Effects of Gasoline Direct Injection Engine Operating Parameters on

Article pubs.acs.org/EF

Effects of Gasoline Direct Injection Engine Operating Parameters on Particle Number Emissions Xin He,* Matthew A. Ratcliff, and Bradley T. Zigler National Renewable Energy Laboratory 1617 Cole Blvd, MS 1634, Golden, Colorado 80401, United States ABSTRACT: A single-cylinder, wall-guided, spark ignition direct injection engine was used to study the impact of engine operating parameters on engine-out particle number (PN) emissions. Experiments were conducted with certification gasoline and a splash blend of 20% fuel grade ethanol in gasoline (E20), at four steady-state engine operating conditions. Independent engine control parameter sweeps were conducted including start of injection, injection pressure, spark timing, exhaust cam phasing, intake cam phasing, and air−fuel ratio. The results show that fuel injection timing is the dominant factor impacting PN emissions from this wall-guided gasoline direct injection engine. The major factor causing high PN emissions is fuel liquid impingement on the piston bowl. By avoiding fuel impingement, more than an order of magnitude reduction in PN emission was observed. Increasing fuel injection pressure reduces PN emissions because of smaller fuel droplet size and faster fuel−air mixing. PN emissions are insensitive to cam phasing and spark timing, especially at high engine load. Cold engine conditions produce higher PN emissions than hot engine conditions due to slower fuel vaporization and thus less fuel−air homogeneity during the combustion process. E20 produces lower PN emissions at low and medium loads if fuel liquid impingement on piston bowl is avoided. At high load or if there is fuel liquid impingement on piston bowl and/or cylinder wall, E20 tends to produce higher PN emissions. This is probably a function of the higher heat of vaporization of ethanol, which slows the vaporization of other fuel components from surfaces and may create local fuel-rich combustion or even pool-fires.

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INTRODUCTION On July 29, 2011, President Obama announced a historic agreement with 13 major automakers to pursue the next phase in the administration’s national vehicle program: increasing fuel economy to 54.5 miles per gallon for cars and light-duty trucks by model year 2025.1 Vehicle manufacturers are turning to advanced engine technologies to improve efficiency and meet strict fuel economy and emissions requirements. Advanced technologies such as variable valve timing and lift, cylinder deactivation, direct fuel injection with turbocharging/supercharging, and integrated starter/generator will be adopted more rapidly in the future to improve engine efficiency. Gasoline direct injection (GDI) coupled with turbocharging is currently one of the most cost-effective technology options for gasoline engines. The major advantages of a GDI engine are increased fuel efficiency and higher specific power output. GDI has better knock resistance than port fuel injection due to the in-cylinder cooling effect of the injected fuel, and the more evenly dispersed mixtures allow for more aggressive ignition timing. GDI coupled with turbocharging/supercharging enables engine downsizing while maintaining the full load performance via boosting. Higher fuel economy is achieved by shifting the speed/load operating point to a more efficient region through the reduction of engine displacement.2 The market share of GDI engines in the United States increased from 0% in 2007 to 8.5% in 2010.3 It is expected that the use of GDI will account for over 35% of the European automotive gasoline engine market by 2013 and 60% of the U.S. market by 2016 .4,5 Although GDI engines are likely to be deployed for their fuel economy and CO2 reduction benefits, GDI technology tends to have higher particulate matter (PM) mass and particle number (PN) emissions than conventional port fuel injection © 2012 American Chemical Society

