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Effect of Coolant Temperature on Performance and Emissions of a Compression Ignition Engine Running on Conventional Diesel and Hydrotreated Vegetable Oil (HVO)

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To meet future goals of energy sustainability and carbon neutrality, disruptive changes to the current energy mix will be required, and it is expected that renewable fuels, such as hydrotreated vegetable oil (HVO), will play a significant role. To determine how these fuels can transition from pilot scale to the commercial marketplace, extensive research remains needed within the transportation sector. It is well-known that cold engine thermal states, which represent an inevitable portion of a vehicle journey, have significant drawbacks, such as increased incomplete combustion emissions and higher fuel consumption. In view of a more widespread HVO utilization, it is crucial to evaluate its performance under these conditions. In the literature, detailed studies upon these topics are rarely found, especially when HVO is dealt with. Consequently, the aim of this study is to investigate performance and exhaust pollutant emissions of a compression ignition engine running on either regular (petroleum-derived) diesel or HVO at different engine thermal states. This study shows the outcomes of warm-up/cool-down ramps (from cold starts), carried out on two engine operating points (low and high loads) without modifying the original baseline diesel-oriented calibration. Results of calibration parameter sweeps are also shown (on the same engine operating points), with the engine maintained at either high or low coolant temperature while combustion phasing, fuel injection pressure, and intake air flow rate are varied one-factor at a time, to highlight their individual effect on exhaust emissions and engine performance. HVO proved to produce less engine-out incomplete combustion species and soot under all examined conditions and to exhibit greater tolerance of calibration parameter changes compared to diesel, with benefits over conventional fuel intensifying at low coolant temperatures. This would potentially make room for engine recalibration to exploit higher exhaust gas recirculation, delayed injection timings, and/or lower fuel injection pressures to further optimize nitrogen oxides/thermal efficiency trade-off.
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Citation: Mancarella, A.; Marello, O.
Effect of Coolant Temperature on
Performance and Emissions of a
Compression Ignition Engine
Running on Conventional Diesel and
Hydrotreated Vegetable Oil (HVO).
Energies 2023,16, 144. https://
doi.org/10.3390/en16010144
Academic Editor: Dimitrios
C. Rakopoulos
Received: 30 November 2022
Revised: 15 December 2022
Accepted: 16 December 2022
Published: 23 December 2022
Copyright: © 2022 by the authors.
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
Effect of Coolant Temperature on Performance and Emissions
of a Compression Ignition Engine Running on Conventional
Diesel and Hydrotreated Vegetable Oil (HVO)
Alessandro Mancarella and Omar Marello *
Energy Department, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy
*Correspondence: omar.marello@polito.it
Abstract:
To meet future goals of energy sustainability and carbon neutrality, disruptive changes to
the current energy mix will be required, and it is expected that renewable fuels, such as hydrotreated
vegetable oil (HVO), will play a significant role. To determine how these fuels can transition from pilot
scale to the commercial marketplace, extensive research remains needed within the transportation
sector. It is well-known that cold engine thermal states, which represent an inevitable portion of a
vehicle journey, have significant drawbacks, such as increased incomplete combustion emissions and
higher fuel consumption. In view of a more widespread HVO utilization, it is crucial to evaluate its
performance under these conditions. In the literature, detailed studies upon these topics are rarely
found, especially when HVO is dealt with. Consequently, the aim of this study is to investigate
performance and exhaust pollutant emissions of a compression ignition engine running on either
regular (petroleum-derived) diesel or HVO at different engine thermal states. This study shows the
outcomes of warm-up/cool-down ramps (from cold starts), carried out on two engine operating
points (low and high loads) without modifying the original baseline diesel-oriented calibration.
Results of calibration parameter sweeps are also shown (on the same engine operating points),
with the engine maintained at either high or low coolant temperature while combustion phasing,
fuel injection pressure, and intake air flow rate are varied one-factor at a time, to highlight their
individual effect on exhaust emissions and engine performance. HVO proved to produce less engine-
out incomplete combustion species and soot under all examined conditions and to exhibit greater
tolerance of calibration parameter changes compared to diesel, with benefits over conventional fuel
intensifying at low coolant temperatures. This would potentially make room for engine recalibration
to exploit higher exhaust gas recirculation, delayed injection timings, and/or lower fuel injection
pressures to further optimize nitrogen oxides/thermal efficiency trade-off.
Keywords:
pollutant emissions; diesel engine; HVO; drop-in fuel; coolant temperature; cold start;
ECU calibration
1. Introduction
Climate change caused by anthropogenic emissions of greenhouse gases (GHG) from
fossil fuels is one of the major global challenges and requires urgent implementation of
effective political, social, and technological solutions in the short, medium, and long term.
The transport sector already plays a decisive role in this challenge. At the European level,
transport accounts for more than a quarter of GHG emissions [
1
], mainly because the
internal combustion engine (ICE) powered by fossil fuels still remains its main source of
energy [
2
]. The global warming potential (GWP) of ICEs is mainly driven by the direct
use of petroleum-derived fuels and their conversion to CO
2
during combustion, although
emissions of CH
4
and N
2
O may have even greater GWP than CO
2
. However, while CH
4
and N
2
O emissions can be curbed by proper after-treatment system (ATS) technology, the
only approach to reduce anthropogenic CO
2
emissions from ICEs is to limit fossil fuel
consumption [
3
]. This can be done by further improving engine thermal efficiency and/or
Energies 2023,16, 144. https://doi.org/10.3390/en16010144 https://www.mdpi.com/journal/energies
Energies 2023,16, 144 2 of 27
using biofuels that rely on renewable feedstocks that absorb CO
2
from the atmosphere
when produced [4].
The EU has been attempting to promote the use of biofuels to reduce GHG emissions
for the past decade. Biofuels can diversify the fuel source for the transport industry, hence
enhancing energy independence and diversifying manufacturing sites. In addition, many
of these biofuels are compatible with existing propulsion systems and fuel infrastructure [
5
].
GHG emissions are certainly a primary legislative driver, but it is also important to consider
other environmental impacts as well, such as air pollution [
6
], which poses major health
risks to human beings [7].
As far as compression ignition (CI) engines are concerned, diesel combustion tends
to produce high nitrogen oxides (NO
x
) and particulate matter (PM). The formation of
nitrogen oxides follows primarily the so-called “thermal mechanism” [
8
,
9
], which is highly
dependent on local in-cylinder temperatures and oxygen concentrations. PM emissions are
governed by the balance of competing soot production and soot oxidation processes [
10
].
The former is determined by the availability of acetylene, the formation of polycyclic
aromatic hydrocarbons (PAHs) and the inception of soot particles, all of which are processes
that are highly dependent on in-cylinder temperatures and air-fuel mixing. The latter is
determined by the availability of hydroxyl radicals, oxygen, and temperature as well [
11
].
All of these mechanisms are difficult to curb; therefore, exhaust pollutant emission targets
are still difficult to meet for CI engines, despite that continuous efforts have been made to
optimize in-cylinder combustion [
12
], engine components [
13
], ATS [
14
], and to develop
combustion control techniques [1519].
Biomass-derived diesel-like fuels offer a viable solution to all these problems, as they
reduce both air pollution and the GHG impact of CI engines. Vegetable oils, animal fats,
and waste cooking oils are some of the renewable feedstocks that can be used to make
diesel substitutes via various production methods. However, the resulting fuels may have
diverse chemical compositions and characteristics [20,21].
First-generation biodiesel, commonly known as biodiesel, is mainly composed of
fatty acid methyl esters (FAME) [
22
]. It is produced, via a transesterification process,
from oil-rich crops such as soybean or rapeseed. FAME provides various advantages over
petroleum-derived diesel, including improved ignition and lower pollutant emissions,
primarily CO, unburned hydrocarbons (HC), and PM [
23
]. Its application, however, is
restricted due to a number of inconveniences, such as its decreased oxidation stability
and unfavorable cold flow properties [
24
]. Indeed, FAME can cause ageing of polymeric
materials commonly used in vehicle fuel systems and corrosion of fuel storage tanks [
25
].
In addition, at low temperatures, FAME tends to create waxy crystals, making its storage
problematic, and to degrade cold engine operation because of its higher viscosity [
26
].
Due to these unfavorable properties, restrictions are generally imposed on the blending of
FAME with conventional petroleum-derived diesel (e.g., for the European standard EN
590, in all EU member states, the maximum FAME concentration is set at 7%-vol) [
27
].
