Improving PM-NOx trade-off with paraffinic fuels: A study towards diesel engine optimization with HVO

Fuel 265 (2020) 116921

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Full Length Article

Improving PM-NOx trade-off with paraffinic fuels: A study towards diesel engine optimization with HVO

T

Athanasios Dimitriadisa, Tine Seljakb, Rok Viharb, Urban Žvar Baškovičb, Athanasios Dimaratosc, ⁎ Stella Bezergiannia, Zissis Samarasc, , Tomaž Katrašnikb a

Chemical Process & Energy Resources Institute (CPERI), Centre for Research and Technology Hellas CERTH, 6km Charilaou-Thermi, Thessaloniki, Greece Faculty of Mechanical Engineering, University of Ljubljana, Aškerčeva 6, SI 1000 Ljubljana, Slovenia c Laboratory of Applied Thermodynamics, Aristotle University Thessaloniki, P.O. Box 458, GR 54124 Thessaloniki, Greece b

A R T I C LE I N FO

A B S T R A C T

Keywords: Renewable diesel HVO PM-NOx trade off Main injection timing Pilot injection timing Injection pressure

The current work investigates the impact of a paraffinic fuel on combustion and emissions of a diesel engine, examining alternative injection strategies for the full exploitation of the fuel characteristics. The paraffinic fuel used was the HVO (Hydrotreated Vegetable Oil) produced by Neste Oil with the brand name NEXBTL. The study was conducted on a light-duty turbocharged and aftercooled common-rail diesel engine, with both HVO and a conventional diesel fuel. Four steady-state operating points were examined, at low and medium engine speeds (1500 and 3000 rpm) and loads (35 and 100 Nm), typical of daily driving of a passenger car. The key contribution of the study is a comprehensive analysis of the phenomena influencing the crank angle resolved incylinder parameters, as well as interlinking the effects of different variations of injection pressure (default and 300 bar higher), pilot injection timing (default and ± 5°CA) and main injection timing (default and ± 2°CA) on gaseous emissions and particulate matter (PM). The findings have shown that at default engine settings the use of HVO results in up to 40% reduction of engine-out PM and HC emissions without appreciable changes in NOx emissions. The significant reduction of engine-out PM levels, facilitates the adoption of measures for NOx emissions limitation. The latter are reduced by up to 20% when the main injection timing is retarded (by 2° CA in the present study), while PM emissions are still kept well below the respective diesel fuel levels.

1. Introduction Oil depletion and air pollution, caused by the extensive use of conventional fossil fuels have triggered research into more environmentally friendly and renewable fuels. First generation biodiesel, known as FAME (Fatty Acid Methyl Ester), is produced via transesterification of vegetable or used cooking oils. In general, the use of FAME biodiesel results in less soot, but higher NOx emissions, while the brake specific fuel consumption (BSFC) is higher due to its lower energy content [1,2]. The major disadvantages of FAME compared to conventional diesel fuel are its lower energy content, high price, the

tendency to form internal injector deposits, higher viscosity, higher cloud and pour point and higher nitrogen oxides (NOx) emissions [3,4]. Further, the oxygen content of FAME raises concerns in terms of maintaining the fuel quality during long term storage and oxidation stability [5,6]. Due to the unfavorable properties of FAME, the transformation of triglyceride containing feedstock by other thermal and/or catalytic processes has been widely explored, especially towards the production of paraffinic and synthetic hydrocarbons. One lipid-based alternative paraffinic fuel is Hydrotreated Vegetable Oil (HVO) [7] that consists mainly of paraffin and is free of aromatics, oxygen and sulfur. Owing to its paraffinic nature, it has a

Abbreviations: A/F, ratio: Air Fuel ratio;; CAD, Crank Angle Degree;; CD, Combustion Duration;; CI, Compression Ignition (engine);; CLD, ChemiLuminescence Detector;; CO, Carbon Oxide;; COV, Coefficient of Variance;; CRDI, Common Rail Direct Injection;; DOC, Diesel Oxidation Catalyst;; DOHC, Double Over-Head Camshaft;; DPF, Diesel Particulate Filter;; EGR, Exhaust gas Recirculation;; FAME, Fatty Acids Methyl Ester;; FID, Flow Indicator Detector;; GFM, Gravimetric Filter Module;; CFPP, Cold Filter Plugging Point;; H/C ratio, Hydrogen to Carbon ratio;; HC, Hydrocarbons (total);; HVO, Hydrotreated Vegetable Oil;; ID, Ignition Delay; IE, Injector Energizing;; IMEP, Indicative Mean Effective Pressure;; IMV, Inlet Metering Valve;; IP, Injection Pressure;; MIT, Main Injection Timing;; MSS, Micro Soot Sensor;; NOX, Nitrogen Oxides;; PIT, Pilot Injection Timing;; PM, Particulate Matter;; pmax, Maximum cylinder pressure;; RoHR, Rate of Heat Release;; SCR, Selective Catalytic Reduction;; SOC, Start of Combustion;; SOI, Start of Injection;; TDC, Top Dead Center; ⁎ Corresponding author at: Aristotle University, Laboratory of Applied Thermodynamics, P.O. Box 458, 54124 Thessaloniki, Greece. Tel.: +302310 996014. E-mail address: [email protected] (Z. Samaras). https://doi.org/10.1016/j.fuel.2019.116921 Received 21 July 2019; Received in revised form 16 December 2019; Accepted 18 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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higher cetane number and higher heating value compared to conventional diesel fuel [8]. It can be produced from many kinds of vegetable oils without compromising fuel quality. Existing farm-based feedstock such as rapeseed, sunflower, and soybean, as well as palm oil, are most commonly used. HVO can be used neat (100%) as a petrodiesel substitute, as a blending agent for petrodiesel, or as a blended fuel with additives in compression ignition (CI) engines [9]. Research on the application of HVO to diesel engines is usually related to fuel spray, combustion and emission characteristics [9,10]. According to the open literature, blends of conventional diesel fuel with HVO result in reductions of both regulated and unregulated emissions. The reduction in CO, HC and PM emissions depends on the proportion of HVO in the fuel blend [11], and it is consistent in both light and heavy-duty engines [12,13]. On the other hand, the literature reports a large spread in NOx emissions results [14–20]. Light-duty engines have shown negligible or even slight increase in NOx emissions [14], while in heavy-duty engines reductions of NOx levels have been observed [15,16]. According to Huang et al. [17], due to the shorter ignition delay caused by the higher cetane number, HVO reduces the energy released in the premixed combustion phase, which in turn reduces the maximum combustion temperature and pressure in the cylinder leading to suppressed NOx formation. Furthermore, the fuel injection system and engine calibration seem to be the most influential parameters for the observed spread in NOx emissions results [18]. According to Sugiyama et al. [19], NOx emissions decreased with HVO in engines with a single injection, but remained unchanged once a pilot injection is added. This can be explained by the fact that the higher cetane number of HVO has no positive effect on shortening the ignition delay of the main injection when a pilot injection is added, as the internal gas temperature at the start of main injection is already very high due to the combustion of the pilot injection. This effect explains why passenger vehicles with HVO fuel exhibit less NOx improvement than heavy-duty engines, which most of the time have no pilot injection [20]. The effect of pilot injection timing for light duty engines running on HVO is likely a key factor in the observed wide range of NOx emission results. In general, pure HVO, used as a drop-in fuel, can reduce PM emissions up to 60% compared to conventional diesel due to the absence of aromatic compounds, while HC and CO emissions can be also up to 50% lower [9,21,22]. Therefore, a proper re-adjustment of the engine settings can lead to significant benefits in terms of the PM-NOx trade off, when the engine runs with HVO fuel. However, there are only a few studies in the open literature that have investigated the potential for emissions reduction through re-adjustments in engine settings when using HVO fuel. Happonen et al. [23] have investigated the impact of variable intake valve closing in combination with different injection timings, EGR rates and injection pressures on NOx and soot emissions. However, in that study the combined effect of the studied parameters was examined. Thus, the quantitative potential of each individual parameter to reduce emissions is missing. Aatola et al. [24] studied the effect of injection timing on emissions of a heavy-duty diesel engine running on HVO. They have noticed that HVO at default engine settings led to 6% lower NOx emissions and 35% lower smoke compared with a sulfur free EN 590 diesel fuel, while, by optimizing injection timing for the paraffinic fuel, even higher NOx or PM emission reductions were achieved, depending on the direction of injection timing alteration. It is observed that most of the studies investigating the effects of HVO fuel focus on heavy-duty engines, resulting in insufficient data on their light-duty counterparts. In addition to that, most studies concentrate on the effect of HVO on emissions without analyzing combustion characteristics. As HVO produces lower PM emissions, re-adjustment of the injection strategy could lead to NOx emissions reductions, while at the same time keeping PM formation below diesel levels. However, there is a gap in the literature on the recalibration potential of a light-duty diesel engine running on HVO. To that purpose, the current research aims at investigating the effect of alternative injection strategies in order to gain full advantage of HVO’s physical and