technology. Reports in the published literature point to GDI PM mass emissions in the range 2−20 mg/mi.5 Studies comparing GDI engines and diesel engines showed higher PN emissions from GDI engines relative to diesel engines equipped with diesel particulate filters (DPFs).6,7 In Europe, a PN emission limit of 6 × 1011 particles/km using the Particle Measurement Programme (PMP) method over the New European Driving Cycle becomes effective at the Euro 5/6 stage for all categories of diesel vehicles. The European Commission issued a proposal for a PN limit for Euro 6 vehicles equipped with GDI engines. The PN limit for GDI vehicles would be phased in in two steps: a limit of 6 × 1012 particles/km (equal to 10 times the Euro 5 + PN limit for diesel vehicles) would become effective at the Euro 6 effective date (2014.09/2015.09 for new types/all models); a limit of 6 × 1011 particles/km (equal to diesel) would become effective three years later (2017.09/2018.09 for new types/all models).8 The California Air Resources Board (CARB) also proposed that, beginning in 2014, all vehicles subject to Low Emission Vehicle (LEV) III through Super Ultra Low-Emission Vehicle (SULEV) requirements must comply with either the Federal Test Procedure (FTP)-weighted PM mass emission limit or FTP-weighted solid PN emission limit of 6.0 × 1012 particles/ mile in 2014 and to 3.0 × 1012 particles/mile in 2017.5 In order to meet these stringent emission standards, future GDI engines need to be optimized for PM and PN emissions. In 2010, the U.S. Environmental Protection Agency (EPA) published the Renewable Fuel Standard (RFS) Program Final Received: December 6, 2011 Revised: February 25, 2012 Published: March 2, 2012 2014

dx.doi.org/10.1021/ef201917p | Energy Fuels 2012, 26, 2014−2027

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shown in Table 1 and Figure 1. Figure 1 also shows the schematic of the SCE setup. In this wall-guided SIDI engine, fuel is injected into the

Rule. The new RFSs increase the total volume of renewable fuel required to be blended into transportation fuel to 36 billion gallons by 2022.9 Over the next several years, ethanol is expected to make up the majority of this requirement. Fuel properties could have significant impacts on PN emissions. Some studies have been conducted to evaluate the impacts of midlevel ethanol blends (ethanol blends up to 20 vol %) on GDI engine PN emissions. However, the results do not reveal a consistent clear picture of ethanol’s effects. Previous studies conducted by the authors using a production General Motors (GM) Ecotec 2.0 L LNF-series GDI engine showed that E10 produced almost the same PN emissions as E0, while E20 reduced PN emission by up to 20%.10,11 Storey et al.12 conducted a similar study using a Pontiac Solstice equipped with a similar GM 2.0 L LNF-series GDI engine by running the vehicle on Federal Test Procedure (FTP) and US06 Supplemental Federal Test Procedure cycles. They found that E20 reduced PM mass emissions by 30% and 42% over the FTP and US06 cycles respectively. Chen et al.13 studied the PM mass emissions from a GDI single-cylinder optical engine for a range of isooctane/ethanol blends at part load operating condition. They found higher PM and PN emissions with increasing ethanol content. They attributed this to poorer homogeneity of air−fuel mixing as a result of the higher specific enthalpy of vaporization of ethanol. Khalek et al.14 studied the engine-out particle emissions from a 2009 GDI engine using three different gasoline fuels. They found that cold-start and acceleration events were major contributors to particle emissions from this GDI engine. Slow droplet evaporation and the presence of fuel enrichment zones during combustion may be the major contributor to PM formation. Most recently, Szybist et al.15 examined the effect of fuel type, engine breathing strategy, and fueling strategy on particle emissions from a naturally aspirated SI engine. They found that fuel injection timing is the engine parameter that has the most influence on particle emissions with direct injection fueling. E85 produces 1−2 orders of magnitude lower particle emissions relative to single direct injection fueling with gasoline and E20. Although these studies provide some insight into the effects of ethanol content and fuel injection timing on PN emissions, very limited information is provided on how to optimize the engine operation parameters to minimize PN emissions. Recently, the authors converted a production GM Ecotec 2.0 L LNF-series spark-ignition direct-injection (SIDI) engine into a single-cylinder research engine. The research engine control system allows fully independent control of spark timing, fuel injection timing, intake and exhaust cam phasing, fuel injection pressure, exhaust back pressure, and air to fuel ratio (AFR). This provides an opportunity for investigating the effect of these specific engine operation parameters on engine performance and emissions. A previous study showed that three-way catalysts (TWCs) used with GDI engines do not significantly alter PN emissions.10 In this paper, special attention is paid to engine-out PN emissions and the development of engine control strategies to reduce them.