Nevertheless, an interesting upside of FAME is its high lubricity, which is beneficial for
components in the injection system that require lubrication from the fuel [28].
Hydrotreated vegetable oil (HVO) could be a viable alternative to FAME. It is a
synthetic liquid biofuel whose chemical composition consists of straight-chain paraffinic
hydrocarbons (i.e., C
n
H
2n+2
alkanes), free of aromatic compounds, oxygen, and sulfur. It is
derived from hydrotreating catalysis of triglyceride-based biomass [
29
] such as vegetable
oils, animal fats, and waste products [
30
]. Hydrotreatment has a number of upsides over
transesterification, including lower processing costs, greater flexibility of raw materials,
and greater compatibility with conventional ICEs and fuel standards [
31
,
32
]: HVO may
be utilized in any proportion in this regard, i.e., either pure or combined with petroleum-
based diesel, with little to no adjustments to existing CI engines [
33
]. Relatively high
cetane number and heating value, lower viscosity, and cloud point as well as better cold
flow properties [
34
,
35
] are some of the benefits HVO can bring over FAME, as thoroughly
explored in the literature [
33
]. In addition, HVO generally features a shorter ignition delay
Energies 2023,16, 144 3 of 27
(ID) and, as a consequence, a more advanced start of combustion (SOC), compared to
conventional diesel [
36
], with a direct impact on engine performance and exhaust pollutant
emissions [37,38].
Most of the available literature agrees that HVO reduces the emission of incomplete
combustion species (CO and HC) when compared to regular diesel [
26
,
27
,
29
,
30
,
33
,
39
,
40
],
due to higher cetane number and better ignition [
26
,
39
]. This might be particularly ben-
eficial at low loads and/or when the engine has not yet warmed up, since incomplete
combustion is likely to occur near relatively colder surfaces of the combustion chamber and
the tailpipe emission of these chemical species cannot be cut down by the after-treatment
system upon cold start, due to poor conversion efficiencies. Proper management and
optimization of the engine behavior during warm-up is, therefore, of paramount impor-
tance [
41
], especially considering that a significant part of the car journeys is done after
the vehicle has been parked for at least 3 to 8 h and may, thus, include a cold start as an
unavoidable part of the daily driving [
42
]. However, in the published literature, there
is only a small number of in-depth research studies on the interactions between engine
thermal level and the combustion process, including an examination of the combined
impacts of coolant temperature and the most important engine calibration parameters,
such as exhaust gas recirculation (EGR) rate, injection time, rail pressure. Furthermore,
there is even less research about this topic using HVO. Therefore, this research has the
goal of examining exhaust emissions and engine performance of an engine operating on
either HVO or conventional diesel, with a focus on the distinct impact of low and high
coolant temperatures when using these two fuels, with a remark on how emissions and
performance of an engine change between a cold start and a test bench condition where the
engine is cooled down by keeping the coolant water temperature “artificially” low.
2. Materials and Methods
2.1. Engine and Experimental Setup
An experimental test campaign was carried out on a fully instrumented 2.3-L four-
stroke prototype diesel engine. This engine, whose basic production version is used
for modern light-duty commercial vehicles, was installed on a dynamic test bench at
Politecnico di Torino’s ICE Advanced Laboratory, equipped with an ELIN AVL APA100
cradle-mounted AC dynamometer with nominal torque and power ratings of 525 Nm and
220 kW, respectively. The main technical specifications of the tested engine are reported
below, in Table 1.
Table 1. Main technical specifications of the tested CI engine.
Number of cylinders 4
Displacement 2.3 L
Compression ratio ~16:1
Valves per cylinder 4
Turbocharger Single-stage VGT
Fuel injection system Common-rail injection system
EGR circuit type Dual-loop, water-cooled
The aforementioned engine has a high-pressure common-rail injection system with
solenoid injectors. On the air/EGR side, the engine is equipped with a variable geometry
turbine (VGT), an intake throttle valve, an exhaust flap, and a dual-loop cooled EGR system,
which consists of both a high-pressure (HP) and low-pressure (LP) EGR circuit. The baseline
(diesel-oriented) calibration of the tested engine uses only the high-pressure EGR circuit.
The ATS installed on the test bench consists of a diesel oxidation catalyst (DOC) and a
diesel particulate filter (DPF). A selective catalytic reduction (SCR) system, which is present
in commercial applications of this engine, was not available in the current configuration.
During the experimental campaign, periodic passive regeneration of the DPF was necessary
to prevent the system from clogging.
Energies 2023,16, 144 4 of 27
Suitable sensors (e.g., pressure transducers, thermocouples, volumetric flowmeters,
etc.) were fitted at various points throughout the engine circuit in order to make low-
frequency measurements. Furthermore, high-frequency Kistler 6058A piezoelectric trans-
ducers were employed to measure, every 0.1 crank angle degree (
CA), the pressure inside
each of the four cylinders of the engine. In addition, an absolute pressure sensor, a Kistler
4007C piezoresistive transducer, was fitted in the intake manifold to reference the four
in-cylinder pressure signals.
As depicted in Figure 1, fuel flow measurement is handled by an AVL KMA 4000
system, which allows continuous measurements of engine fuel consumption with an
accuracy of 0.1%, while an AVL AMAi60 exhaust gas analyzer was used to measure
NO
x
/NO, HC, CO, CO
2
, and O
2
volumetric concentrations upstream and downstream of
the ATS, as well as CO
2
concentrations in the intake manifold (in order to estimate the EGR
rate). Soot emissions were measured, under steady-state conditions, using an AVL 415S
smoke-meter.
Energies 2023, 16, x FOR PEER REVIEW 4 of 30
The ATS installed on the test bench consists of a diesel oxidation catalyst (DOC) and
a diesel particulate filter (DPF). A selective catalytic reduction (SCR) system, which is pre-
sent in commercial applications of this engine, was not available in the current configura-
tion. During the experimental campaign, periodic passive regeneration of the DPF was
necessary to prevent the system from clogging.
Suitable sensors (e.g., pressure transducers, thermocouples, volumetric flowmeters,
etc.) were fitted at various points throughout the engine circuit in order to make low-
frequency measurements. Furthermore, high-frequency Kistler 6058A piezoelectric trans-
ducers were employed to measure, every 0.1 crank angle degree (°CA), the pressure inside
each of the four cylinders of the engine. In addition, an absolute pressure sensor, a Kistler
4007C piezoresistive transducer, was fitted in the intake manifold to reference the four in-
cylinder pressure signals.
As depicted in Figure 1, fuel flow measurement is handled by an AVL KMA 4000
system, which allows continuous measurements of engine fuel consumption with an ac-
curacy of 0.1%, while an AVL AMAi60 exhaust gas analyzer was used to measure
NO
x
/NO, HC, CO, CO
2
, and O
2
volumetric concentrations upstream and downstream of
the ATS, as well as CO
2
concentrations in the intake manifold (in order to estimate the
EGR rate). Soot emissions were measured, under steady-state conditions, using an AVL
415S smoke-meter.
Figure 1. Schematics of the engine test bench.
All of the aforesaid measurement equipment was managed by AVL PUMA Open 2
software, while IndiCom and AVL CONCERTO 5 were used for indicating measurements
and data postprocessing, respectively. ETAS INCA was also used for real-time monitor-
ing, calibration, and recording of data through the ETK (German acronym for emulator
test probe) interface of the engine electronic control unit (ECU).
Tables 2 and 3 report the available data to establish the accuracy of the measured
pollutant emission values. Previous works [43] have shown that the expanded uncertain-
ties of pollutant emission measurements taken at this engine test facility fall within a 2–
4% range. As far as the extended uncertainties pertaining to the brake specific emissions
are concerned, the fuel flow rate system accuracy (0.1% over a 0.28–110 kg/h fuel flow rate
measurement range) and the maximum errors of the engine speed (1.50 rpm at full scale)
and torque (0.30 Nm at full scale) also have to be considered [13].
Figure 1. Schematics of the engine test bench.
All of the aforesaid measurement equipment was managed by AVL PUMA Open 2
software, while IndiCom and AVL CONCERTO 5 were used for indicating measurements
and data postprocessing, respectively. ETAS INCA was also used for real-time monitoring,
calibration, and recording of data through the ETK (German acronym for emulator test
probe) interface of the engine electronic control unit (ECU).
