Study of Miller timing on exhaust emissions of a hydrotreated vegetable oil (HVO)-fueled diesel engine

Abstract

The effect of intake valve closure (IVC) timing by utilizing Miller cycle and start of injection (SOI) on particulate matter (PM), particle number, and nitrogen oxide (NO_x) emissions was studied with a hydrotreated vegetable oil (HVO)-fueled nonroad diesel engine. HVO-fueled engine emissions, including aldehyde and polyaromatic hydrocarbon (PAH) emissions, were also compared with those emitted with fossil EN590 diesel fuel. At the engine standard settings, particle number and NO_x emissions decreased at all the studied load points (50%, 75%, and 100%) when the fuel was changed from EN590 to HVO. Adjusting IVC timing enabled a substantial decrease in NO_x emission and combined with SOI timing adjustment somewhat smaller decrease in both NO_x and particle emissions at IVC −50 and −70 °CA points. The HVO fuel decreased PAH emissions mainly due to the absence of aromatics. Aldehyde emissions were lower with the HVO fuel with medium (50%) load. At higher loads (75% and 100%), aldehyde emissions were slightly higher with the HVO fuel. However, the aldehyde emission levels were quite low, so no clear conclusions on the effect of fuel can be made. Overall, the study indicates that paraffinic HVO fuels are suitable for emission reduction with valve and injection timing adjustment and thus provide possibilities for engine manufacturers to meet the strictening emission limits.

Implications:

NO_x and particle emissions are dominant emissions of diesel engines and vehicles. New, biobased paraffinic fuels and modern engine technologies have been reported to lower both of these emissions. In this study, even further reductions were achieved with engine valve adjustment combined with novel hydrotreated vegetable oil (HVO) diesel fuel. This study shows that new paraffinic fuels offer further possibilities to reduce engine exhaust emissions to meet the future emission limits.

Supplementary Materials: Supplementary materials are available for this paper. Go to the publisher's online edition of the Journal of the Air & Waste Management Association for a complete list of analysed PAH compounds.

Introduction

Engine manufacturers have to fulfill stricter emission limits and, at the same time, lower the fuel consumption. The next step in heavy-duty diesel engine emission limits in Europe is the EURO VI limit (takes effect in 2013), which lowers nitrogen oxide (NO_x) emission to 0.4 g/kWhr (from 2.0 g/kWhr) and particulate matter (PM) emission to 0.01 g/kWhr (from 0.02 g/kWhr). EURO VI also limits particle number emission (European Parliament and European Council, 2009). To achieve these limits, engine manufacturers are forced to use a diesel particulate filter (DPF) for particle emissions and selective catalytic reduction (SCR) or exhaust gas recirculation (EGR) for NO_x emissions. It is known that there is a trade-off between particle and NO_x emissions, which challenges the manufacturers to find a balance between these emissions (Ladommatos et al., 1998). The urge to lower CO₂ emissions and to reduce fossil oil dependence has increased the use of biobased alternative fuels. New HVOs (paraffinic hydrotreated vegetable oils; e.g., NExBTL by Neste Oil) and synthetic F-T (Fischer-Tropsch) diesel fuels (e.g., GTL [gas-to-liquid] by Shell), which have very similar properties, are reported to lower the regulated as well as the unregulated engine exhaust emissions compared with regular diesel fuel (e.g., Tilli et al., 2010; Abu-Jrai et al., 2009; Heikkilä et al., 2009b; Knothe et al., 2010; Murtonen et al., 2010; Aatola et al., 2008). The absence of polyaromatic hydrocarbons (PAHs), the higher cetane number, and the lower distillation endpoint of HVO fuel compared with conventional EN590 (sulfur content <10 ppm) fuel are usually proposed as the main characteristics for lower NO_x and PM emissions (Gill et al., 2011; Nylund et al., 2011). Nylund et al. (2011) performed a large-scale field test with a variety of buses of different engine types fueled with 30% and 100% HVO blends without engine modifications. They concluded that, on average, the 100% blending of HVO fuel reduced regulated emissions (CO −30%, total hydrocarbons [THC] −40%, PM −30%, NO_x −10%) and CO₂ (−5%), whereas volumetric fuel consumption was slightly (+4.5%) increased. Armas et al. (2010) studied the effect of alternative fuels (F-T, soybean methyl ester [SME], and fossil diesel fuel) on exhaust emissions with matched combustion phasings. F-T fuel produced the lowest NO_x, HC, PM, and CO emissions, whereas particle number emission was the lowest with regular ultra-low-sulfur fuel. They speculated that high paraffinic content and low aromatic content are the main reasons for low emission of regulated compounds with the F-T fuel.