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Table 1. Engine Specifications engine

single-cylinder GDI engine

displacement (L) bore (mm) stroke (mm) connecting rod length (mm) compression ratio

0.5 86.0 86.0 145.5 9.2

Figure 1. Schematic of the SCE setup. combustion chamber from a side-mounted injector. The fuel−air mixture is then guided toward the spark plug by a special piston recess, assisted by reverse tumble. Fuel injection typically occurs during the intake stroke to ensure sufficient fuel−air mixing before combustion starts. The engine is connected to a 75-hp AC dynamometer provided by Sakor, Inc. An engine controller developed by Drivven, Inc., is used to provide fully flexible and independent control of fuel injection timing, spark timing, and intake and exhaust cam-phaser positions. It also controls the high-pressure fuel delivery cart, critical orifice air supply system, and exhaust valves. The air flow is controlled and measured by the critical orifice system provided by Flow Systems, Inc. The system includes six nozzles with orifice diameters of 0.044 in., 0.063 in., 0.088 in., 0.125 in., 0.177 in., and 0.25 in. These provide 2-fold increments in orifice areas, giving a wide range of air flow control by selecting the appropriate orifices and controlling the pressure upstream of the orifice nozzles. The nozzles are calibrated so that the air flow rate can be calculated based on the upstream pressure. The Drivven independent engine control system determines the selection of the nozzles based on desired intake air pressure. The high-pressure fuel delivery cart was designed and manufactured by IHCdirect. This system provides fuel to the fuel rail with stable and desired pressure and is used in lieu of the production cam-driven highpressure fuel pump. All system components are compatible with biofuels, such as ethanol. The fuel temperature is self-controlled by the fuel delivery cart. Fuel pressure can be either controlled by a standalone control panel or by the Drivven independent controller. The emissions analysis bench includes four California Analytical Instruments, Inc. (CAI) 600 series analyzers and is designed for measuring raw (undiluted) exhaust gas. Nitrogen oxide (NOx) is measured using a CAI 600 HCLD NO/NOx digital chemiluminescence analyzer. Total hydrocarbon (THC) is measured using a CAI 600 HFID flame ionization analyzer. Carbon monoxide (CO) and carbon dioxide (CO2) are measured using a CAI 600 nondispersive infrared (NDIR) digital analyzer, which includes a low CO channel, a high CO channel, and a CO2 channel. Oxygen (O2) is measured using another CAI 600 NDIR digital analyzer. The exhaust sample is

EXPERIMENTAL SETUP

Single Cylinder Gasoline Direct Injection Engine. The research single cylinder engine (SCE) was developed from a production 2009 model year GM Ecotec 2.0 L LNF-series engine, which uses a wall-guided, stoichiometric SIDI combustion system. The engine specifications and a schematic diagram of the system setup are 2015


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cooled the sample gas to about 40 °C before entering the FMPS. The estimated residence time in the sample line is about 3 s. The engineout dry-based CO2 was measured using the emission bench described above. The postdilution exhaust was resampled after the FMPS for CO2 measurement using a dedicated CAI CO2 analyzer (CAI Model 300). Because no water condensed during exhaust dilution, the postdilution CO2 is a wet-based measurement. Because the dilution gas is CO2-free air, the dilution ratio, α, can be calculated based on the pre- and post-FMPS CO2 concentration using eq 1, where (H/C) is the fuel hydrogen to carbon ratio.