Tables 2and 3report the available data to establish the accuracy of the measured
pollutant emission values. Previous works [
43
] have shown that the expanded uncertainties
of pollutant emission measurements taken at this engine test facility fall within a 2–4%
range. As far as the extended uncertainties pertaining to the brake specific emissions are
concerned, the fuel flow rate system accuracy (0.1% over a 0.28–110 kg/h fuel flow rate
measurement range) and the maximum errors of the engine speed (1.50 rpm at full scale)
and torque (0.30 Nm at full scale) also have to be considered [13].
2.2. Tested Fuels
The fuels employed in the experimental campaign presented in this research were
conventional diesel B7, derived from petroleum (with up to 7% biodiesel, in compliance
with EN 590 standard) and HVO. The main properties of both fuels are listed in Table 4.
Energies 2023,16, 144 5 of 27
This table includes information such as density at 15
C (lower for HVO), cetane number
(higher for HVO owing to its paraffinic nature), and average chemical composition.
Table 2.
Composition of the gas calibration cylinders and extended uncertainty (95% confidence interval).
Composition of the Gas Calibration Cylinder and Extended Uncertainty
NO (lower range) [ppm] 89.7 ±1.7
NO (higher range) [ppm] 919 ±18
CO (lower range) [ppm] 4030 ±79
CO (higher range) [%] 8.370 ±0.097
CO2(lower range) [ppm] 4.980 ±0.067
CO2(higher range) [%] 16.78 ±0.15
C3H8(lower range) [ppm] 88.8 ±1.8
C3H8(higher range) [ppm] 1820 ±36
Table 3. Manufacturer’s data for the measurement errors of emission analyzers.
Measurement Errors of Emission Analyzers
Linearity
1% of full-scale range
2% of measured value
whichever is smaller
Drift 24 h 1% of full-scale range
Reproducibility 0.5% of full-scale range
Table 4. Diesel vs. HVO main properties.
Parameter Unit EN590 Diesel HVO
Density at 15 Ckg/m3830.6 777.8
Kinematic viscosity mm2/s 2.969 2.646
Dynamic viscosity Pa·s2.47 ×1032.06 ×103
Cetane number - 54.6 79.6
Monoaromatic %v/v20.1 0.50
Polyaromatic %v/v3.00 0
Total aromatic %v/v23.1 0
Flammability C 74.0 60.5
Lower heating value MJ/kg 42.65 44.35
Hydrogen %m/m 13.72 15.00
Carbon %m/m 85.67 85.00
Oxygen %m/m 0.61 0
Sulphur mg/kg 6.50 0.53
FAME %v/v5.00 0.05
Approx. formula - C13H24O0.06 C13H28
2.3. Exerimental Test Procedure
The experimental campaign consisted of two distinct types of tests, each of which
was intended to investigate the behavior of the engine running on HVO or diesel under
different boundary conditions. In the first test type (test type #1), the engine is warmed
up to 85
C from a cold start and the baseline diesel-oriented calibration of the ECU is left
unchanged for both fuels. In the second test type (test type #2), single-parameter sweeps
are performed to determine how variations to some of the most important calibration
parameters affect exhaust emissions and engine performance when running on either fuel.
Separate descriptions of both test procedures are provided below.
2.3.1. Test Type #1: Warm-Up/Cool-Down Ramps
For test type #1, a cold engine is required. Before each test series, the engine was,
therefore, soaked at room temperature overnight (at least 12 h). After properly warming
Energies 2023,16, 144 6 of 27
up all measuring devices (e.g., the emission analyzers), the engine was started, idled for a
few seconds, and was then brought to the desired steady-state engine operating point. The
engine was then allowed to “naturally” warm up until the coolant temperature at the engine
outlet reached the nominal set value of 85
C. At this point, the engine coolant temperature
was “artificially”decreased by regulating the amount of water (from the laboratory facilities)
flowing through the coolant water cooler using a PID-controlled electrovalve. The procedure
was repeated for both fuels on two steady-state engine operating points, with rotational
speeds of 1250 and 2000 rpm and brake mean effective pressure (bmep) values of 2 and 9 bar,
respectively. They will be referred to as 1250
×
2 and 2000
×
9 from here on out. At each
engine operating point, constant engine speed and bmep values were maintained by letting
the engine test bench controller adjust the injected fuel supply accordingly.
The baseline (diesel-oriented) calibration of the engine remained unchanged through-
out the entire test series. This means that the engine was free to operate with all of its
actuations, strategies, and corrections as if no calibration tools (i.e., ETAS INCA) were avail-
able at the test bench to potentially tune calibration parameters on-the-fly. This implies that
some engine calibration setpoints (rail pressure, SOI
Main
, ecc.) may vary slightly through-
out the warm-up period, mostly due to the varying accelerator pedal positions required for
the engine to produce constant bmep. In this way, it is possible to examine the differences in
engine performance and emissions between conventional (petroleum-derived) diesel and
HVO as a “drop-in” fuel, i.e., without adjusting the baseline calibration of the engine.
Figure 2depicts the temporal evolution of the coolant temperature for the 1250
×
2 ramp.
As can be seen, the engine is first “naturally” warmed up to 85
C before the coolant
temperature is “artificially” decreased to 40
C. The color gradient from dark to light green
represents the elapsed time along the ramps. Darker shades represent earlier time, lighter
shades represent later time.
Energies 2023, 16, x FOR PEER REVIEW 7 of 30
Figure 2. Evolution of the coolant temperature during a warm-up ramp at 1250 × 2. The dark-to-
light green color palette is the function of the elapsed time and the same is used for Figure 3.
Figure 3. CO emissions for the 1250 × 2 ramp with diesel (a) and HVO (b) as a function of coolant
outlet temperature. Dark and light green shades are the function of elapsed time (it is the same color
palette as Figure 2). Black squared markers highlight data points used in the warm-up analysis in
Section 3.1, whereas black cross markers highlight the results obtained by steady-state points ac-
quired during the sweep-tests analysis in Section 3.2 and show test repeatability between ramps and
sweeps.
2.3.2. Test Type #2: Calibration Parameter Sweeps
For test type #2, the effects of varying some of the main engine calibration parame-
ters, i.e., rail pressure (p
rail
), electric start of the main injection (SOI
Main
), and intake in-cyl-
inder air quantity (q
air
), were studied for both diesel and HVO and for the same two engine
operating points mentioned previously, at different coolant temperature values. Specifi-
cally, two steady-state temperature levels (40 °C and 85 °C at 1250 × 2 and 60 °C and 85 °C
at 2000 × 9) were identified. Single-variable sweeps were performed at each coolant
Figure 2.
Evolution of the coolant temperature during a warm-up ramp at 1250
×
2. The dark-to-light
green color palette is the function of the elapsed time and the same is used for Figure 3.
Energies 2023,16, 144 7 of 27
Energies 2023, 16, x FOR PEER REVIEW 7 of 30
Figure 2. Evolution of the coolant temperature during a warm-up ramp at 1250 × 2. The dark-to-
light green color palette is the function of the elapsed time and the same is used for Figure 3.
Figure 3. CO emissions for the 1250 × 2 ramp with diesel (a) and HVO (b) as a function of coolant
outlet temperature. Dark and light green shades are the function of elapsed time (it is the same color
palette as Figure 2). Black squared markers highlight data points used in the warm-up analysis in
Section 3.1, whereas black cross markers highlight the results obtained by steady-state points ac-
quired during the sweep-tests analysis in Section 3.2 and show test repeatability between ramps and
sweeps.
2.3.2. Test Type #2: Calibration Parameter Sweeps
For test type #2, the effects of varying some of the main engine calibration parame-
ters, i.e., rail pressure (p
rail
), electric start of the main injection (SOI
Main
), and intake in-cyl-
inder air quantity (q
air
), were studied for both diesel and HVO and for the same two engine
operating points mentioned previously, at different coolant temperature values. Specifi-
cally, two steady-state temperature levels (40 °C and 85 °C at 1250 × 2 and 60 °C and 85 °C
at 2000 × 9) were identified. Single-variable sweeps were performed at each coolant
Figure 3.
CO emissions for the 1250
×
2 ramp with diesel (
a
) and HVO (
b
) as a function of coolant
outlet temperature. Dark and light green shades are the function of elapsed time (it is the same
color palette as Figure 2). Black squared markers highlight data points used in the warm-up analysis
in Section 3.1, whereas black cross markers highlight the results obtained by steady-state points
acquired during the sweep-tests analysis in Section 3.2 and show test repeatability between ramps
and sweeps.