As the paraffinic fuels differ from regular diesel in chemical and physical properties, there might be potential to lower emissions further with better engine optimization for new fuels. There have been several studies showing the potential of engine optimization to reduce emissions. According to some studies, engine optimization might be necessary in order to avoid problems related to the new-generation biodiesel fuels. For example, Lapuerta et al. (2011) pointed out that high HVO concentration in fuel blend might induce problems related to the high cetane number. Reason for this is that high cetane number would lead to much earlier ignition, which might further affect adversely emissions, efficiency, and thermal and mechanical loads. They pointed out that high-HVO blends may require engine adjustments to compensate for the change in ignition. On the other hand, they also proposed that low- and medium-HVO blends may improve combustion. Adjusting valve and injection timings is one method to optimize the combustion phasing for specific fuel properties. Hong et al. (2004) provided an extensive review of different valve timing strategies mainly for spark ignition (SI) engines, including early intake valve closing, the same strategy applied in this study. Imperato et al. (2010) studied the effect of the Miller cycle (Miller and Lieberherr, 1957) to NO_x emission of a single-cylinder medium-speed research engine with two different intake valve closure (IVC) timings (−35 °CA and −42 °CA). They reported that NO_x emission reduced by 25–45% depending on load and on the IVC timing of the Miller cycle, but filter smoke number (FSN) increased simultaneously. Happonen et al. (2012) studied the effect of advanced intake valve closing and fuel injection timings on exhaust emissions with an engine run with HVO fuel. They obtained notable reductions with all tested loads in PM and NO_x emissions by combining advanced intake valve closing and fuel injection timings with EGR. Benajes et al. (2009) studied the potential of the Atkinson cycle (comparable to the Miller cycle, but without constant intake air mass flow) and EGR to lower NO_x and other pollutant emissions. They used regular fossil diesel fuel and concluded that EGR is a better method to reduce the NO_x emission than the Atkinson cycle, although they noted that the results would be better in terms of soot, CO, and fuel consumption if the intake air pressure was increased to achieve the original air/fuel ratio.

In this study, the aim was to lower NO_x and particle emissions (PM and particle number) of an engine run with HVO fuel by adjusting IVC and the start of injection (SOI) timings. The emissions of aldehydes and PAHs are also reported.

Experimental

Engine, fuels, and driving modes

Measurements were performed on an engine dynamometer. The engine used in the measurements was a single-cylinder research engine based on a commercial 6-cylinder off-road common rail diesel engine (cylinder diameter 111 mm, stroke 145 mm, compression ratio 17:1). No aftertreatment was used. The charge air was supplied by an external air compressor and the system had an air heater and a cooler to adjust the charge air temperature. The mass flow was measured with a Micro Motion coriolis mass flow meter and controlled by a control valve. To simulate the effect of a turbo charger, there was a back-pressure control valve in the exhaust system. Three steady-state loads (50%, 75%, and 100%) were used while keeping engine speed constant at 1500 rpm.

The control system allowed total control over engine speed, charge air pressure and temperature, exhaust back pressure, and fuel injection timing, pressure, and duration. The engine also had an electrohydraulic valve actuator system that allowed the operator to control valve opening and closing timings, opening duration, and opening magnitude.

The studied engine parameters were advanced IVC and the SOI. Advancing IVC while keeping the intake air mass flow constant is called Miller cycle (Miller and Lieberherr, 1957) and it is used to reduce NO_x emissions by lowering the effective compression ratio and thus reducing the charge temperature at the end of the compression stroke. Measurements were performed with IVC advanced by 30, 50, and 70 °CA relative to the standard setting. The charge air mass flow was kept constant by increasing the charge air pressure respectively to the advanced IVC. Measurements with SOI advanced by 2 °CA relative to the standard setting were also performed at the 100% load. The details of each engine condition are shown in Table 1.