Table 1 Properties of conventional diesel (D2) and renewable HVO fuel. Property

Unit

D2 (EN 590)

HVO

Method

FAME content Density at 60 °C Kinematic viscosity Flash point Cloud point Sulfur content Cetane number Ash content Water content

% v/v kg/m3 mm2/s °C °C ppm wt – % m/m mg/kg

7 832.4 3.236 59 −5.0 9.1 56.5 0.002 < 30

0 778.7 2.820 83 −22.2 < 5.0 76.3 < 0.001 20

Polyaromatic hydrocarbons CFPP Lower heating value Lower heating value Oxidation stability Distillation A/Fs Oxygen content Hydrogen content Carbon content C/H ratio

% m/m

2.2

0

EN 14078:2014 ASTM 4052 ASTM D445 ASTM D0093 ASTM D2500 ASTM 5453 ASTM D7170-14 EN ISO 10370 Pr EN ISO 12937:1996 IP 391

°C MJ/kg MJ/l hr °C – % wt % wt % wt –

−5 42.8 35.6 >6 191–357 14.2 0.77 12.0 87.2 7.26

−21 44.0 34.3 > 22 189–301 15.2 0 15.4 84.6 5.49

ASTM IP309 ASTM D4809 ASTM D4809 EN 15751 ASTM D86 – – ASTM 4629 ASTM 5291 –

Table 2 Engine specifications. Number of cylinders

4 (DOHC)

Bore × stroke (mm) Cubic capacity Maximum power (kW) Maximum torque (Nm) Compression ratio

75.0 × 88.3 1560 66.2 @ 4000 rpm 215 @ 1750 rpm 18

chemical properties. More specifically, the individual effects of main injection timing, pilot injection timing and injection pressure were examined for both HVO and commercial diesel fuel. The research is focused on engine-out emissions, correlating the results with combustion characteristics of a light-duty engine running on HVO, and covering in that way gaps in the open literature. This approach enables a solid basis for interpretation of combustion related and in-cylinder derived parameters, thus offering an insight into upstream phenomena. At the same time, engine-out emissions serve as the most important input data for justifying the use and also development of exhaust aftertreatment systems. In this way, engine-out emissions feature high relevance and may serve several research fields dealing with HVO. Therefore, this study provides insight into the causal relations, thus forming a solid basis for future research on engine optimization with paraffinic fuels. The findings show that by careful re-adjustment of the main injection timing, simultaneous reduction of PM and NOx emissions can be achieved with HVO compared to conventional diesel fuel. 2. Methodology 2.1. Fuel properties Two fuels were examined in this study, a commercial market diesel (D2), as the reference fuel, and a second generation HVO fuel, produced via two stage catalytic hydroprocessing of vegetable oil. A first hydrotreating step followed by a second isomerization process was used to transform heavy-chain hydrocarbons of vegetable oil to lighter ones in the diesel range. This fuel is fully paraffinic, and thus, the contents of aromatics, sulfur and oxygen were below the detectable limit. The HVO was provided by Neste oil Corporation in Finland, under the brand name NEXBTL, and it complies with the EN 15940 standard for paraffinic diesel fuels [25,26]. The reference fuel D2 complies with the 2

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Fig. 1. Schematic diagram of the experimental setup. Table 3 Steady-state operating points and default engine settings. Operating points

Speed (rpm)

Torque (Nm)

bmep (bar)

PIT (°CA aTDC)

MIT (°CA aTDC)

IP (bar)

1 2 3 4

1500 1500 3000 3000

35 100 35 100

2.82 8.05 2.82 8.05

−25 −33 −46 −50

4 −4 −19 −24

650 840 550 950

Table 5 IMEP and coefficient of variance % (COV(IMEP)) over 100 consecutive combustion cycles. D2 Speed [rpm]/Torque [Nm] IMEPmin (bar) IMEPmax (bar) IMEPmean (bar) COV(IMEP) %

HVO

D2

HVO

D2

HVO

D2

HVO

1500/35

1500/100

3000/35

3000/100

3.95 4.14 4.01 0.95

8.91 9.04 8.99 0.33

5.00 5.18 5.07 0.89

10.38 10.65 10.51 0.63

3.98 4.18 4.04 1.04

8.89 9.03 8.96 0.29

5.03 5.24 5.12 1.02

10.18 10.45 10.31 0.67

Table 4 Test protocol (bold values indicate the parameter being varied). Table 6 Measuring equipment used in experiments.