2.3.2. Test Type #2: Calibration Parameter Sweeps
For test type #2, the effects of varying some of the main engine calibration parameters,
i.e., rail pressure (p
rail
), electric start of the main injection (SOI
Main
), and intake in-cylinder air
quantity (q
air
), were studied for both diesel and HVO and for the same two engine operating
points mentioned previously, at different coolant temperature values. Specifically, two
steady-state temperature levels (40
C and 85
C at 1250
×
2 and 60
C and 85
C at 2000
×
9)
were identified. Single-variable sweeps were performed at each coolant temperature level
and with both fuels, that is, a “one-factor-at-a-time” approach, while keeping the others
(including the boost pressure, which was not included in the parameter sweeps) fixed and
equal for both fuels. Table 5contains the main engine parameters used as “central points”
throughout these variable sweeps. These values would have been (slightly) different if
the original engine calibration had been let completely free (as in test type #1), depending
on the specific fuel and on the actual coolant temperature, since the ECU applies some
corrections to engine calibration parameters based on coolant temperature measurement (for
example, it advances the fuel injection pattern if the coolant temperature declines). For a
more meaningful comparison between fuels, each single-parameter sweep was carried out
holding all the other parameters constant and fuel-independent. The objective was to examine
differences in engine behavior attributable to fuel and coolant temperature only (isolating
them as much as possible from potential calibration differences) as well as to analyze the
engine response to specific changes in engine calibration parameters with both fuels, possibly
identifying useful guidelines for engine recalibration during cold HVO operations.
Table 5.
Setpoint values for the “central points” along calibration parameter sweeps. Setpoints are
fuel independent.
SOIMain pRail qair
[CA bTDC] [mbar] [mg/str]
1250 ×2 HOT 2.8 610 316
1250 ×2 COLD 1.4 570 316
2000 ×9 HOT 2.4 1350 590
2000 ×9 COLD 2.1 1320 595
Energies 2023,16, 144 8 of 27
2.3.3. Additional Observations on Experimental Test Procedures
It should be noted that because the ramps for test type #1 required overnight soaking,
they had to be carried out at the start of different workdays (upon engine start). Sweep
tests (test type #2), however, had to be performed with the engine running for several hours
on the test bench for the remainder of the days, after one of the ramps pertaining to test
type #1 had been completed.
Sweep tests with lower coolant temperatures (40 C at 1250 ×2 and 60 C at 2000 ×9)
had to be carried out by keeping the coolant temperature “artificially” low for an extended
period of time. This makes the results of low temperature sweeps inherently different from
what could be obtained if the sweeps were performed on an engine running with the same
coolant temperature, but just started up, primarily owing to the thermal inertia of the engine
metal parts and different walltemperature gradients. However, this is the only test procedure
that can achieve a proper degree of repeatability in low-temperature tests. That is to say, it
would be impossible to carry out meaningful sweep tests while the engine is “naturally”
warming up, because of the inherent transient behavior of such an operating condition.
The results from the ramps performed for test type #1 can help identify and quantify
the differences, at the same coolant temperature, between a cold engine “naturally” warm-
ing up and an engine running on the test bench whose coolant water temperature is kept
“artificially” low. Figure 3depicts engine-out CO emissions as a function of coolant outlet
temperature during the “natural” warm-up/“artificial” cool-down ramps at 1250
×
2,
for diesel (Figure 3a) and HVO (Figure 3b). CO was selected as an example, but similar
conclusions can be drawn from other pollutant emissions/combustion metrics, which
are not reported here for conciseness reasons. The dark-to-light green color palette of
Figure 3is identical to that of Figure 2, allowing the elapsed time along the ramp to be
derived from the same plot. Figure 3highlights variations in the engine behavior at a same
coolant temperature, depending on how that thermal level was reached (hysteresis pattern).
Specifically, at the coolant outlet temperature approximately 40
C during the first “natural”
warm-up phase of the ramps, engine-out CO is around 1000 for diesel and 450 ppm for
HVO, respectively, whereas, at the same coolant temperature, engine-out CO is around 650
and 350 ppm, respectively, if that temperature is “artificially” decreased and maintained
low. These latter conditions are, incidentally, exactly how the low-temperature sweep tests
(test type #2) were carried out.
In Figure 3, the black cross-shaped symbols (referred to as “repetition points” in
the legend) represent the baseline calibration points around which the sweep tests were
conducted (at low and high coolant temperatures). They are numerous because they
represent repetition steady-state tests (“central points”) carried-out during the test type #2
phase in order to assess the consistency and variability of these tests. As can be seen, black
cross-shaped symbols referring to low coolant temperature “central points” overlap the
end of the “artificial” cool-down portion of test type #1 ramps, whereas symbols referring
to high coolant temperature “central points” overlap the “warmed-up” engine portion of
the same ramps, suggesting that the engine thermal state during these tests is comparable.
Figure 3also includes black square-shaped symbols (referred to as “sample points” in the
legend) that represent sampled points extracted from the ramps (during the “natural” warm-up
phase), which will be used in the following sections to analyze the results of test type #1.
3. Experimental Test Analysis
3.1. HVO vs. Conventional Diesel Oil: “Natural” Warm-Up Operation (Test Type #1)
Based on the previously described test procedure, this section analyzes the results in
terms of exhaust emissions and engine performance during “natural” warm-up operation.
As stated previously, the engine was run on either diesel or HVO while allowing its
standard baseline calibration to run free. For each ramp performed, several data points,
one every 5
C, were sampled during the “natural” warm-up phase and will be shown in
the following Figures 46.
Energies 2023,16, 144 9 of 27
Energies 2023, 16, x FOR PEER REVIEW 9 of 30
warm-up phase of the ramps, engine-out CO is around 1000 for diesel and 450 ppm for
HVO, respectively, whereas, at the same coolant temperature, engine-out CO is around
650 and 350 ppm, respectively, if that temperature is “artificially” decreased and main-
tained low. These latter conditions are, incidentally, exactly how the low-temperature
sweep tests (test type #2) were carried out.
In Figure 3, the black cross-shaped symbols (referred to asrepetition pointsin the
legend) represent the baseline calibration points around which the sweep tests were con-
ducted (at low and high coolant temperatures). They are numerous because they represent
repetition steady-state tests (“central points”) carried-out during the test type #2 phase in
order to assess the consistency and variability of these tests. As can be seen, black cross-
shaped symbols referring to low coolant temperature “central points” overlap the end of
the “artificial cool-down portion of test type #1 ramps, whereas symbols referring to high
coolant temperature “central points overlap the “warmed-up” engine portion of the
same ramps, suggesting that the engine thermal state during these tests is comparable.
Figure 3 also includes black square-shaped symbols (referred to as “sample points”
in the legend) that represent sampled points extracted from the ramps (during the natu-
ral warm-up phase), which will be used in the following sections to analyze the results
of test type #1.
3. Experimental Test Analysis
3.1. HVO vs. Conventional Diesel Oil: “Natural” Warm-Up Operation (Test Type #1)
Based on the previously described test procedure, this section analyzes the results in
terms of exhaust emissions and engine performance during “natural” warm-up operation.
As stated previously, the engine was run on either diesel or HVO while allowing its stand-
ard baseline calibration to run free. For each ramp performed, several data points, one
every 5 °C, were sampled during the “natural” warm-up phase and will be shown in the
following Figures 4–6.
Before proceeding with the results analysis, a brief description of how the following
figures display the outcomes of the “natural” warm-up tests is provided. A circle denotes
diesel tests, while a star denotes HVO tests. The color distinction makes the coolant tem-
perature at which the tests were performed visually intuitive. Light and dark greens rep-
resent 1250 × 2, pink and purple represent 2000 × 9.
Figure 4. SOI
Main
(a) and EGR rate (b) measured at various coolant temperatures along the “natural”
warm-up ramps of the engine. Comparison between diesel and HVO at 1250 × 2 and 2000 × 9. Light
and dark greens represent 1250 × 2, pink and purple represent 2000 × 9. Circles represent diesel,
while stars represent HVO.
Figure 4.