extracted before the exhaust surge tank. A heated filter (Unique Heated Products, Inc.) is used to remove particles. Both NOx and THC analyzers measure the hot exhaust sample, providing wet-based NOx and THC results. The exhaust sample is dried using a refrigerated heat exchanger before entering the NDIR analyzers for dry-based CO, CO2, and O2 measurements. Combustion analysis and crank angle-based high-speed data acquisition (DAQ) are performed using an AVL IndiMODUL system. The engine crank angle position is identified using an AVL encoder type 365C01. Cylinder pressure is measured using a Kistler piezoelectric pressure transducer type 6125C. Intake manifold pressure and fuel rail pressure are measured using Kistler pressure transducer types 4005BA5FA2 and 4065A500A2. Exhaust manifold pressure is measured using an OMEGA pressure transducer type PX303-050A5 V. Tektronix current probes (Model A622) are used to measure the spark and fuel injection signals. Two-stage Dilution System and TSI Fast Mobility Particle Sizer. The PN size distribution is measured using a TSI, Inc. fast mobility particle sizer (FMPS) spectrometer (Model 3091). The FMPS spectrometer performs particle size classification based on differential electrical mobility classification. The charging of the aerosol is accomplished through two unipolar diffusion chargers. The charged particles are deflected radially outward and collected on electrically isolated electrodes located at the outer wall. The PN concentration is determined by measuring the electrical current collected on the electrodes. An inversion algorithm is used to deconvolute the data and make corrections for image charges (created by particles that flow past the detection stages but do not contact the electrode rings) and time delays in the column, converting currents from the electrometers into 32 channels of output. This process allows the maximum resolution of the instrument to be represented by output channels that are equally spaced on a log scale between 5.6 and 560 nm.16 In this study, the data were collected at a sampling rate of 1 Hz for 3 min. The results presented are the averaged PN over 180 sample points with the error bar representing the standard deviation. PN measurement is very sensitive to the sample’s relative humidity, especially when measuring small particles with diameter less than 30 nm.17 To prevent water condensation and to minimize semivolatile nucleation in the sampling system, the exhaust samples were diluted using a two-stage dilution system, achieving a total dilution ratio of

α=

⎛ [CO2dry ]engineout /⎜100 + [CO2dry ]engineout × ⎝

( HC )/2⎞⎠ ⎟

[CO2wet ]afterdilution

(1) Fuels. In this study, two fuels were tested, including EPA Tier II EEE U.S. federal emission certification fuel (E0) purchased from Johann Haltermann Ltd. as a baseline comparison and splash blended E20 (20 vol % fuel grade ethanol with E0) by the authors. Table 2 summarizes selected fuel properties.

Table 2. Test Fuel Properties ASTM test method density lower heating value (LHV) research octane number (RON) motor octane number (MON) (RON + MON)/2 ethanol carbon hydrogen oxygen H/C O/C T50 T90 a

D4052 D240 D2699

units

E0

E20

[kg/L] [MJ/kg]

0.744 42.68 97

0.754 40.52 103.8

88.5

90.8

92.75

97.3 19.98 79.39 13.02 7.3 1.954 0.069 164.7 307.9

D2700

D5599 E191/D5291a E191/D5291a D5599

[vol %] [wt %] [wt %] [wt %]

86.51 13.38 1.847

D86 D86

[°F] [°F]

223 319

E20 tested by ASTM D5291.

Engine Operating Conditions. Table 3 lists the engine operating conditions conducted in this investigation. The notation “speed-net mean effective pressure (NMEP)” will be used here to represent an engine operation condition. For example, “1000-3” means the engine was running at 1000 rpm and 3 bar NMEP. Experiments were conducted on both “cold” engine and “hot” engine conditions by maintaining the engine-out coolant temperature at 30 and 88 °C, respectively. For this SCE, the engine coolant was used as cooling fluid for the oil heat exchanger. Thus, the engine oil temperature was always slightly higher than the coolant temperature. Previous studies have found that the majority of PN were created during engine cold start when driving on the FTP cycle.14 In this study, the “cold” engine experiments simulated engine operation during an engine cold start. Parametric sweeps from these baseline conditions were conducted for the various studies discussed in this paper.

Figure 2. Schematic of exhaust sampling system.