Parameters adjustment Case 1

Advance MIT (°CA aTDC)

Default MIT (°CA aTDC)

Retard MIT (°CA aTDC)

Speed/Load

PIT

MIT

PIT

MIT

PIT

MIT

1500 rpm/35Nm 1500 rpm/ 100Nm 3000 rpm/35Nm 3000 rpm/ 100Nm

−25 −33

2 −6

−25 −33

4 −4

−25 −33

6 −2

−46 −50

−21 −26

−46 −50

−19 −24

−46 −50

−17 −22

Case 2

Advance PIT (°CA aTDC) PIT MIT −30 4 −38 −4

Default PIT (°CA aTDC) PIT MIT −25 4 −33 −4

Retard PIT (°CA aTDC) PIT MIT −20 4 −28 −4

−51 −55

−46 −50

−41 −45

Speed/Load 1500 rpm/35Nm 1500 rpm/ 100Nm 3000 rpm/35Nm 3000 rpm/ 100Nm Case 3 Speed/Load 1500 rpm/35Nm 1500 rpm/ 100Nm 3000 rpm/35Nm 3000 rpm/ 100Nm

Component

Measuring device

Accuracy

Measuring range

THC

Horiba OBS-2200

± 2.5% FSO

NOX PM

Horiba OBS-2200 AVL PM-PEMS 494

± 2.5% FSO 5̴ μg/m3

0–1000 ppm to 0–10000 ppm (C1) 0–100 ppm to 0–3000 ppm Up to 1000 mg/m3

*FSO: Full Scale Output.

Injection pressure (bar) Default 650 840

High 950 1140

specifications of EN-590 standard [27] as a low sulfur market diesel, sourced from a fuel station. The properties of the two examined fuels are shown in Table 1. More detailed information on the characteristics of HVO fuel can be found in [15,28]. Due to the paraffinic nature and the low final boiling point, the density of HVO is lower than that of D2 fuel, resulting in a decrease in heating value per volume unit. However, HVO has a higher heating value per mass unit compared to that of D2 fuel, due to its higher H/C ratio, while its aromatic content is almost zero (< 0.1%wt) [24]. HVO’s stability is higher compared to conventional diesel, meaning that there is no risk to vehicles or stationary engines if they are not used for extended periods and for longtime storage.

550 950

850 1250

2.2. Engine setup and control

−19 −24

−19 −24

−19 −24

The tests were conducted in a 4-cylinder, 4-stroke, turbocharged and after-cooled 1.6 L light-duty diesel engine, coupled to a Zöllner B350AC eddy-current dynamometer controlled by a Kristl, Seibt & Co 3

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Fig. 2. Effect of main injection timing on in-cylinder pressure, heat release rate and in-cylinder temperature at four steady state operating points (1500 rpm/35Nm, 1500 rpm/100Nm, 3000 rpm/35 Nm and 3000 rpm/100Nm).

signals to set start and duration of injectors energizing by the driven system, which can be set in a PC graphic user interface. A driven system generated inlet metering valve signals (IMV signal) and injector energizing signals (IE signal) to control the common rail pressure and fuel injection duration. It was connected to the PC, and the energizing characteristics for the injectors along with other driven parameters were set with CalView software. For the experiments, two separate fuel tanks were used with individual fuel filters, one for D2 (reference fuel) and the second for HVO fuel. The fuels were supplied to the common-rail pump by two separate lines, one for D2 fuel through an AVL Fuel balance Consumption Meter and one for HVO through Micromotion Coriolis flow meter (model CMFS015M). In the current engine, the common rail pump was equipped with a build-in low-pressure pump. The measurements were conducted at thermally stabilized steadystate conditions at two different engine speeds and loads for both tested

control system Kristl, Seibt Advanced Data Acquisition and Control system. The main technical characteristics of the engine are presented in Table 2. A Kistler CAM UNIT Type 2613B shaft encoder provided an external trigger and an external clock at 0.1 deg CA for data acquisition and for an injection control system. In-cylinder pressure was measured with a calibrated piezo-electric pressure transducer AVL GH12D in combination with a charge amplifier AVL MICROIFEM, connected to a 16-bit, 4-channel National Instruments data-acquisition system with a maximum sampling frequency 1 MHz per channel. Top Dead Center (TDC) was determined by a capacitive sensor COM Type 2653. Fig. 1 presents a schematic layout of the test installation. The original control unit (ECU) was by-passed and the engine was controlled externally with real-time hardware. Data acquisition and an injection control embedded system (“NI cRIO system”) were based on a National Instruments cRIO 9024 processing unit and 9114 chassis. Along with an indication of in-cylinder pressure traces, it generated digital output

4

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Fig. 3. Burned mass fraction at various MIT. Table 7 Effect of main injection timing (MIT) on start of combustion (SOC), ignition delay (ID) and combustion duration (CD). MIT

Advance 2°CA

Speed[rpm]/ Torque [Nm]

SOC (°CA aTDC)

ID (°CA)

CD (°CA)

SOC (°CA aTDC)

ID (°CA)

CD (°CA)

SOC (°CA aTDC)

ID (°CA)

CD (°CA)

−16.5 −23.7 −24.0 −30.7

8.5 9.3 22.0 19.3

43.5 31.0 37.5 42.1

−16.5 −23.7 −23.7 −30.4

8.5 9.3 22.3 19.6

46.1 32.3 38.1 42.4

−16.5 −23.3 −23.8 −31.1

8.5 9.7 22.2 18.9

48.4 32.9 38.5 45.6

−17.5 −25.0 −26.5 −32.4

7.5 8.0 19.5 17.6

48.1 31.4 39.8 44.3

−17.4 −24.9 −26.2 −32.5

7.6 8.1 19.8 17.5

50.3 32.5 39.9 44.8

−17.5 −25.0 −26.5 −32.6

7.5 8.0 19.5 17.4

53.8 33.6 40.9 49.0

D2 1500/35 1500/100 3000/35 3000/100 HVO 1500/35 1500/100 3000/35 3000/100

Default

Table 8 Maximum cylinder pressure.

Table 9 Boost pressure during different main injection timings for both fuels.

MIT

Advance 2°CA

Default

Speed[rpm]/ Torque [Nm]

pmax (bar)

pmax (bar)

D2 1500/35 1500/100 3000/35 3000/100 HVO 1500/35 1500/100 3000/35 3000/100

°CA aTDC

Retard 2°CA

Retard 2°CA °CA aTDC

pmax (bar)

Boost pressure [bar]

°CA aTDC

56 92 95 154

0 9 4 3

56 86 89 149

0 11 5 4

56 78 87 144

0 13 6 5

57 94 96 156

−1 9 4 3

57 87 91 151

−1 11 5 4

57 80 88 147

−1 13 6 5

MIT

Advance 2°CA

Default

Retard 2°CA

Speed[rpm]/ Torque [Nm]

D2

HVO

D2

HVO

D2

HVO

1500/35 1500/100 3000/35 3000/100

1.1 1.2 1.6 2.0

1.1 1.3 1.6 2.0

1.1 1.3 1.6 2.0

1.1 1.3 1.6 2.0

1.1 1.3 1.6 2.0

1.1 1.3 1.6 2.0

fuels, as shown in Table 3. These points were selected in order to cover both low and high engine speeds in low and medium loads. The engine was initially started with D2 fuel and warmed up for 15 min. In-cylinder pressure and exhaust emissions were measured when stable engine

5

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Fig. 4. Effect of main injection timing on emission characteristics and bsfc.