Table 1. Measurement matrix showing fuel, load, indicated power, relative IVC and SOI timings, injection pressure, specific fuel consumption (SFC), and indicated efficiency

Fuel	Load (%)	Indicated Power (kW)	Relative IVC (°CA)	Relative SOI (°CA)	Injection Pressure (bar)	SFC (g/kWhr)	Indicated Efficiency
EN590	50	19	0	0	460	226.9	36.2%
EN590	75	29	0	0	600	229.9	36.4%
EN590	100	38	0	0	900	221.7	37.5%
HVO	50	19	0	0	460	221.1	36.9%
HVO	75	28	0	0	600	223.7	36.8%
HVO	100	38	0	0	900	217.4	37.7%
HVO	100	37	−30	0	900	217.7	37.6%
HVO	100	38	−50	0	900	217.3	37.7%
HVO	100	37	−70	0	900	218.8	37.4%
HVO	100	37	−30	−2	900	207.6	39.4%
HVO	100	37	−50	−2	900	205.9	39.7%
HVO	100	37	−70	−2	900	206.4	39.7%

Two different fuels were used as 100% blends: regular sulfur-free (4 ppm sulfur) EN590 fossil diesel fuel and commercial HVO diesel fuel (NExBTL, Neste Oil). HVO had cetane number of 88 and EN590 54.5, sulfur levels were low (<4 ppm). Total aromatic content in the EN590 fuel was 18.7 and 0.3 wt% in the HVO fuel. Both fuels had low polyaromatic content, 1.1 wt% in the EN590 fuel and <0.1 wt% in the HVO fuel. The values of fuel characteristics are presented in Table 2. Lubricating oil was SAE 10W-40.

Table 2. Fuel characteristics

Fuel Specification	EN590	HVO
Cetane number	54.5	88.2
Total aromatics (wt%)	18.7	0.3
Polyaromatics (wt%)	1.1	< 0.1
Sulfur content (mg/kg)	3.6	< 3.0
Carbon C (wt%)	85.9	84.5
Hydrogen H (wt%)	13.5	15.2
C/H ratio	6.4^a	5.6
Ash (wt%)	<0.001	<0.001
Density at 15 °C (kg/m^3)	837.3	779.9
Viscosity at 40 °C (mm^2/sec)	3.6	3.0
Lubricity at 60 °C (HFRR) (μm)	309.0	351.0
Effective heating value (MJ/kg)	43.2	44.0
Distillation 95 vol% (°C)	360.5	297.6
Note: ^aCalculated value, not analyzed.

Size distribution measurements

The measurement setup is shown in Figure 1. To dilute the exhaust sample, a porous tube diluter (Mikkanen et al., 2001) was used. Downstream of the porous tube was an aging chamber and an ejector-type diluter. The dilution ratio of the porous tube was set to 12 and the temperature of the dilution air was kept at 30 °C. The dilution ratio of the ejector diluter was 10 according to the manufacturer (Dekati Inc., Tampere, Finland) and the sample air temperature was the same as the surrounding temperature (i.e., 20–25 °C). The total dilution ratio was calculated from the raw and the diluted CO₂ concentrations. Similar sampling systems have been used in several vehicle and engine exhaust studies (e.g., Heikkilä et al., 2009b; Rönkkö et al., 2007; Vaaraslahti et al., 2006).

Figure 1.Measurement setup in the engine laboratory.

A thermodenuder was used to study particle volatility. In the thermodenuder, the diluted exhaust sample is first led through a heated tube (temperature = 265 °C) where the volatile compounds are evaporated. The evaporated compounds are gradually cooled after the heater and absorbed into active charcoal. Renucleation or condensation of vaporized compounds after the thermodenuder treatment can be assumed to be negligible due to the high total dilution ratio of the sample entering the thermodenuder. All particle size distributions measured with the thermodenuder were corrected using a penetration curve for the thermodenuder published in an earlier study (Heikkilä et al., 2009a).