SOI
Main
(
a
) and EGR rate (
b
) measured at various coolant temperatures along the “natural”
warm-up ramps of the engine. Comparison between diesel and HVO at 1250
×
2 and 2000
×
9. Light
and dark greens represent 1250
×
2, pink and purple represent 2000
×
9. Circles represent diesel,
while stars represent HVO.
Energies 2023, 16, x FOR PEER REVIEW 10 of 30
Figure 5. Exhaust gas temperature (a) and MFB50 (b) measured at various coolant temperatures
along the “natural” warm-up ramps of the engine. Comparison between diesel and HVO at 1250 ×
2 and 2000 × 9. Light and dark greens represent 1250 × 2, pink and purple represent 2000 × 9. Circles
represent diesel, while stars represent HVO.
Figure 5.
Exhaust gas temperature (
a
) and MFB50 (
b
) measured at various coolant temperatures
along the “natural” warm-up ramps of the engine. Comparison between diesel and HVO at 1250
×
2
and 2000
×
9. Light and dark greens represent 1250
×
2, pink and purple represent 2000
×
9. Circles
represent diesel, while stars represent HVO.
Before proceeding with the results analysis, a brief description of how the following
figures display the outcomes of the “natural” warm-up tests is provided. A circle denotes
diesel tests, while a star denotes HVO tests. The color distinction makes the coolant
temperature at which the tests were performed visually intuitive. Light and dark greens
represent 1250 ×2, pink and purple represent 2000 ×9.
Energies 2023,16, 144 10 of 27
Energies 2023, 16, x FOR PEER REVIEW 11 of 30
Figure 6. HC emissions (a), CO emissions (b), CO
2
emissions (c), NO
x
emissions (d), bsfc (e) and
engine thermal efficiency (f) measured at various coolant temperatures along the “natural” warm-
up ramps of the engine. Comparison between diesel and HVO at 1250 × 2 and 2000 × 9. Light and
dark greens represent 1250 × 2, pink and purple represent 2000 × 9. Circles represent diesel, while
stars represent HVO.
Figure 6.
HC emissions (
a
)
,
CO emissions (
b
), CO
2
emissions (
c
), NO
x
emissions (
d
), bsfc (
e
) and
engine thermal efficiency (
f
) measured at various coolant temperatures along the “natural” warm-up
ramps of the engine. Comparison between diesel and HVO at 1250
×
2 and 2000
×
9. Light and dark
greens represent 1250
×
2, pink and purple represent 2000
×
9. Circles represent diesel, while stars
represent HVO.
Energies 2023,16, 144 11 of 27
3.1.1. Effects on Engine Combustion
During warm-up operation, engine combustion is not only affected by lower tem-
peratures, but also by variations in ECU calibration parameters. The accelerator pedal
position set by the test bench controller for the engine to produce constant bmep varies as a
result of the increasing thermal efficiency of the engine as it warms up and of differences
in fuel behavior. Consequently, some engine calibration setpoints change slightly during
the warm-up phase. In addition, the ECU makes calibration corrections to compensate for
lower coolant temperatures. For example, as can be observed in Figure 4a, which shows
the variation of SOI
Main
along the two warm-up ramps, the ECU tends to advance injection
timings at low coolant temperatures to compensate for lower in-cylinder temperature at
the time of injection, delayed combustion evolution caused by longer ignition delays and
higher gas-wall heat exchanges.
The increase in coolant temperature along the “natural engine warm-up is accom-
panied by an increase in exhaust gas temperature (cf. Figure 5a). The main reason for
this should be related to decreasing gas-wall heat exchange as coolant temperature rises.
Moreover, delayed combustion phasing may increase exhaust temperatures. However, as is
evident in the 1250
×
2 diesel ramp, combustion at coolant outlet temperatures of 30
C and
85
C exhibits nearly the same combustion barycenter (represented by MFB50 values, which
are around 8
CA aTDC, cf. Figure 5b) but different exhaust temperatures, indicating that
the primary factor influencing exhaust temperature is, in fact, heat transfer. MFB50 values
at 2000
×
9, however, are not constant but exhibit monotonic delaying trends as coolant
temperature rises, for both diesel and HVO. This is primarily caused by SOI
Main
corrections
implemented by the ECU as coolant temperature varies (MFB50 delay patterns are very
similar to SOI
Main
delays, cf. Figures 4a and 5b) and there is no substantial difference in
behavior between HVO and diesel in this regard. In contrast, at 1250
×
2, HVO has a more
advanced combustion barycenter than diesel. Moreover, even though SOI
Main
corrections
along the ramps are nearly the same for the two fuels, combustion barycenter advance with
HVO is more pronounced at the lowest coolant temperatures (at coolant outlet temperature
of 30
C, for example, it has a combustion barycenter advanced by around 2
CA compared
to diesel). Despite the fuel injection advance at low coolant temperature, diesel features
nearly constant MFB50 values around 8
CA aTDC at 1250
×
2 (cf. Figure 5b). Nevertheless,
SOI
Main
advance fully translates into MFB50 advance for HVO, presumably due to its greater
ignitability. This suggests that HVO is less susceptible to ignition delays at low coolant
temperatures than diesel. Thus, a dedicated HVO calibration at low coolant temperatures
would presumably require smaller SOIMain corrections than conventional diesel.
3.1.2. Effects on Exhaust Pollutant Emissions and Engine Performance
Before delving into exhaust pollutant emissions and engine performance analyses, it is
important to reiterate that SOI
Main
is gradually delayed by the ECU as coolant temperature
rises along the “natural” warm-up ramps. Furthermore, as shown in Figure 4b, the EGR
rate decreases (as a result of changing engine thermal state) while the engine warms up,
influencing the subsequent results.
HC and CO Emissions
As depicted in Figure 6a, lowering coolant temperatures increases HC emissions to a
great extent, with both diesel and HVO. This is most likely due to an enhancement to over-
leaning and flame quenching phenomena, as in-cylinder and wall temperatures decline
with colder thermal states, as these are two common mechanisms that cause HC emissions
in diesel engines [
8
]. Furthermore, low temperatures may also inhibit HC oxidation in the
cylinder and at the exhaust. It can be seen that a coolant temperature of 30
C during the
cold start of the 1250
×
2 diesel ramp corresponds to an HC emission level of 2.4 g/kWh,
which is more than three times the value at 85
C (0.68 g/kWh). HVO, however, experiences
a significantly smaller increase in engine-out HC as coolant temperature is dropped, rising
from 0.43 to just under 0.78 g/kWh. This is likely due to its enhanced ignition properties,
Energies 2023,16, 144 12 of 27
and similar conclusions can be drawn for the 2000
×
9 ramps. In fact, HVO emits less
engine-out HC than diesel regardless of engine operating point and coolant temperature.
In terms of CO emissions (Figure 6b), HVO outperforms diesel along the 1250
×
2 ramps
regardless of coolant temperature. At 85
C, HVO reduces CO emissions by around 30%
compared to diesel, from 2.10 to 1.45 g/kWh. At 30
C, the advantage of HVO increases
to 60%, reducing CO emissions from 16.5 to 6.32 g/kWh. In contrast, at 2000
×
9, the
relative change in CO between HVO and diesel tends to be negligible, as do their absolute
values, which are relatively low due to the high in-cylinder temperatures involved in the
combustion process at this higher load.
In general, HVO reduces emissions from incomplete combustion due to its high cetane
number and narrow distillation range, which improve ignition behavior. In compression
ignition engines, fuel evaporation is critical, especially at low load and during cold start
operation. Typically, fuels with low distillation curves, such as HVO, exhibit improved
evaporation (hence mixing) with the intake charge and higher reactivity, particularly at
low combustion temperatures [
44
]. Indeed, the more pronounced HC reduction brought
about by HVO is clearly obtained at 1250
×
2 and coolant outlet temperature of 30
C
(
67% compared to diesel). When the engine is warmed-up, however, the reduction is
less pronounced in relative terms, as HC emissions are cut by 37%. HVO still outperforms
diesel at 2000
×
9, but HC emissions at this higher load are generally lower for both fuels
(below 0.3 g/kWh), diminishing the significance of the differences.
It is worth noting that the discussed effects of fuel properties and coolant temperature
on HC and CO emissions outweigh any other possible effect of variations in engine and
calibration parameters along the ramps (e.g., SOI
Main
and EGR rate). Progressive delays
in SOI
Main
(cf. Figure 4a) as the engine warms up would, if anything, contribute to the
opposite direction of the visible HC and CO trends. In contrast, EGR rate reduction
(cf. Figure 4b) as coolant temperature rises would be consistent with the declining trends of
HC and CO. However, if reference is made to the 1250
×
2 ramps, the EGR rate at 30
C
and 35
C coolant temperatures is relatively flat and comparable for the two fuels, yet their
HC and CO emissions are significantly different.