2.3. Parameter variation

operating point was achieved, indicated by exhaust manifold temperature. After the completion of the measurement, the load was adapted to change the engine operating point. After the measurements with conventional diesel (D2), the same procedure was repeated for the HVO fuel.

The effects of the Main Injection Timing (MIT) (Case 1), Pilot Injection Timing (PIT) (Case 2) and Injection Pressure (IP) (Case 3) were examined for both tested fuels (Table 4). For every operating point, three main injection timings were investigated, the default one and ± 2°CA (pilot injection timing was kept constant to default values, as shown in Table 4). Furthermore, for every operating point, three

6

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Fig. 5. Effect of main injection timing, trade-off between PM and NOX emissions.

pilot injection timings (PIT) were tested, the default one and ± 5°CA (main injection timing was kept constant to default values, as shown in Table 4). It is noted that the dwell time between the pilot and main injection varied in each case, accordingly. Finally, for every operating point two injection pressures were tested, the default one and 300 bars higher compared to default values. At each operating point, the engine speed and torque remained constant, independent of the variation of the injection characteristics, by adjusting accordingly the fuel supply, while the exhaust gas recirculation (EGR) valve was fully closed (position set to 0%) in order not to affect the results of the examined parameters. The variation of engine parameters was selected on the basis of preliminary screening where it was found that larger variations lead to unstable combustion.

COV (IMEP )[%] =

σ (IMEP )(100cycles) μ (IMEP )(100cycles)

∙100

where “σ” and “μ” are the standard deviation and the mean value of IMEP, respectively, over 100 consecutive cycles. COV was chosen as the combustion stability indicator to evaluate cycle to cycle variation. According to Bytnner et al. [30] and Heywood [31], vehicle drivability issues may occur in cases where the COV (IMEP) exceeds 10%. However, there is other research that has suggested 3% as a threshold value for COV [32]. According to Table 5, it is observed that the use of HVO fuel does not affect the cyclic variability (irregularity) as compared to the baseline fuel (D2). In any case, the cyclic combustion irregularity of the tested engine is very low. The mean in-cylinder pressure trace was calculated by averaging 100 cycles of the individual operating points, as averaging significantly eliminates point to point variations due to signal noise [33]. In addition, a low pass FIR filter was applied to the average pressure trace in order to eliminate pressure oscillations in the combustion chamber. The pressure trace was further processed via the BURN functionality of the AVL Boost software [34], and the gross heat release rate (HRR) and mean gas temperature were calculated via the AVL 2000 model. The heat transfer during gas exchange strongly influences the volumetric efficiency of the engine, especially at low speeds. The AVL 2000 model is a modified Woschni heat transfer equation that takes this into account.

2.4. Combustion analysis In-cylinder pressure was recorded over 100 successive cycles at a sampling rate of 0.1°CA. Very good repeatability of the individual cycles was proven by overlapping pressure oscillations that correspond to the lowest excitation frequency of the gas in the combustion chamber over consecutive cycles [29]. Table 5 presents the minimum, the maximum, and the average value of IMEP over 100 consecutive cycles at default engine settings for the four tested points and both fuels (D2 and HVO). In addition, the coefficient of variance (COV) is given, which is defined as the standard deviation of IMEP divided by its mean value, as follows [30]:

2.5. Emissions measurements The exhaust gas analysis was conducted with a Horiba OBS-2200 7

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Fig. 6. Effect of main injection timing on thermal efficiency (%).

and an AVL PM-PEMS 494. Two separate sampling fittings were welded on the exhaust pipe and connected to the measuring devices. A heated sampling pipe was used to guide wet exhaust gases to a Horiba gas analyzer that measures total hydrocarbons (THC) in an FID cell and nitrogen oxides (NOx) in a CLD cell. The PM-PEMS device uses the photoacoustic method for real-time measurement of soot concentrations (performed by a micro soot sensor MSS) and a gravimetric method with filter loading (performed with a gravimetric filter module, GFM) to calibrate the real-time results of soot concentrations. Sampling of particulate matter (PM) was done with a heated line and diluted with filtered air according to the sensitivity of the micro soot sensor (MSS) and the desired filter loading rate for the gravimetric filter module (GFM). The measuring range and the accuracy of the devices are presented in Table 6. It is noted that the aim of the current investigation is to study the effect of the HVO fuel on engine-out emissions without aftertreatment devices.

of combustion (SOC) is defined as the crank angle where the HRR (Fig. 2) crosses the horizontal axis and transitions to positive values. The ignition delay was calculated as the time interval between the start of first injection (SOI), presented in Table 4, and the start of combustion (SOC) presented in Table 7, while the combustion duration (CD) is defined as the period from 10% to 90% of the burned mass fraction (MFB), as presented in Fig. 3, the latter determined by the cumulative gross heat released. At default engine settings, HVO shows a shorter ignition delay (ID) compared to D2 at all the operating points examined, as observed in Table 7 and Figs. 2 and 3. The shorter ID of HVO emanates from its higher cetane number compared to D2 fuel. This means that the pilot injected fuel of HVO is burned earlier compared to D2 case, prolonging the diffusion combustion part (main injection), leading ultimately to longer combustion durations (Table 7). Table 8 summarizes the maximum cylinder pressure (pmax) of both fuels with the angle where it occurs. As cylinder pressure is directly translated to torque build-up, a significantly distorted pressure curve would result in an unfavorable torque profile. For this reason, pmax and the angle where it appears are important factors when comparing two different fuels in terms of engine operability. In general, the engine is optimized by the manufacturer for diesel fuel. From Table 8, it is observed that although HVO shows a higher pmax compared to D2, pmax occurs at the same angle for both fuels. Table 9 presents the boost pressure, where it is observed that this remains the same between the fuels in all the examined operating conditions. This means that the intake conditions were the same for both fuels. Fig. 2 presents the cylinder pressure and temperature, as well as the gross HRR, for all the examined operating conditions with both fuels. At 1500 rpm and 35 Nm, pilot injection starts at −25°CA aTDC where the temperature is high, while main injection starts after TDC (4°CA aTDC). This leads to a first pressure and temperature peak that is reduced before the combustion of main injection. Furthermore, at this operating

3. Results and discussion The results are grouped according to the injection characteristics under examination. Firstly, the results of modified MIT (Main Injection Timing) are presented followed by the results of varied PIT (Pilot Injection Timing) and IP (Injection Pressure). 3.1. Effect of main injection timing (MIT) 3.1.1. Effect of MIT on combustion The two tested fuels (HVO and D2) were compared in terms of incylinder pressure and temperature and gross Heat Release Rate (HRR), in order to examine the effect of different main injection timings (MIT) on combustion characteristics. The results are presented in Figs. 2 and 3 and Tables 7–9. Table 7, presents the start of combustion (SOC), the ignition delay (ID) and the combustion duration (CD) in °CA. The start 8

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Fig. 7. Effect of pilot injection timing by 5°CA on in-cylinder pressure, heat release rate and in-cylinder temperature at four steady state operating points (1500 rpm/ 35Nm, 1500 rpm/100Nm, 3000 rpm/35Nm and 3000 rpm/100Nm).