Particle size distributions were measured with two scanning mobility particle sizers (SMPS) (Wang and Flagan, 1990; Chen et al., 1998) and an electrical low-pressure impactor (ELPI; Dekati) (Keskinen et al., 1992), with the filter stage (Marjamäki et al., 2002) installed. The ELPI measured the particle size distribution with 1-Hz time resolution and it was used to confirm that the particle emissions were stabilized before the actual measurements. One SMPS was equipped with a DMA 3085 and a CPC 3025 (nano-SMPS; TSI Inc., St. Paul, MN) and the other with a DMA 3071 and a CPC 3025 (SMPS; TSI), called “long-SMPS” from now on. The nano-SMPS and the long-SMPS had measurement ranges from 3 to 60 nm and from 10 to 400 nm, respectively. Up scan time of 120 sec was used in both SMPS instruments.

PM, OC/EC, NO_x, PAH, and aldehyde measurements

PM emission was measured with an AVL SPC 472 (Mainz-Kastel, Germany) microdilution tunnel according to the International Organization for Standardization ISO 8178–1:2006 standard. The sample was taken from raw exhaust gas and diluted on one stage before sample filters. The PM samples were collected on 70-mm-diameter Pallflex TX40HI20-WW filters (Pall Corp., Port Washington, NY). The filter face velocity was 48 cm/sec (at 47 °C) and the diluted sample gas temperature was kept between 42 and 52 °C. The dilution ratio set point was 6 for the 50% and 75% loads and 7 for the 100% load. Filters were weighted in a humidity- and temperature-controlled climate chamber before and after the measurements with a Sartorius SE2-F ultramicrobalance (Goettingen, Germany).

The samples for the organic carbon/elemental carbon (OC/EC) analysis were collected on 70 mm Munktell MK360 quartz fiber filters (Falun, Sweden) using an AVL microdilution tunnel with the same sampling parameters that were used for PM collection. The OC/EC analysis of the collected samples was performed with an OC/EC analyzser model 4L (Sunset Laboratory Inc., Tigard, OR) in accordance with the National Institute for Occupational Safety and Health (NIOSH) Method 5040. Before the measurements, the sample filters were heat treated in an oven for 2 hr at 800 °C to eliminate possible carbon residuals on the filters. After the heat treatment, the filters were stabilized in a climate room before weighing them for the measurements.

NO_x emission was measured using an Eco Physics CLD 822 S h analyzer (Duernten, Switzerland) and the sample was taken from the raw exhaust through a heated sample line.

Aldehyde samples were collected simultaneously with PM samples from the AVL microdilution tunnel using dinitrophenylhydrazine (DNPH) cartridges. The DNPH derivatives were extracted with acetonitrile and the extraction was diluted (1:1) with water before the analysis. Altogether 11 aldehydes were analyzed with high-performance liquid chromatography (HPLC) (HP 1050; Agilent Technologies, Santa Clara, CA; ultraviolet [UV] detector, Nova-Pak C18 column). The main attention was given to formaldehyde (FA) and acetaldehyde (AA). Other aldehydes analyzed were acrolein, propionaldehyde, crotonaldehyde, methacrolein, butyraldehyde, benzaldehyde, valeraldehyde, m-tolualdehyde, and hexanal.

To analyze PAHs, the PM sample filters were extracted with toluene using the Soxhlet method. The PAH analysis was performed from the extracted part of particle mass, soluble organic fraction (SOF). The list of analyzed PAHs is presented in Table A1 (supplementary materials). Table A1 also shows total PAH versus PM as a mass ratio. The PAH analysis was performed using gas chromatography with selected ion monitoring mass spectrometric detection (GC/SIM-MS) after liquid chromatographic purification of the extract. There are several lists of priority PAHs. In this article, the sum of all analyzed PAHs and lists of 14 and 7 priority PAHs are reported. The list of 14 priority PAHs is based on different PAH listings (e.g., U.S. Environmental Protection Agency [EPA], European Union [EU], NIOSH) and the 7 priority PAHs are defined as probable carcinogenic compounds by the EPA (Table A1). As PAH compounds are measured only from the particulate phase, the interpretation of the results should be conducted with caution. A major portion of 2- and 3-ring PAH compounds can be in the gaseous phase and, thus, they are not detected when only PM samples are analyzed. Murtonen and Aakko-Saksa (2009) reported that the emission of semivolatile PAH compounds, collected on polyurethane filters, is lower with paraffinic fuel compared with conventional diesel fuel.