NOxEmissions
In addition to a predictable increase in NO
x
emissions with increasing load for both
fuels, Figure 6d indicates that NO
x
emissions increase with rising coolant temperatures as
well. Hotter coolant results in an increase in in-cylinder gas temperatures due to a decrease
in heat transfer between in-cylinder gases and wall, both in the compression phase (leading
to higher compressed gas temperatures at the onset of combustion) and during combustion
(leading to higher in-cylinder peak combustion temperatures, which are highly correlated
with NOxformation mechanisms along with intake O2) [45,46].
Regarding differences between fuels, NO
x
emission levels of HVO and diesel appear
comparable. As a function of coolant outlet temperature, there is no discernible trend
indicating that one fuel emits consistently more (or less) NO
x
than the other. This is consistent
with the existing literature on the subject [
22
,
30
], which suggests that it is still uncertain
whether HVO decreases or increases NO
x
emissions relative to diesel. The higher the cetane
number, the shorter the ID and the faster the combustion. However, a shorter ID does not
necessarily guarantee NO
x
reduction [
47
], and results may vary depending on the actual
engine load, coolant temperature, and/or calibration-specific parameters. Specifically, NO
x
variations appear to be much more influenced by EGR variations than any other parameter,
and this will be discussed in greater detail based on the results of test type #2.
Fuel Consumption and Engine Thermal Efficiency
In addition to the previously observed reductions in engine-out pollutant emissions,
Figure 6e shows how, due to its higher heating value (44.35 vs. 42.65 MJ/kg, cf. Table 4),
HVO also reduces brake specific fuel consumption (bsfc) in comparison to the reference
diesel fuel. The same plot also demonstrates that bsfc is worse at low coolant temperatures
Energies 2023,16, 144 13 of 27
because of increased friction and less efficient combustion (details on this aspect and the
effects of oil and coolant water temperatures on frictions have been thoroughly studied
in [
48
]). Similar conclusions can be drawn from Figure 6f, which depicts engine thermal
efficiency (
ηengine
) as a function of coolant outlet temperature. For both fuels,
ηengine
is better
at warmed-up coolant temperatures and worse at reduced coolant temperatures. However,
owing to its increased reactivity and ignitability, which allows HVO combustion to develop
faster even during cold starts, the efficiency drop of HVO at low coolant temperatures is
less pronounced than that of diesel, particularly at low load. For the 1250
×
2 ramps, HVO
is nearly 2% more efficient than diesel at 30
C coolant outlet temperature, with this benefit
declining as the engine warms up. When the engine has warmed up, the trend flips over,
with diesel displaying higher efficiency than HVO, albeit very slightly (+0.2%). For the
2000
×
9 ramps,
ηengine
turns out to be slightly higher for diesel along the whole ramp, and
efficiency degradation as coolant temperature decreases turns out to be less for both fuels.
Finally, as shown in Figure 6c, even when
ηengine
of HVO is lower than that of diesel due
to the differences in chemical properties and heating value of the two fuels, engine-out CO
2
emissions are comparable or slightly lower for HVO. Nevertheless, the real potential of HVO
in terms of CO
2
emission reduction in the atmosphere is not directly linked to engine-out CO
2
(i.e., tank-to-wheel CO
2
) but has tobe estimated through a well-to-wheel analysis, considering
HVO is produced from renewable feedstocks that absorb CO2while growing [4].
3.2. HVO vs. Conventional Diesel Oil: Calibration Parameter Sweep Tests (Test Type #2)
This chapter examines the results in terms of exhaust emissions and engine perfor-
mance along calibration parameter sweeps, based on the test procedure #2 described in
Section 2.3.2. Sweep tests were performed at 1250
×
2 with coolant outlet temperatures
of 40
C (for cold conditions) and 85
C (for warmed-up conditions), but at 2000
×
9 the
coolant temperature could not be reduced below 60
C for cold conditions (due to insuffi-
cient cooling power of the test bench heat exchanger). Warmed-up coolant temperature did
not change (85
C). It is worthwhile repeating that, since these sweep tests were carried out
by “artificially” controlling the coolant outlet temperature, they are inherently unable of
capturing all the effects that occur during a natural warm-up of the engine. Nevertheless,
they can still be thought as representative of drawbacks an engine would endure at colder
thermal states compared to warmed-up conditions.
To investigate the effects of the selected calibration parameters (SOI
Main
, p
rail
and q
air
)
one at a time, single-parameter sweeps were carried out, while keeping fixed the setpoints
values for all the other parameters (in addition to fixed boost pressure, which was not
included in the sweeps). These setpoint values were retrieved from the baseline calibration
of the engine and made equal for HVO and diesel, as shown in Table 5.
Before proceeding with the results analysis, a brief description of how the following
figures display the outcomes of the sweep tests is provided. A circle denotes diesel tests,
while a star denotes HVO tests. The color distinction makes the coolant temperature at
which the tests were performed visually intuitive. Blue and cyan (cool colors) represent low
coolant temperature tests (40
C at 1250
×
2, 60
C at 2000
×
9), while red and orange (warm
colors) represent high coolant temperature tests (85
C). Black-edged symbols represent
the “central point” (referring to the fixed baseline calibration, cf. Table 5) around which the
sweeps were carried out.
3.2.1. SOIMain Sweep
Combustion phasing is a crucial calibration parameter and has a direct influence on fuel
consumption, pollutant emissions, and global engine performance. It is normally set to obtain
the lowest possible bsfc (for a given braking power) while ensuring engine-out pollutant
emissions stay below reasonable limits and/or engine performance does not degrade. In this
Subsection, SOIMain sweeps were carried out (i.e., SOIMain was advanced or delayed relative
to the baseline value) to investigate how adjusting combustion phasing may result in different
outcomes for both investigated fuels, taking into consideration their distinct features.
Energies 2023,16, 144 14 of 27
Engine-Out HC and CO Emissions
As shown in Figure 7, HVO combustion produces significantly lower levels of CO
and HC at the engine exhaust than diesel combustion, especially at low load, where this
reduction is more interesting, due to potential low catalytic efficiency of after-treatment
systems. At 2000
×
9, however HC levels are extremely low, for both fuels, while CO
levels remain appreciable and exhibit a clear “u-shaped” trend as a function of SOI
Main
.
When SOI
Main
is too delayed, a greater amount of fuel misses the piston bowl (the spray
trajectory is too wide to enter the bowl, due to the distant position of the piston at the
time of the main injection) and is injected towards the cylinder walls and the piston head,
near the squish region [
34
]. This results in inefficient use of the oxygen present within the
piston bowl volume, contributing to CO emissions. Furthermore, delayed injection timings
reduce the amount of time available to complete CO oxidation reactions at the end of the
expansion stroke. However, when a significant portion of the injected spray is targeted on
the piston surface and splits evenly into two parts, one entering the squish region and the
other entering the piston bowl, thus optimizing oxygen usage, a minimum in CO trends as
a function of SOI
Main
can be detected [
49
]. At 1250
×
2 this “u-shaped” CO trend is less
evident, presumably because of higher in-cylinder oxygen availability at low load, which
makes this phenomenon contribute less to CO formation [8].
Energies 2023, 16, x FOR PEER REVIEW 16 of 30
Figure 7. Engine-out HC and CO emissions along SOI
Main
sweep tests at 1250 × 2 (a,b) and 2000 × 9
(c,d). Comparison between diesel and HVO at high and low coolant temperatures. Warm colors (red
and orange) represent high coolant temperatures, while cool colors (blue and cyan) represent low
coolant temperatures. Circles represent diesel, while stars represent HVO.
HC and CO reduction provided by HVO is certainly noticeable in warmed-up con-
ditions (within 20 to 30% at 1250 × 2, even below 10% at 2000 × 9), but it is even more
significant at lower coolant temperatures, with diesel generating up to twice as much CO
as HVO along the whole SOI
Main
sweep at 1250 × 2. In terms of CO, not only does diesel
have higher overall emission values, but it also appears to be more sensitive to SOI
Main
variations than HVO, particularly at low coolant temperatures, with more pronounced
diverging trends as the injection pattern is advanced. For example, at 1250 × 2, an injection
advance from 7 to 2 °CA bTDC results in a 33% CO increase (from 6 to 8 g/kWh), whereas
the same SOI
Main
variation for HVO results in a 14% CO increase (from 3.5 to 4 g/kWh
only).
