingement
on the piston top and cylinder wall, which can lead to the transport of fuel trough the piston
ring pack into the crank case, where the blending of fuel and engine oil can occur. The unburnt
hydrocarbon content of the exhaust gas which is influenced by late injection timing [1], [2] and
exhaust gas recirculation can contribute to a similar phenomenon (Fig. 1.) [3]. In addition to
the design and control strategy of the engine, environmental variable also can have an effect on

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oil degradation. Cold climate, driver behaviour and traffic conditions can contribute to an
elevated amount of fuel dilution [4].
Figure 1. Overview of fuel absorption and transport processes taking place in-cylinder
and on the cylinder liner according to Shayler et al. [3]
Fuel dilution and oil aging can have diverse effects on the internal combustion engine on a
system level, which could induce complex processes and impact several components. Lubricity
and viscosity can be affected through fuel dilution, which lead to a thinner elasto-hydrodynamic
film between lubricated surfaces [5] that can lead to higher wear under elevated load. Degraded
lubricant inside the combustion chamber can increase the occurrence of stochastic pre-ignition
(SPI), a phenomenon of knocking which can lead to damaged or failed components [6].
Standardized procedures for engine oil testing are designed to quantize the oxidation stability
of fully formulated lubricants under certain circumstances. However, these procedures rarely
incorporate long-term effects in association with fuel dilution or extend beyond the chemical
evaluation of the aged lubricant. Utilizing dynamometer or field tests to produce used engine
oil would be the most realistic approach in order to analyse oil degradation and its implications
on the engine [7]. However, the time and cost associated with these methods prevents their
usage in the first phase of development, where a broad number of base oils, additives, fuels and
materials in combination with each other have to be proven regarding compatibility. Coupling
temperatures up to 300°C with short reaction times can yield fast results which can be relevant
to the simulation of thermal shock or bearing seizure scenarios, however the chemical changes
taking place due to cyclic energy input cannot be assessed [8]. A cyclic oxidation test under
moderate temperatures around 160°C followed by friction and wear experiments on a ball-on-
disc model system can produce results for a variety of cases under reasonable time [9]. This
procedure could incorporate fuel dilution with minor modifications to the artificial aging setup
and with a modified yet still simple friction test setup which better represents boundary
conditions inside and engine.
A standard procedure for the integrated analysis of the long-term effects of fuel in engine oil
and fuel induced oil degradation is yet to be developed. The goal of this study is to establish a
basic understanding of artificial aging and design a procedure which will be capable of
producing aged samples comparable in composition and performance to field aged engine oil.
Results presented in this paper represent the basis for the development of an artificial oil aging
and analysis procedure which will be capable of incorporating fuel dilution and assessing aged
samples through friction and wear testing on a piston ring – cylinder liner model system.

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1. Methodology
1.1 Laboratory oil aging procedure
In order to reduce cost and time needed for investigating several oil formulations with diverse
fuel samples regarding thermal and oxidation stability a laboratory procedure was established
based on Singer et al [10]. The chosen method combines the effects of high temperature and
constant gas flow inside the internal combustion engine with a cyclic load structure similar to
real-life vehicle use in order to induce chemical alterations of the lubricant.
A thermal-cycling rig (Fig. 2.) was designed with 6 individual heaters for the simultaneous
aging of different samples. The heater stages are able to accept round bottom flasks up to 500
ml and utilize a heater mantle for direct heat transfer. The flasks are connected to a continuous
air supply for plain air circulation through the oil samples using Drechsel bottle heads. The
gases exiting the flasks are collected and fed through two Drechsel bottles filled with tap water
for filtration purposes. The setup is placed inside a purpose-built cabinet which consist of an
aluminium profile frame, polycarbonate side and top panels and a stainless-steel vat for spillage
collection. The cabinet is connected to an explosion-proof air extraction unit.
Figure 2. Schematics and photograph of the thermal-cycling apparatus based on Singer
et al. [10]
The built-in temperature control unit of the heater has no temperature logging capability; hence
an Arduino data logging system was designed to monitor temperature. This setup also allows
for future automation of the thermal-cycling equipment with precise temperature control,
automated cycle start and stop, electronic air flow monitoring and control for each heater stage.
Temperature is measured with type-K thermocouples fitted between the corresponding flasks
and heater mantle of 5 of the 6 available heater stages.
Six batches of 150 ml SAE 0W-20 grade commercially available engine oil were subjected to
a 96-hour long cyclic aging experiment in order to check the control stability of the equipment.
Each heater was set to achieve a mantle temperature of 160°C ± 5°C. Three of the six samples
were chosen for friction and wear testing based on the temperature profiles collected during the
aging process.
1.2 Friction and wear testing
Friction and wear measurements were carried out on an Optimol SRV5 high frequency
reciprocating rig (HFRR) in a ball-on-disc configuration utilizing 100Cr6 steel specimens
supplied by the manufacturer. To minimize measurement error and maximize reproducibility
the testing of the aged oil samples was carried out according to the ISO 19291:2016 standard.