Results and Discussion

Emissions with standard engine running parameters

Emissions with standard valve and injection timings were measured with both fuels. With the HVO fuel, specific fuel consumption (g/kWhr) was slightly lower and efficiency higher than with the EN590 fuel due to the higher gravimetric heating value of the HVO fuel (volumetric heating value is lower than that of EN590, because of the lower density).

Figure 2 presents PM and NO_x emissions for both fuels with standard engine running parameters on different loads. With the HVO fuel, the PM emissions were lower than with the EN590 fuel, as reported in earlier studies (e.g., Kuronen et al., 2007). The absence of aromatic compounds and in some cases higher cetane number of the HVO fuel are usually reported as the main factors for lower PM emissions (Aatola et al., 2008; Gill et al., 2011). The NO_x emission at all load points was somewhat lower with the HVO fuel than with the EN590 fuel. One reason for lower NO_x emissions with the HVO fuel is probably the absence of aromatics. Aromatic compounds have higher adiabatic flame temperature and lower H/C ratio, therefore producing higher combustion temperature (Glaude et al., 2010), which again promotes NO_x production. Higher cetane number of the HVO fuel, on the other hand, promotes earlier ignition, which in turn shortens the premixed combustion phase, resulting in lower NO_x emission. Aatola et al. (2008) suggested that as fuel injection lasts longer with the HVO fuel, because of lower volumetric heating value of the fuel, the combustion duration becomes longer and the peak temperature lower, leading to reduced NO_x emission.

Figure 2. PM and NO_x emissions for both fuels on the studied loads with standard engine settings.

Figure 3 shows particle size distributions (Figure 3a–c) and particle number emission (Figure 3d) at different loads and with the two fuels used. Particle size distributions were measured with both the nano- and the long-SMPS. Only the distributions measured with the long-SMPS are shown because neither the nucleation mode nor a considerable amount of particles smaller than 10 nm were observed at any point. The porous tube diluter used as the primary dilution allows nucleation mode formation similar to real-world dilution conditions (Rönkkö et al., 2006). All distributions are corrected with a dilution factor to raw exhaust concentrations. Particle concentrations are lower with higher loads (Figure 3a–c). The main reason for this is the higher injection pressure of fuel with higher loads. Higher fuel injection pressure decreases fuel droplet size and enhances mixing of fuel with air. The effect of fuel injection pressure on the particle size distribution was studied by Lähde et al. (2011).

Figure 3. Particle number size distributions and emission on the studied loads with standard engine settings. Note the different scale in y-axes.

The HVO fuel reduces both the particle number emission and particle size when compared with the EN590 fuel. When the sample was treated with the thermodenuder, no significant difference was observed compared with the size distributions presented above on any load. The relative difference in PM emissions was larger between the two fuels than the difference in particle number emission. This can partly be explained by particle size. With the EN590 fuel, the emitted particles are larger than with the HVO fuel and this shows in the mass-based measurements more strongly.

Lower soot particle number emissions and smaller particle size with the HVO fuel are in line with earlier studies (e.g., Heikkilä et al., 2009b; Lapuerta et al., 2010). With the HVO fuel, lower particle emissions (PM and number) are mainly due to the absence of polyaromatic (PAH) compounds, as aromatic compounds are considered as the precursors for the soot formation process (Marchal et al., 2009). Higher cetane number of the HVO fuel shortens the ignition delay and thus changes the combustion phasing (Aatola et al., 2008; Nishiumi et al., 2004; Ickes et al., 2006). The shorter duration of the premixed combustion phase in general leads to increased particle emissions, but in the case of the HVO, the total effect of fuel properties (e.g., the absence of aromatics) seems to have a stronger effect on lowering particle emissions.