the already burning fuel of the pilot injection for both fuels. The burned mass fraction of all examined cases is depicted in Fig. 3. Advancement of MIT reduces combustion duration for both fuels, especially at low loads, as shown in Table 7 and Fig. 3, with the HVO still showing longer durations. Advancing MIT by −2°CA, the SOC of the main injection starts earlier, resulting in an increase of pmax, which appears also earlier compared to default MIT (Table 8), while the HRR of the main injection also moves closer to TDC (Fig. 2). On the other hand, retardation of MIT results in longer combustion durations for both fuels (Table 7). This is due to the fact that the main injection fuel is injected in a later phase of the compression stroke, shifting the complete process later in the expansion and leading also to lower pmax (Table 8 and Fig. 2). Furthermore, changes on MIT ( ± 2°CA) do not affect the SOC of pilot injection, as the PIT remains constant (Table 7).

point, HVO shows a higher HRR peak during the combustion of the pilot injection and a lower one during the combustion of the main injection. This can be attributed to the lower CN of conventional diesel (D2), which might lead to overmixing of some fuel from the pilot injection, going outside of the ignition limits, such that the fuel does not burn during the pilot injection, but rather subsequently during the combustion of the main injection. At 1500 rpm and 100 Nm, there are few differences in the HRR of the combustion during the main injection between the HVO and the D2. Furthermore, at low engine speed (1500 rpm), the ID of the main injection is similar between the fuels, indicating that the combustion of the pilot injected fuel has raised the gas temperature to a high enough value that the cetane number does not affect the combustion of the main injection, as was also found by Bohl et al. [34]. At 3000 rpm and 35 Nm, HVO is characterized by a higher heat release peak during the combustion of the pilot injection compared to D2 fuel. Finally, it is observed in Fig. 2 that at 3000 rpm and 100 Nm a part of the main injection fuel quantity is injected during

3.1.2. Effect of MIT on emissions and engine performance The results of emissions at various MIT are presented in Fig. 4. At 9

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Fig. 8. Burned mass fraction at various PIT. Table 10 Effect of pilot injection timing (PIT) on start of combustion (SOC), ignition delay (ID) and combustion duration (CD). PIT

Advance 5°CA

Speed[rpm]/ Torque [Nm]

SOC (°CA aTDC)

ID (°CA)

CD (°CA)

SOC (°CA aTDC)

ID (°CA)

CD (°CA)

SOC (°CA aTDC)

ID (°CA)

CD (°CA)

−19.8 −25.5 −25.2 −33.5

10.2 12.5 25.8 21.5

35.5 32.1 37.1 43.7

−16.5 −23.7 −23.7 −30.4

8.5 9.3 22.3 19.6

46.1 32.3 38.1 42.4

−12.5 −20.0 −22.0 −29.4

7.5 8.0 19.0 15.6

46.6 32.6 38.7 46.2

−21.4 −27.3 −27.6 −34.7

8.6 10.7 23.4 20.3

52.2 32.3 40.3 46.5

−17.4 −24.9 −26.2 −32.5

7.6 8.1 19.8 17.5

50.3 32.5 39.9 44.8

−13.7 −21.4 −24.7 −30.8

6.3 6.6 16.3 14.2

48.5 33.5 42.6 52.0

D2 1500/35 1500/100 3000/35 3000/100 HVO 1500/35 1500/100 3000/35 3000/100

Default

default engine settings, both fuels show similar NOx emissions, with only a slight increase observed for HVO, as compared to D2. According to Sugiyama et al. [19], NOx emissions are reduced with HVO in engines with a single injection, but remain practically unchanged once a pilot injection is added. The reason is that when a pilot injection takes place the higher cetane number of HVO does not make a difference in ID of the main injection, as the gas temperature at the start of the main injection is already high due to the combustion of pilot injection. PM reduction with HVO fuel, ranging from 10 to 40%, is observed in all operating points that were tested compared to D2, which is in agreement with the results of Rantanen et al. [11] and Murtonen et al. [12] (Fig. 4). The reduction of PM emissions is due to the shorter molecular chain and lower C/H ratio of HVO, which contains almost exclusively paraffinic hydrocarbons and not any aromatic components, sulfur and other mineral impurities that enhance PM formation [35]. Furthermore, the lower viscosity, distillation temperature and density

Table 11 Maximum cylinder pressure. PIT

Advance 5°CA

Default

Speed[rpm]/ Torque [Nm]

pmax (bar)

°CA aTDC

pmax (bar)

°CA aTDC

pmax (bar)

°CA aTDC

D2 1500/35 1500/100 3000/35 3000/100

55 83 92 151

0 11 5 3

56 86 89 149

0 11 5 4

56 84 94 149

0 11 4 4

HVO 1500/35 1500/100 3000/35 3000/100

56 84 92 151

−1 11 5 4

57 87 91 151

−1 11 5 4

57 86 95 150

−1 11 4 4

Retard 5°CA

Retard 5°CA

10

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Fig. 9. Effect of pilot injection timing on emission characteristics and bsfc.

also limits this source of PM formation. Furthermore, the lighter paraffinic hydrocarbons of HVO and its higher cetane number result in a shorter ignition delay, which increases the residence time of the combustible mixture at high temperatures and at the same time prolongs

of HVO compared to conventional diesel (Table 1), reduce fuel spray penetration length. This leads to a faster evaporation rate and most likely a more uniform A/F ratio distribution throughout the fuel cloud. At the same time, it reduces the fuel jet wall impingement rate, which 11

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Fig. 10. Effect of pilot injection timing, trade-off between PM and NOX emissions.

Fig. 11. Effect of pilot injection timing on thermal efficiency (%).

12

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Table 12 Effect of PIT on injected fuel mass and engine power. PIT

Advance 5°CA

Speed/Load (rpm/Nm)

Injected fuel mass (kg/hr)

1500/35 1500/100 3000/35 3000/100

Default

Diesel

HVO

1.59 3.61 3.54 7.77

1.56 3.56 3.50 7.73

Power (kW)

Retard 5°CA

Injected fuel mass (kg/hr)

5.5 15.7 11.1 31.3

Diesel

HVO

1.57 3.56 3.52 7.57

1.55 3.52 3.54 7.48

Default

Speed[rpm]/ Torque [Nm]

SOC (°CA aTDC)

ID (°CA)

CD (°CA)

SOC (°CA aTDC)

ID (°CA)

CD (°CA)

D2 1500/35 1500/100 3000/35 3000/100

−16.5 −23.7 −23.7 −30.4

8.5 9.3 22.3 19.6

46.1 32.3 38.1 42.4

−17.9 −25.5 −24.9 −36.3

7.1 7.5 21.1 13.7

24.6 30.1 37.1 51.5

HVO 1500/35 1500/100 3000/35 3000/100

−17.4 −24.9 −26.2 −32.5

7.6 8.1 19.8 17.5

50.3 32.5 39.9 44.8

−18.2 −25.6 −27.2 −34.4

6.8 7.4 18.8 15.6

34.5 31.1 38.7 51.9

300 bars Higher

Table 14 Maximum cylinder pressure. IP

Default

Speed[rpm]/ Torque [Nm]

pmax (bar)