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This procedure utilizes a Ø24 mm flat disc specimen and a Ø10 mm ball specimen which travels
a 1 mm stroke with 50 Hz frequency for 120 minutes. The calculated initial Hertzian contact
pressure based on the dimension, material and load of the ball and disc specimen is 2.09 GPa.
Measurements were carried out at 100°C specimen temperature. The details of the used
parameter set are described in Table 1.
Table 1. Load set for friction and wear testing
Parameter
Value
Parameter
Value
Lubricant volume [ml]
0.3
Run-in load [N]
50
Stroke length [mm]
1.0
Run-in time [s]
30
Frequency [Hz]
50
Normal load [N]
300
Specimen temp. [°C]
100
Total test time [s]
7230
Each oil sample was tested three times consecutively in order to have a statistical representation
of its lubricating performance. The coefficient of friction (COF) is characterized through the
integral of the registered values of friction during the sampling time of 1 second. The averaged
wear scar diameter (AWSD) of each ball specimen was determined through optical microscopy
as an average of two perpendicular diameter measurements.
2. Results and discussion
2.1 Thermal cycling experiment
Registered temperatures during the first aging test are plotted on Figure 3, which shows a
significant difference between measured values. This can be attributed to the error of probe
placement or more specifically the conformity of thermal contact between flask, probe and
mantle.
Figure 3. Temperature profile of selected heaters during the first aging experiment
The placement of the temperature probes has to be altered for more relevant measurement in
the future. Placing the sensor between the flask and the mantle was considered in order to avoid
aging processes resulting from direct metal contact. This is a necessary simplification of the
experiment during the commissioning phase to increase reliability and repeatability. However,
the placement of the probe and clamping force of the flask has a high impact on measured
temperatures, which vary from 160°C to over 200°C.
It is clear that placing the probe inside the oil sample is necessary for setup and calibration
purposes. Maintaining full function with the desired probe placement requires thermocouples
with a tip diameter of 0.25 mm. Opening the air circulation system to fit a larger probe through

106
the neck of the flask would result in differing flow rates through the flasks, which would affect
oxidation and aging of the samples. A thin probe could be fed through the Drechsel heads’
exhaust side without altering the gas flow and would yield more stable temperature readings.
Another possible solution could be the adaptation of three-necked round bottom flasks with a
dedicated neck for thermocouple feedthrough. This option would also facilitate automated
contaminant dosage in later experiments.
Samples from heaters #2, #3 and #4 were selected for further testing based on the temperature
logs. The temperatures of heaters #2 and #3 are similar and are close to the setpoint while heater
#4 has the largest deviation from the group and registered the highest temperature. Temperature
data of heater #1 shows a similar trend as #2 and #3, while heater #6 shows a temperature trend
similar to #4, hence they are not investigated further in this experiment.
2.2 Lubricating performance of aged samples
The lubricating performance of aged samples shows a variation similar to the measured
temperatures during the thermal cycling procedure. Figure 4 presents scatter bands of COF
curves and calculated mean AWSD values with the corresponding variance. Since no significant
difference can be found regarding the wear phenomena taking place on the surfaces, the
qualitative analysis of the wear test specimens was excluded from this study.
Figure 4. Scatter bands of friction curves and calculated mean AWSD values measured
with selected oil samples
Comparing the scatter bands of friction curves shows that the values of COF increased
compared to the reference oil, which is presumably the consequence of increased viscosity due
to thermal load and oxidation. During oxidation the molecules of the base oil and oxygen react
and build numerous chemical species including aldehydes, ketones, hydroperoxides and
carboxylic acids, resulting in the breakdown of the base oil molecules [11]. This breakdown
and cleavage of oil molecules results in a decreased viscosity. Over time these new species can
combine and form larger molecular species with higher molecular weight and cause an increase
in viscosity. This process is denoted as polymerization and could result in the formation of
deposits and sludge, which can lead to clogging and contribute to damage or failure of a real-

107
life tribosystem e.g. the journal bearings of an internal combustion engine. The phenomenon of
changing oil viscosity was expected based on former studies conducted on degraded lubricant
samples and the work of other researchers [9], [12].
The mean values and deviation of AWSD varies in accordance with the scatter bands. However,
the significantly lower amount of wear with sample #4 compared to the other samples can be
an indicator of the variance in the grade of degradation of aged samples due to bad thermal
contact through inappropriate thermocouple placement. The altered viscosity of the lubricant
as a result of the thermal cycling can affect the ability of the liquid to form a hydrodynamic
film and introduce a variation in the wear reduction capability of the oil. Since no ferrous
catalytic agent was utilized, the degradation of lubricant additives can be more subtle compared
to real-life use.
The low COF and AWSD values of the reference oil can be attributed to the intact base oil and
activation of anti-wear and friction modifier additives, which could have undergone different
processes in each flask due to the difference in thermal contact. Another factor influencing the
aging process is the amount of air fed through the samples during the aging procedure, which
was not regulated in this case. A desired variance of such an experiment would be one that
resembles the results with sample #4 and the reference oil. To achieve this, the air throughput
during aging has to be regulated as well to a constant value with equal flow rate through each
flask, regardless the number of samples aged in one cycle.
3. Conclusion
A thermal-cycling apparatus for laboratory aging of liquid lubricants was established in order
to investigate engine oil aging and degradation. Six batches of a commercially available SAE
0W-20 grade oil were subjected to a 90 hours long cycle consisting of 12 hours phases of
tempering at 160°C followed by 12 hours of cool-down phases at room temperature. The
temperature of the heating mantles was registered. Three batches were selected for friction and
wear testing on a reciprocating rig and compared to results of a clean reference lubricant. A
ball-on-disc model system was used with 100Cr6 steel specimens under standardized conditions
and loads.
The investigation of coefficient of friction curves and averaged wear scar diameters showed
that there is significant variation between the batches of aged oil samples which is presumably
a consequence of the difference of thermal contact between mantle and flask due to the
placement of the thermocouple probes for temperature monitoring. Another influencing factor
is believed to be the amount of air circulated through the samples which was not regulated
during the experiment.
Acknowledgements
EFOP-3.6.1-16-2016-00017 Internationalization, initiatives to establish a new source of
researchers and graduates, and development of knowledge and technological transfer as
instruments of intelligent specializations at Széchenyi University
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Reviewer: Tóth-Nagy, Csaba, associate professor, Széchenyi István University