When the sample was treated with the thermodenuder, no significant differences in the measured size distributions were observed compared with the untreated sample, indicating a relatively low fraction of volatile material in the particles. It has to be noted that there might be compounds with low vapor pressure that are not evaporated at the temperature of the thermodenuder (265 °C). Also, the low fraction of OC compared with EC emission (see Table 3) supports this conclusion. The measurement on the 100% load with the HVO fuel is an exception, with the OC/EC ratio of 53%. With the HVO fuel, the change in the OC/EC ratio was mainly due to the change in the EC emission, as the OC emission varied much less. Thus, the changes in PM emissions between different loads were mainly due to nonvolatile particle matter, that is, soot.

Table 3. Organic carbon (OC), elemental carbon (EC), aldehyde, and PAH emissions on standard engine settings

Fuel/Measurement Point	Load (%)	OC (mg/kWhr)	EC (mg/kWhr)	OC/EC	FA (mg/kWhr)	AA (mg/kWhr)	Other than FA and AA (mg/kWhr)	Total PAH (μg/kWhr)	PAH14 (μg/kWhr)	PAH7 (μg/kWhr)
EN590 reference	50	—	—	—	23.7	6.6	4.8	61.6	37.9	1.3
EN590 reference	75	21.0	89.8	23%	3.1	0.9	0.3	60.0	31.9	1.0
EN590 reference	100	13.1	48.9	27%	0.9	0.4	0.1	26.7	13.0	0.9
HVO reference	50	—	—	—	18.9	5.3	4.1	29.4	16.9	0.6
HVO reference	75	15.5	55.3	28%	3.8	0.9	0.5	26.3	13.6	0.6
HVO reference	100	12.1	23.0	53%	1.2	0.2	0.2	12.5	6.6	0.6

Aldehyde emissions (FA, AA) were slightly higher with the HVO fuel than with the EN590 fuel on the 75% and 100% loads. The 50% load point is an exception; aldehyde emissions were lower with the HVO fuel than with the EN590 fuel. According to Lin et al. (2009), paraffinic fuels reduce aldehyde emissions at low-load conditions. Thus, our results are in line with this previous observation. Total PAH emissions were reduced over 50% (52–56% depending on load) with the HVO fuel compared with the EN590 fuel. With both fuels the amount of the seven priority PAHs was negligible. The absence of aromatics in the HVO fuel is the most probable cause for the difference in PAH emissions between the fuels (Mi et al., 2000).

Emissions with advanced IVC and SOI

All values with advanced IVC and SOI timings were measured on the 100% load and with the HVO fuel. Figure 4 shows PM and NO_x emissions with IVC advanced 70 °CA while keeping SOI as standard and IVC advanced 70 °CA combined with SOI advanced 2 °CA. Emissions with the standard engine settings and both fuels are shown for reference.

Figure 4. PM and NO_x emissions with advanced IVC and SOI.

Advancing IVC by 70 °CA lowered NO_x emission from 5.20 to 4.01 g/kWhr with the HVO fuel, but at the same time PM emission increased from 44 to 72 mg/kWhr.

When both IVC and SOI were advanced (respectively by 70 and 2 °CA), the PM emission dropped to 36 mg/kWhr and NO_x emission to 4.44 g/kWhr, the OC and EC emissions were reduced to 11.0 mg/kWhr (−9%) and to 18.6 mg/kWhr (−19%), respectively. Specific fuel consumption (SFC) was reduced 5.1% compared with the standard IVC and SOI timings (Table 1).

Advancing SOI usually reduces particle emission and produces more NO_x due to higher combustion temperature and pressure. This could also have an effect to the premixed combustion phase. According to the measured heat release curves, no considerable difference in the premixed combustion phase between the standard and the advanced SOI timings was found.

Figure 5 shows particle number and NO_x emissions and the geometric mean diameter (GMD) of the measured particle number size distributions with different engine running parameters on the 100% load with the HVO fuel. Advancing IVC reduced NO_x emission but both particle number emission and particle size increased. Advancing IVC with 70 °CA reduced NO_x emission by 23%, as mentioned previously. However, particle number emission was increased by 81% (from 4.0 × 10¹⁵/kWhr to 7.2 × 10¹⁵/kWhr). Combining advanced SOI with IVC −70 °CA resulted in 16% and 15% reduction in particle number and NO_x emission, respectively. GMD was also reduced, from 59 to 56 nm, due to the modified SOI and IVC timings. Advancing SOI by 2 °CA and IVC by 50 °CA reduced particle number emission by 40% (from 4.0 × 10¹⁵/kWhr to 2.4 × 10¹⁵/kWhr), whereas NO_x emission was only slightly reduced (−4%) compared with the standard.