°CA aTDC

pmax (bar)

°CA aTDC

D2 1500/35 1500/100 3000/35 3000/100

56 86 89 149

0 11 5 4

53 91 97 163

0 10 4 3

HVO 1500/35 1500/100 3000/35 3000/100

57 87 91 151

−1 11 5 4

56 92 97 161

−1 10 3 3

5.6 15.8 10.9 31.3

Injected fuel mass (kg/hr) Diesel

HVO

1.57 3.56 3.53 7.75

1.52 3.51 3.53 7.60

Power (kW)

5.6 15.8 10.9 31.5

results in earlier start of combustion of the main injection leading to higher in-cylinder pressures and temperatures, promoting the formation of thermal NOx (Fig. 4) [36]. In comparison to diesel, HVO shows slightly higher NOx emissions, an effect which is more intense at advanced MIT, due to the higher temperatures. In case of PM emissions, retardation of MIT results in higher PM emissions for both fuels, due to the lower in-cylinder temperature, which inhibits the oxidation of PM particles. On the other hand, advancement of MIT causes a reduction in PM emissions due to higher incylinder pressures and temperatures, as well as due to more time available for mixing with air. The same trends were observed for both tested fuels, however, absolute concentrations of PM were much lower (10–40% lower) for HVO in all cases. Fig. 5 illustrates the effect of MIT on the PM-NOx trade-off, where the grey color denotes the target area in which both PM and NOx emissions are lower compared to default engine settings with D2 fuel. As HVO produces lower PM emissions, retardation of MIT (by 2° CA) leads to a reduction of NOx emissions by up to 20%, compared to default settings with diesel, while PM emissions were still kept well below D2 levels. The findings show that although the engine is designed for optimum operation with diesel fuel, significant further advancement in lowering exhaust emissions are possible through utilization of alternative fuels with favorable fuel properties and proper engine control. Therefore, a proper MIT adjustment with HVO fuel, can lead to significant benefits in terms of PM and NOx emissions. Furthermore, when the MIT is retarded, the fuel of main injection is injected later in the compression stroke, in an environment of higher pressure and temperature leading to shorter ignition delay of MIT. As a result, the engine operates with slightly richer local air–fuel mixtures, which leads to a lower degree of premixing and higher HC formation. On the other hand, advanced MIT leads to reductions in HC emissions due to the higher in-cylinder temperatures and longer residence times of the mixture [37]. The effect of MIT is similar for both tested fuels; however, HVO fuel showed lower HC emissions for all the tested operating conditions of the engine (Fig. 4). As far as the engine performance is concerned, bsfc (brake specific fuel consumption) is presented in Fig. 4 and thermal engine efficiency in Fig. 6. HVO shows slightly lower absolute thermal efficiencies compared to D2 fuel at any given MIT, load and speed due to the lower fuel density. However, the difference in thermal efficiency between the two fuels is very small, and in practice might not be noticeable by the end-user. The expected effect of MIT on thermal efficiency is observed, i.e., efficiency increases at advanced MIT and decreases at retarded MIT. As was already discussed in Section 3.1.1, at advanced MIT the pressure peak and thus the HRR are higher resulting in higher thermal efficiencies. The opposite trend is observed during retardation of MIT. Further, the absolute value of bsfc of HVO is slightly lower compared to D2. As HVO is characterized by lower density and higher mass heating value compared to D2, the mass-based fuel consumption of HVO is lower. Advanced MIT results in bsfc reductions due to higher thermal efficiencies, while the opposite effect is observed for retarded MIT. The trends are similar for both tested fuels. Finally, it should be noted that most of the diesel fuel vehicles are equipped with DPF’s. As HVO results

Table 13 Effect of injection pressure (IP) on start of combustion (SOC), ignition delay (ID) and combustion duration (CD). IP

Power (kW)

300 bars Higher

the soot oxidation interval, reducing PM emissions. HC emissions at default engine settings are lower for HVO fuel at all operating points (Fig. 4). The reduction ranges from 15 to 45% depending on the operating point of the engine. One of the main reasons is the higher HRR observed during the combustion of pilot injected fuel. This is caused by the higher cetane number of the HVO, and also by presumably increased atomization quality, ensuring fast mixture formation due to differences in density, distillation temperature and viscosity of diesel and HVO. The results are in agreement with the results of Kousoulidou et al. [14] and Pflaum et al. [21]. Furthermore, overleaning of fuel injected during the ignition delay period is a significant source of HC emissions, especially when the ignition delay is long. As a result, the shorter ignition delay of HVO compared to the D2 fuel and the absence of aromatics are two more reasons for its lower HC emissions [31]. As far as the effect of MIT is concerned, the trends for NOx emissions are similar for both tested fuels for all examined points. Retardation of MIT decreases the peak pressure (Table 8) and reduces the mean temperature, thus NOx emissions diminish, while advancement of MIT, 13

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Fig. 12. Effect of injection pressure on in-cylinder pressure, heat release rate and in-cylinder temperature at four steady state operating points (1500 rpm/35 Nm, 1500 rpm/100 Nm, 3000 rpm/35 Nm and 3000 rpm/100 Nm).

and 100 Nm increases the combustion duration for the pilot injection (Fig. 7), resulting in higher temperatures at the SOC for the main injection, and thus the peak of HRR for the main injection increased (Fig. 7). At higher engine speeds (3000 rpm), part of the main injection fuel is injected during the combustion of pilot injected fuel and from Fig. 7, it is observed that, advancement of PIT results in similar or even slightly higher pmax for both fuels. In contrast, for retarded PIT, fuel is injected at higher in-cylinder pressures and temperatures resulting in shorter ignition delay (Table 10 and Fig. 8) and a higher HRR peaks during combustion for the pilot injection (Fig. 7). Furthermore, retardation of PIT reduces the dwell time between pilot and main injection. At 3000 rpm, more quantity of pilot injected fuel is burned with the main injected fuel resulting in more intense combustion (higher HRR peak). Finally, it should be noted that changes of PIT by ± 5°CA do not affect the time (angle) where pmax occurs (Table 11) showing that changes of PIT also do not influence the engine behavior in terms of operability and drivability.

in lower PM emissions, the interval time between DPF regenerations increases, which might offset the fuel consumption penalty of HVO fuel. However, this effect is beyond the scope of this study. 3.2. Effect of pilot injection timing (PIT) 3.2.1. Effect of PIT on combustion This section presents the effect of different Pilot Injection Timings (PIT) on engine combustion. The results are presented in Figs. 7 and 8 and Tables 10 and 11. When the PIT is advanced, the fuels are injected in an environment of lower pressures and temperatures and a lower peak of HRR during combustion for the pilot injection is observed. The lower pressure and temperature at the start of pilot injection result in longer ignition delays (ID) (Table 10). In the case of D2 at the 1500 rpm and 35 Nm point, the peak of HRR for the main injection increased as the quantity of unburned pilot injected fuel increased and burned during the combustion of the main injection. However, this did not happen in the case of the HVO fuel, which is characterized by a higher cetane number. On the other hand, advancement of PIT at 1500 rpm 14