Engine-Out NO
x
and Soot Emissions
As shown in Figure 8, HVO consistently outperforms diesel in terms of soot emis-
sions, regardless of combustion phasing and coolant temperatures. As a function of
SOI
Main
, both fuels exhibit similar soot trends. At 1250 × 2, there is a slight soot increase at
first as SOI
Main
is advanced from very delayed values, followed by a gradual decrease as
Figure 7.
Engine-out HC and CO emissions along SOI
Main
sweep tests at 1250
×
2 (
a
,
b
) and 2000
×
9
(
c
,
d
). Comparison between diesel and HVO at high and low coolant temperatures. Warm colors (red
and orange) represent high coolant temperatures, while cool colors (blue and cyan) represent low
coolant temperatures. Circles represent diesel, while stars represent HVO.
Energies 2023,16, 144 15 of 27
HC and CO reduction provided by HVO is certainly noticeable in warmed-up conditions
(within 20 to 30% at 1250
×
2, even below 10% at 2000
×
9), but it is even more significant at
lower coolant temperatures, with diesel generating up to twice as much CO as HVO along
the whole SOI
Main
sweep at 1250
×
2. In terms of CO, not only does diesel have higher
overall emission values, but it also appears to be more sensitive to SOI
Main
variations than
HVO, particularly at low coolant temperatures, with more pronounced diverging trends as
the injection pattern is advanced. For example, at 1250
×
2, an injection advance from
7 to
2
CA bTDC results in a 33% CO increase (from 6 to 8 g/kWh), whereas the same SOI
Main
variation for HVO results in a 14% CO increase (from 3.5 to 4 g/kWh only).
Engine-Out NOxand Soot Emissions
As shown in Figure 8, HVO consistently outperforms diesel in terms of soot emissions,
regardless of combustion phasing and coolant temperatures. As a function of SOI
Main
, both
fuels exhibit similar soot trends. At 1250
×
2, there is a slight soot increase at first as SOI
Main
is advanced from very delayed values, followed by a gradual decrease as combustion is
further advanced. At 2000
×
9, however, trends of soot emissions are similar to trends of
CO, with a “u-shaped” minimum particularly visible especially at low coolant temperatures.
This implies that similar explanations, related to improved air-fuel mixing when the fuel
spray properly targets the edge of the piston bowl, might still be valid for soot formation
mechanisms, at higher load. It is interesting to note, however, how diesel at low coolant
temperatures and high load produces significantly more soot than HVO (up to +50%),
despite both fuels exhibit similar trends.
Soot formation is a complex phenomenon influenced by a number of factors, including
fuel properties (cetane number, density, viscosity, and the presence of aromatic and pol-
yaromatic compounds) and EGR rate. Because EGR rate does not vary significantly enough
across the entire SOI sweeps to justify such large soot differences (because intake air flow
rate and boost pressure are fixed at each coolant temperature value), smoke differences
between HVO and diesel can be attributed almost entirely to their distinct fuel properties
and the absence of aromatic chemical compounds (which tend to act as soot precursors)
in HVO composition. In addition, compared to conventional diesel, HVO has a lower
density and viscosity, as well as a narrower distillation temperature range, as stated in
previous subsections. This presumably promotes faster evaporation and a more uniform
air-fuel mixture throughout the fuel cloud [
26
], hence, further contributing to reduce soot
formation. Nevertheless, the amount of soot produced at low load is generally small, so
reduction in soot is of much more interest at 2000 ×9.
Engine-out NO
x
levels are very similar for HVO and diesel, across all SOI sweep tests.
Only when the engine is warmed up does HVO appear to emit slightly less NO
x
than diesel,
at both tested engine operating points. This slight difference may be attributable to minor
EGR differences (reported, as an example, in Figure 9, at 2000
×
9) that can occur during
testing, even if intake air flow rate and boost pressure are kept fixed. This is most likely
because HVO produces lower exhaust gas temperatures than diesel, resulting in lower
temperatures of the residual gas in the combustion chamber at the end of combustion,
higher density of the gas recirculated in the intake manifold and a lower intake temperature.
For very delayed SOI
Main
, this results in an increase in EGR of about 2% for HVO, which
presumably causes the abovementioned slight NO
x
reduction. While this minor increase
in EGR benefits HVO by lowering NO
x
, it has little effect on HC, CO, or soot emissions,
which are much more linked to the chemical properties of the fuel.
Engine Thermal Efficiency and Fuel Consumption
HVO shows lower bsfc than diesel across the entire SOI
Main
sweeps (due to its lower
heating value). Engine thermal efficiency, however, is very similar, as shown in Figure 10.
In fact, advancing SOI
Main
diesel seems to slightly outperform HVO, with this effect more
evident at high coolant temperature. In contrast, for very delayed SOI
Main
, the trend flips
over, with HVO resulting in slightly higher efficiencies.
Energies 2023,16, 144 16 of 27
Figure 8.
Soot and NO
x
emissions along SOI
Main
sweep tests at 1250
×
2 (
a
,
b
) and 2000
×
9 (
c
,
d
).
Comparison between diesel and HVO at high and low coolant temperatures. Warm colors represent
high coolant temperatures, while cool colors represent low coolant temperatures. Circles represent
diesel, while stars represent HVO.
Energies 2023, 16, x FOR PEER REVIEW 18 of 30
stated in previous subsections. This presumably promotes faster evaporation and a more
uniform air-fuel mixture throughout the fuel cloud [26], hence, further contributing to
reduce soot formation. Nevertheless, the amount of soot produced at low load is generally
small, so reduction in soot is of much more interest at 2000 × 9.
Engine-out NO
x
levels are very similar for HVO and diesel, across all SOI sweep tests.
Only when the engine is warmed up does HVO appear to emit slightly less NO
x
than
diesel, at both tested engine operating points. This slight difference may be attributable to
minor EGR differences (reported, as an example, in Figure 9, at 2000 × 9) that can occur
during testing, even if intake air flow rate and boost pressure are kept fixed. This is most
likely because HVO produces lower exhaust gas temperatures than diesel, resulting in
lower temperatures of the residual gas in the combustion chamber at the end of combus-
tion, higher density of the gas recirculated in the intake manifold and a lower intake tem-
perature. For very delayed SOI
Main
, this results in an increase in EGR of about 2% for HVO,
which presumably causes the abovementioned slight NO
x
reduction. While this minor
increase in EGR benefits HVO by lowering NO
x
, it has little effect on HC, CO, or soot
emissions, which are much more linked to the chemical properties of the fuel.
Figure 9. EGR rate along SOI
Main
sweep tests at 1250 × 2 and 2000 × 9. Comparison between diesel
and HVO at high and low coolant temperatures. Warm colors represent high coolant temperatures,
while cool colors represent low coolant temperatures. Circles represent diesel, while stars represent
HVO.
Engine Thermal Efficiency and Fuel Consumption
HVO shows lower bsfc than diesel across the entire SOI
Main
sweeps (due to its lower
heating value). Engine thermal efficiency, however, is very similar, as shown in Figure 10.
In fact, advancing SOI
Main
diesel seems to slightly outperform HVO, with this effect more
evident at high coolant temperature. In contrast, for very delayed SOI
Main
, the trend flips
over, with HVO resulting in slightly higher efficiencies.
Figure 9.
EGR rate along SOI
Main
sweep tests at 1250
×
2 and 2000
×
9. Comparison between diesel and
HVO at high and low coolant temperatures. Warm colors represent high coolant temperatures, while
cool colors represent low coolant temperatures. Circles represent diesel, while stars represent HVO.
Energies 2023,16, 144 17 of 27
Energies 2023, 16, x FOR PEER REVIEW 19 of 30
Figure 10. bsfc and engine efficiency along SOI
Main
sweep tests at 1250 × 2 (a,b) and 2000 × 9 (c,d).
Comparison between diesel and HVO at high and low coolant temperatures. Warm colors represent
high coolant temperatures, while cool colors represent low coolant temperatures. Circles represent
diesel, while stars represent HVO.