Figure 5. Particle number and NO_x emissions with different IVC and SOI timings. Geometric mean diameters of measured particle size distributions are also shown.

Advancing IVC reduced NO_x emission, but at the same time particle number emission and the GMD of the particle size distribution increased. Similar results have been reported by Imperato et al. (2010), though they measured filter smoke number (FSN) to study particle emission. Advancing SOI seems to have a notable effect on particle emission by making combustion phasing more optimal, as discussed earlier. There are many valve timing strategies as proposed by Hong et al. (2004) but, as Lancefield et al. (2000) point out, mechanical aspects limit the applicability to modern diesel engines. Modern light-duty diesel engines have high compression ratios and, therefore, very little clearance between the piston and valves when the piston is close to the top dead center. Therefore, adjusting intake valve closing and exhaust valve opening might be the only realistic options to such diesel engines.

Particle number and NO_x emissions compared with the standard settings as the reference at different engine running parameters are shown in Table 4. It can be seen that, with different running parameters, it is possible to reduce NO_x or particle emission, or to find a suitable balance between the two emissions.

Table 4. Comparison of PM, particle number, and NO_x emissions with different IVC and SOI timings

Engine Running Parameters/HVO Fuel	PM Emission vs. HVO Fuel Reference	Particle Number Emission vs. HVO Fuel Reference	NOx Emission vs. HVO Fuel Reference
IVC −30 °CA	—	+7%	−3%
IVC −50 °CA	—	+33%	−13%
IVC −70 °CA	+64%	+81%	−23%
IVC −30 °CA, SOI −2 °CA	—	−45%	+7%
IVC −50 °CA, SOI −2 °CA	—	−40%	−4%
IVC −70 °CA, SOI −2 °CA	−17%	−16%	−15%

Conclusions

The effects of IVC and the SOI timings on particle (PM and particle number) and NO_x emissions of a single-cylinder nonroad diesel test engine run with HVO fuel were studied. Also, the emissions of OC/EC, PAHs, and aldehydes were studied with standard engine settings run with both the tested fuels.

Particle number, mass, and NO_x emissions were lower with the HVO fuel compared with the EN590 fuel with standard engine settings for all the studied load conditions. Aldehyde emissions were slightly higher with the HVO fuel at the high loads (75% and 100%), although the absolute emission level was low. At the medium (50%) load, the aldehyde emission was lower with the HVO fuel. PAH emission was reduced substantially with the HVO fuel, mainly due to the absence of aromatics.

Advancing IVC lowered NO_x emissions, but at the same time PM and particle number emission and particle size were increased. Advanced IVC (−50 and −70 °CA) combined with advanced SOI decreased both particle number and NO_x emission, whereas with IVC −30 °CA and SOI −2 °CA NO_x emission was slightly increased. The optimum combination of the engine parameters depends on the required emission level and on the applied aftertreatment system. If, for example, a DPF is used, the raw particle number emission can be higher and the NO_x emission can be reduced with advanced IVC. If, on the other hand, an SCR is used, the NO_x raw emission can be higher, and advanced SOI can be used to lower the particle number emission.

This study shows that changing the fuel from EN590 to HVO diesel fuel decreases exhaust emissions but there is a great potential for further emission reductions in optimizing the engine specifically for the HVO diesel fuel. By combining more optimal timings of IVC and SOI for the fuel (HVO in this case), it is possible to either reduce NO_x or particle emissions substantially or to reduce both of these emissions to a smaller extent. Engines and vehicles generally use regular fuel that varies in the limits of specifications. Engine optimization for a certain fuel can be accomplished, for example, with centrally fueled vehicle fleet, such as buses or a company's trucks. Another option is to produce engines with systems that automatically adjust engine parameters according to the fuel used. In conclusion, HVO and F-T fuels, having similar properties, seem to provide more possibilities for engine manufacturers to meet the strictening emission limits.

Acknowledgments

This study was funded by Tekes (decision 40445/08).