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Fig. 13. Burned mass fraction at various IP.

responsible for an increase in the PM production rate are: the higher temperature of the gases during the PM growth phase, the presence of local zones with poor mixing of the fuel with air, and direct interference between the hot flames of the pilot injection and the main injection. At advanced PIT and 35 Nm (1500 rpm and 3000 rpm), PM emissions decrease for both tested fuels. At these points, the fuels blend with the air in the cylinder and form a more uniform combustible mixture, smoothing the combustion process. However, at 100 Nm (1500 rpm and 3000 rpm), at advanced PIT, PM emissions increase and this behavior, according to d’ Ambrosio et al. [40], can be attributed to inhomogeneity in the combustion chamber, with the presence of spots of burned gases derived from the pilot combustion event, which interfere with the main injection fuel spray. Advancing PIT at 1500 rpm and 35 Nm with HVO fuel leads to lower PM and NOx emissions simultaneously (Fig. 10). However, at 3000 rpm and 100 Nm, the default PIT is the optimum in terms of the trade-off between NOx and PM. The engine-out HC emissions tend to increase as PIT is advanced and tend to decrease as PIT is retarded. At earlier PIT, the occurrence of overmixing is more likely, causing an increase in HC emissions [40]. The pilot fuel is injected into a cooler environment, leading to an increase in the ignition delay, which in turn promotes overmixing of the fuel with air [42]. Besides, the absolute values of HC emissions are lower for HVO compared to D2 by an almost constant difference of 30 to 40% over the entire operating range of the engine. As far as the engine performance is concerned, advanced PIT results in a slightly lower thermal efficiency (Fig. 11) for both fuels increasing in that way the bsfc (Fig. 9). This is expected as the injected fuel mass increased (Table 12). On the other hand, when the dwell time between pilot and main injection is reduced, the pilot and main combustions are linked more smoothly, or are at least closer, and thus enhancing combustion efficiency, and thus bsfc improvement [40]. A reduced dwell

3.2.2. Effect of PIT on emissions and engine performance The effect of pilot injection timing on emissions of regulated pollutants (NOx, PM and HC) is presented in Fig. 9. At low speed operation (1500 rpm), NOx emissions are reduced for advanced PIT due to lower in-cylinder pressures and temperatures that were observed in the previous Section 3.2.1. However, at retarded PIT, the opposite trend for NOx is observed for both fuels as the temperature and pressure increased. At 3000 rpm, the fuel for the main injection is injected in the burning fuel of the pilot injection, as a result retardation of PIT lead to higher temperatures promoting NOx formation. When the pilot injection is too close to the start of main injection combustion, the interval time is shorter for sufficient fuel-air mixing [38]. The higher combustion temperature and over-rich regions are relatively wide, resulting in relatively high NOx emissions [39]. As far as advanced PIT at 3000 rpm is concerned, the earlier the pilot injection timing, the lower the heat release rate (HRR) peak of the pilot injection (Fig. 7), and thus the more moderate the pilot combustion. This shows that, NOx emissions produced during pilot combustion, are limited by the advanced PIT. On the other hand, advanced PIT boosted NOx production during the main combustion leading to higher total NOx emissions [40]. On the other hand, at retarded PIT, PM decreases (with the exception of 3000 rpm and 100 Nm point) as the high temperature in the cylinder during the combustion of pilot injection boosted the combustion of the main injection, enhancing soot oxidation [41]. When the dwell time between pilot and main injection is very short, the velocity of the injector needle is higher, during the nozzle opening phase of the main injection, leading to a better spray atomization [40]. This can significantly improve the premixing phase of the main injected fuel with air and thus enable a reduction in the PM emissions [40]. When the PIT is retarded, HVO fuel produces even less PM emissions compared to D2 at default PIT values. In general, the main factors 15

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Fig. 14. Effect of injection pressure on emission characteristics and bsfc.

16

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Fig. 15. Effect of injection pressure, trade-off between PM and NOX emissions.

change > 2 °CA, which shows that there should not be any drivability and operability engine problems. Similar trends were observed for both fuels, with HVO having a shorter ignition delay in all cases compared to D2 fuel (Table 13). However, D2 is more sensitive to injection pressure changes than HVO. More specifically, at medium load conditions (1500 rpm/100 Nm and 3000 rpm/100 Nm) and higher IP, the start of combustion during the main injection, due to faster evaporation of fuels, occurs earlier and closer to TDC in an environment of higher in-cylinder pressures and temperatures resulting in a higher HRR peak. For all analyzed points, an increase of fuel IP led to a higher and narrower heat release peak [31], which results in shorter combustion duration (Table 13 and Fig. 13) and also higher pmax and temperatures (Table 14 and Fig. 12). An exception is only noticed at 1500 rpm and 35 Nm for both fuels, where pmax is lower at higher IP. At this operating point, pilot injection occurs later in the compression stroke (at −25°CA aTDC), where the temperature and pressure are high, as a result pmax, which occurred at 0°CA, is a result of the combustion for the pilot injection. An increase of IP leads to faster evaporation and thus faster burning of the pilot injected fuel, reducing in that way the duration and pmax of combustion during pilot injection. In contrast, the main injection, at 1500 rpm and 35 Nm, starts after TDC (4°CA aTDC), where in-cylinder pressure and temperature have started decreasing (see Table 14 and Fig. 12), however, pmax of the combustion for the main injection increased with the increase of IP due to faster evaporation of fuels, occurs earlier and closer to TDC in an environment of higher in-cylinder pressures and temperatures resulting in a higher HRR peak.

Table 15 Injected fuel mass and power for D2 and HVO in all examined points at default and high IP. PIT

Default

Speed/Load (rpm/Nm)

Injected fuel mass (kg/hr)

1500/35 1500/100 3000/35 3000/100

300 bars Higher

Diesel

HVO

1.57 3.56 3.52 7.57

1.55 3.52 3.54 7.48

Power (kW)

5.6 15.8 10.9 31.3

Injected fuel mass (kg/hr) Diesel

HVO

1.58 3.83 3.54 10.98

1.54 3.77 3.52 9.64

Power (kW)

5.5 15.7 10.9 31.8

time makes the overall combustion occur over a shorter time, and this determines a decrease in bsfc. All in all, HVO showed lower bsfc in most cases compared to D2 fuel due to its lower density. 3.3. Effect of injection pressure (IP) 3.3.1. Effect of IP on combustion This section investigates the effect of increased Injection Pressure (IP) on engine combustion. Table 13 presents the effect of IP on ignition delay and combustion duration. It is observed that increasing IP leads to shorter ignition delay and shorter combustion duration. Higher fuel injection pressure provides more energy to break up fuels into smaller droplets, leading to faster evaporation at the periphery of the fuel sprays, superior dispersion, finer atomization and faster ignition of the fuel vapor [1]. Furthermore the shorter ID, results in an increase of pmax (Table 14) and the HRR peak, and thus combustion temperature (Fig. 12). However, the time (angle) where pmax occurred do not

3.3.2. Effect of IP on emissions and engine performance The effect of injection pressure on engine emissions is presented in Fig. 14. Increasing IP boosted NOx emissions due to higher pressures, 17

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Fig. 16. Effect of injection pressure on thermal efficiency.