3.2.2. p
Rail
Sweep
Rail pressure is a crucial calibration parameter that has a strong influence on air-fuel
mixture formation, which results in a significant impact on engine-out pollutant emissions
and engine performance. In this subsection, p
Rail
sweeps carried out (i.e., p
Rail
was in-
creased or reduced relative to the baseline value) to investigate how adjusting rail pres-
sure may result in different outcomes for both investigated fuels, taking into consideration
their distinct features are presented.
Engine-Out HC and CO Emissions
As shown in Figure 11, confirming the results of previous subsections, engine-out
CO and HC emissions from HVO combustion are significantly lower than conventional
diesel regardless of fuel injection pressure, especially at low load. At 1250 × 2, HC (regard-
less of coolant temperature) and CO (in warmed up conditions) emissions are relatively
unaffected by changes within investigated p
Rail
ranges, with both diesel and HVO. How-
ever, at low coolant temperatures, HVO clearly highlights the tendency to maintain low
sensitivity to changes in rail pressure, unlike diesel, which exhibits a substantial increase
Figure 10.
bsfc and engine efficiency along SOI
Main
sweep tests at 1250
×
2 (
a
,
b
) and 2000
×
9 (
c
,
d
).
Comparison between diesel and HVO at high and low coolant temperatures. Warm colors represent
high coolant temperatures, while cool colors represent low coolant temperatures. Circles represent
diesel, while stars represent HVO.
3.2.2. pRail Sweep
Rail pressure is a crucial calibration parameter that has a strong influence on air-
fuel mixture formation, which results in a significant impact on engine-out pollutant
emissions and engine performance. In this subsection, p
Rail
sweeps carried out (i.e., p
Rail
was increased or reduced relative to the baseline value) to investigate how adjusting
rail pressure may result in different outcomes for both investigated fuels, taking into
consideration their distinct features are presented.
Engine-Out HC and CO Emissions
As shown in Figure 11, confirming the results of previous subsections, engine-out CO
and HC emissions from HVO combustion are significantly lower than conventional diesel
regardless of fuel injection pressure, especially at low load. At 1250
×
2, HC (regardless of
coolant temperature) and CO (in warmed up conditions) emissions are relatively unaffected
by changes within investigated p
Rail
ranges, with both diesel and HVO. However, at low
coolant temperatures, HVO clearly highlights the tendency to maintain low sensitivity to
changes in rail pressure, unlike diesel, which exhibits a substantial increase in this emission
level. For example, Figure 11b shows that, at low coolant temperatures, increasing rail
pressure from 400 to 800 bar generally increases CO emissions, most likely due to prevailing
Energies 2023,16, 144 18 of 27
over-leaning phenomena with improved fuel atomization [
8
]. However, the CO increase
for diesel goes from 5.5 to 8 g/kWh, whereas only from 3 to 4 g/kWh for HVO.
Energies 2023, 16, x FOR PEER REVIEW 20 of 30
in this emission level. For example, Figure 11b shows that, at low coolant temperatures,
increasing rail pressure from 400 to 800 bar generally increases CO emissions, most likely
due to prevailing over-leaning phenomena with improved fuel atomization [8]. However,
the CO increase for diesel goes from 5.5 to 8 g/kWh, whereas only from 3 to 4 g/kWh for
HVO.
CO trends at 2000 × 9 differ from low load conditions, with increasing rail pressure
resulting in decreasing CO emissions. At higher loads, increasing fuel injection pressure
improves the air-fuel mixing, and since the dominant effect on CO formation at higher
loads is linked to oxygen deficiency, this results in lower CO levels. Nevertheless, HVO
still outperforms diesel as far as CO are concerned, while HC trends, throughout the
trade-off, are negligible.
Figure 11. HC and CO emissions along p
Rail
sweep tests at 1250 × 2 (a,b) and 2000 × 9 (c,d). Compar-
ison between diesel and HVO at high and low coolant temperatures. Warm colors represent high
coolant temperatures, while cool colors represent low coolant temperatures. Circles represent diesel,
while stars represent HVO.
Engine-Out NO
x
and Soot Emissions
As shown in Figure 12, NO
x
and soot emissions are not only affected by engine loads
and particular fuel used, but also by rail pressure variations. Increased p
Rail
improves fuel
atomization, enlarges the interface between fuel spray particles and air, and decreases
Figure 11.
HC and CO emissions along p
Rail
sweep tests at 1250
×
2 (
a
,
b
) and 2000
×
9 (
c
,
d
).
Comparison between diesel and HVO at high and low coolant temperatures. Warm colors represent
high coolant temperatures, while cool colors represent low coolant temperatures. Circles represent
diesel, while stars represent HVO.
CO trends at 2000
×
9 differ from low load conditions, with increasing rail pressure
resulting in decreasing CO emissions. At higher loads, increasing fuel injection pressure
improves the air-fuel mixing, and since the dominant effect on CO formation at higher
loads is linked to oxygen deficiency, this results in lower CO levels. Nevertheless, HVO still
outperforms diesel as far as CO are concerned, while HC trends, throughout the trade-off,
are negligible.
Engine-Out NOxand Soot Emissions
As shown in Figure 12, NO
x
and soot emissions are not only affected by engine loads
and particular fuel used, but also by rail pressure variations. Increased p
Rail
improves
fuel atomization, enlarges the interface between fuel spray particles and air, and decreases
evaporation time. As a consequence, the air entrainment into the fuel spray and the mixture
formation process are greatly enhanced and the fuel distribution is more uniform. All of
these factors hinder soot formation mechanisms. Furthermore, increased p
Rail
results in
Energies 2023,16, 144 19 of 27
higher in-cylinder pressure and temperature values during combustion, which ultimately
favor NOxformation mechanism, setting up a clear NOx/soot trade-off.
Energies 2023, 16, x FOR PEER REVIEW 21 of 30
evaporation time. As a consequence, the air entrainment into the fuel spray and the mix-
ture formation process are greatly enhanced and the fuel distribution is more uniform. All
of these factors hinder soot formation mechanisms. Furthermore, increased p
Rail
results in
higher in-cylinder pressure and temperature values during combustion, which ultimately
favor NO
x
formation mechanism, setting up a clear NO
x
/soot trade-off.
Figure 12. Soot and NO
x
emissions along p
Rail
sweep tests at 1250 × 2 (a,b) and 2000 × 9 (c,d). Com-
parison between diesel and HVO at high and low coolant temperatures. Warm colors represent high
coolant temperatures, while cool colors represent low coolant temperatures. Circles represent diesel,
while stars represent HVO.
As far as differences between fuels are concerned, NO
x
variations can be mainly at-
tributed to slightly different EGR levels across the tests (despite boost pressure and intake
air quantity setpoints are kept constant), as discussed in the previous subsections. Regard-
ing soot, if the highest rail pressure is maintained (800 bar), smoke levels at 1250 × 2 are
bounded within a narrow range for all tests (diesel and HVO, hot and cold coolant). Con-
versely, at low coolant temperatures and low fuel injection pressure, HVO outperforms
di esel o nce ag ain, by up to 40% . This means that, at low coolant temperatures, diesel seems
to be more sensitive to changes in fuel injection pressure than HVO. At 2000 × 9, HVO
outperforms diesel across the entire p
Rail
trade-off. Diesel soot levels increase from 0.25 to
0.45 g/kWh when starting from the highest rail pressure and decreasing it, whereas HVO
levels increase from 0.15 to 0.27 g/kWh.
Figure 12.
Soot and NO
x
emissions along p
Rail
sweep tests at 1250
×
2 (
a
,
b
) and 2000
×
9 (
c
,
d
).
Comparison between diesel and HVO at high and low coolant temperatures. Warm colors represent
high coolant temperatures, while cool colors represent low coolant temperatures. Circles represent
diesel, while stars represent HVO.
As far as differences between fuels are concerned, NO
x
variations can be mainly
attributed to slightly different EGR levels across the tests (despite boost pressure and
intake air quantity setpoints are kept constant), as discussed in the previous subsections.
Regarding soot, if the highest rail pressure is maintained (800 bar), smoke levels at 1250
×
2
are bounded within a narrow range for all tests (diesel and HVO, hot and cold coolant).
Conversely, at low coolant temperatures and low fuel injection pressure, HVO outperforms
diesel once again, by up to 40%. This means that, at low coolant temperatures, diesel seems
to be more sensitive to changes in fuel injection pressure than