As far as HC emissions are concerned, it was observed from Fig. 14 that an increase of IP with HVO results in lower HC emissions in the case of low load (35 Nm) and higher HC emissions in case of medium load (100 Nm). At low loads, the injected fuel mass is not affected by the variation of IP. As a result, the more uniform A/F ratio with higher IP aids the reduction of HC as the combustion rate is mostly limited by the mixture formation rate. At 100 Nm, the influence of a lower overall air-fuel ratio compared to 35 Nm becomes visible. In this case, the injected fuel mass increased at higher IP (Table 15), as a result the engine operates with slightly richer local air–fuel mixtures, which leads to a lower degree of premixing and higher HC formation. In contrast, combustion of D2 with a lower cetane number is limited by chemical kinetics, thus longer premixing, combined with a longer ignition delay lead to entrapment of the mixture to colder parts of combustion chamber, leading to increased HC emissions at all points with the exception of 3000 rpm and 35 Nm. HC emissions of D2 are reduced at 3000 rpm and 35 Nm due to the merging of the combustion of the pilot and main injection. In this case, the ignition delay of the main injection is negligible, preventing escape of the mixture to areas where combustion is not possible. Finally, Figs. 14 and 16 present the effect of IP on bsfc and thermal efficiency, respectively. As the IP increased, more fuel mass is injected in the cylinder (Table 15). As a result, the engine in order to counterbalance this effect reduces the injection duration of both the pilot and main injection (Table 16). However, the injected fuel mass is still higher compared to the default IP for both fuels. As a result, the thermal efficiency of the engine reduces.

Table 16 Injection duration (μs) of pilot and main injection. IP

Default

300 bars Higher

Speed[rpm]/ Torque [Nm]

Pilot (μs)

Main (μs)

Pilot (μs)

Main (μs)

D2 (injection duration) 1500/35 1500/100 3000/35 3000/100

230 280 250 250

475 710 650 690

180 230 180 220

385 585 455 610

D2 (injection duration) 1500/35 1500/100 3000/35 3000/100

230 280 250 250

480 720 640 690

181 230 180 220

380 595 450 610

and thus higher temperatures that occurred during combustion. The same trends are observed for all tested operating points, with the exception of 1500 rpm and 35 Nm for HVO fuel (Fig. 14), due to the differences observed at this point, during the combustion between the two fuels, for the reasons that were analysed in Section 3.1.1. At this operating point (1500 rpm and 35 Nm), NOx emissions of HVO are almost similar for both examined injection pressures as there are no strong differences on in-cylinder temperature between the two IP (Fig. 12). At higher IP, the smaller fuel droplets (lower diameter) and faster fuel evaporation rate lead to improved fuel air mixing [43], higher incylinder pressures and temperatures (Fig. 12), favoring the oxidation process of soot particles and leading to lower PM emissions of up to 80% for both tested fuels. The trade-off plots between NOx-PM emissions for the tested IP are presented in Fig. 15. It is observed that, by reducing PM emissions, strong penalties in NOx emissions up to 60% are inevitable for both fuels.

4. Summary and conclusions HVO (hydrotreated vegetable oil) is a very promising renewable fuel for the transportation sector due to its favorable properties. However, there is a gap in the literature on the recalibration potential of light 18

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duty diesel engines running on HVO. To that purpose the impact of main injection timing, pilot injection timing and injection pressure was investigated in a turbocharged diesel engine operating with HVO fuel to gain full advantage of its properties. HVO was compared with a conventional EN 590 diesel fuel. The aim was to achieve PM and NOx reduction with HVO without strong penalties on thermal efficiency and brake specific fuel consumption. Based on a steady-state analysis and experimental data, HVO was found to provide lower engine-out PM emissions up to 40%, lower HC emissions up to 45%, as well as lower mass-based FC up to 3%, while engine-out NOx emissions were similar or slightly higher to commercial diesel. The findings have shown that although the engine is designed for optimum operation with diesel fuel significant, further advancements in lowering exhaust emissions are possible through utilization of alternative fuels with favorable fuel characteristics and proper engine control. As HVO produces lower PM emissions compared to diesel fuel at the same engine control setting, there is a big potential for NOx emission reductions through a proper engine re-adjustment aiming to significantly improve the PM-NOx trade-off. For the examined engine, the optimum approach was to retard the main injection timing (by 2°CA). In that case, in addition to the reductions instigated by the simple replacement of D2 with HVO, further emission reductions were achieved leading to 20% lower engine-out NOx emissions, while PM emissions were still kept well below D2 levels (up to 30% lower).

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CRediT authorship contribution statement Athanasios Dimitriadis: Methodology, Formal analysis, Writing original draft, Writing - review & editing. Tine Seljak: Investigation, Validation. Rok Vihar: Investigation, Data curation. Urban Žvar Baškovič: Investigation, Data curation. Athanasios Dimaratos: Conceptualization, Methodology, Validation, Writing - review & editing. Stella Bezergianni: Conceptualization. Zissis Samaras: Conceptualization, Methodology, Validation, Writing - review & editing. Tomaž Katrašnik: Conceptualization, Methodology, Validation, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work has been partially funded by COST Action SMARTCATs (CM 1404), supported by COST (European Cooperation in Science and Technology, www.cost.eu) and by Slovenian Research Agency (research core funding No. P2-0401). The authors also wish to express their appreciation for the HVO (NEXBTL) fuel provided by Neste Oil Corporation and the AVL that provided the software for combustion analysis. References [1] Labecki L, Ganippa LC. Effects of injection parameters and EGR on combustion and emission characteristics of rapeseed oil and its blends in diesel engines. Fuel 2012;98:15–28. https://doi.org/10.1016/j.fuel.2012.03.029. [2] US EPA – United States Environmental Protection Agency. Published date 2003. A comprehensive analysis of biodiesel impacts on exhaust emissions. [3] Demirbas A. New liquid biofuels from vegetable oils via catalytic pyrolysis. Energy Educ Sci Technol 2008;21:1–59. [4] Demirbas A. Importance of biodiesel as transportation fuel. Energy Policy 2007;35:4661–70. https://doi.org/10.1016/j.enpol.2007.04.003. [5] Basheer D, Abdul AAR, Daud WMAW, Chakrabarti MH. Performance evaluation of biodiesel from used domestic waste oils: a review. Process Saf Environ Protect 2012;90:164–79. https://doi.org/10.1016/j.psep.2012.02.005. [6] Swain PK, Das LM, Naik SN. Biomass to liquid: a prospective challenge to research and development in 21st century. Renew Sustain Energy Rev 2011;15:4917–33.

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