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about 10:1. Figure 2 shows the schematic of the exhaust sampling system. The sample lines in red are heated to 190 °C. Engine-out samples were extracted 50 cm downstream of the engine exhaust valves. The dilution gas was generated by a Parker Balston purge gas generator (Model 75-45-12VDC), which provided clean, dry CO2-free air. Two mass flow controllers (MFCs) control the dilution gas flow rate. The first stage dilution ratio was about 5:1, and the dilution gas was heated above 130 °C to minimize semivolatile nucleation. The second stage dilution gas was at ambient temperature to cool the sample gas. Stainless steel tubing after the second stage dilution further

RESULTS AND DISCUSSION PN size distribution and total PN emissions are presented for each operating condition. The PN concentration is normalized to the bin width and displayed on the y-axis as dN/dlogDp (number/cm3).18 dN is the number of particles in the range (total concentration). dlogDp is the difference in the log of the channel width, which is calculated by subtracting the log of the lower channel diameter from the log of the upper channel 2016


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Table 3. Baseline Engine Operation Conditions engine speed (rpm)

engine NMEP (bar)

1000

3

1500

6

1500 2000

13 14.4

cold hot cold hot hot hot

coolant T (°C)

oil T (°C)

intake air T (°C)

spark timing (dBTDCa)

inject. timing (dBTDCa)

30 88 30 88 88 88

35 89.5 40 90 90 92.5

29 33 29 33 30 30

22.6 22.6 28 33 10.4 14.4

269 269 269 269 278 290

inject. pressure (bar)

intake cam phasing (CADb)

exhaust cam phasing (CADb)

33

16.4

15.6

52

40.4

30.6

68 68

35.6 33.6

25 27.1

a Degrees before top dead center (dBTDC), referenced to the top of the combustion stroke. bCrank angle degree (CAD) relevant to the cam phaser parked position. “0” CAD means the default cam phaser parked position, which is the most retarded position for the intake cam phaser and the most advanced position for the exhaust cam phasing, as shown in Figures 9 and 12.

Figure 3. Impact of SOI on PN emissions at 1000-3-cold.

Figure 4. Impact of SOI on PN emissions at 1500-13-hot.

by agglomeration of nucleation mode particles and may also include condensed or adsorbed volatile material. Engine Operating Parameter Effects. To illustrate the range of engine operating parameter effects on PN emissions, comparisons are made of measurements from two disparate engine operating conditions using E0. One set of conditions was at low speed (1000 rpm), low load (3 bar NMEP), with a cold engine (1000-3-cold). The other set of conditions was at slightly higher speed (1500 rpm), but high load (13 bar NMEP), with a hot engine (1500-13-hot). Independent sweeps of engine operating parameters were performed, including start

diameter for each channel. In this case, the normalized PN concentration is calculated by multiplying the PN concentration by 16, the number of channels per decade.16 A bimodal particle size distribution was observed at all conditions with peaks at 10 nm and ∼70 nm. In this paper, small particles less than 30 nm in diameter are referred to as “nucleation mode” particles.19 They are formed during combustion and dilution, usually through homogeneous and heterogeneous nucleation mechanisms. Nucleation mode particles are more sensitive to humidity, dilution, and sampling system factors.20 Particles larger than 30 nm diameter are referred to as “accumulation mode” particles, which are formed 2017


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Figure 5. Impact of injection pressure on PN emissions at 1000-3-cold.

Figure 6. Impact of injection pressure on PN emissions at 1500-13-hot.

was retarded beyond 249 dBTDC, and at 209 dBTDC, a significant increase in nucleation mode particles was observed. This may be the result of an insufficient mixing time and thus less homogeneity of the fuel−air mixture at the start of combustion. This interpretation is supported by the observed higher CO and THC emissions and higher NMEP coefficient of variation at the 209 dBTDC condition. These results show that SOI can be optimized to obtain the lowest total PN emissions. At this “ideal” SOI, fuel impingement on the piston is avoided while fuel injection is also early enough to provide adequate time for fuel−air mixing before the start of combustion. A similar response was observed at 1500-13-hot conditions, as illustrated in Figure 4. A sharp increase in total PN emissions was observed when SOI occurred before 278 dBTDC. For SOI at 308 dBTDC, the peak of accumulation mode particle size distribution was observed at 80 nm. By retarding the SOI to 278 dBTDC, the peak of accumulation mode particle distribution shifted to 60 nm. From these examples, it is clear that SOI can significantly impact PN emissions. The results support the hypothesis that high PN emissions from a production engine at idle and 1000-3 are due to the fuel impingement on the piston bowl. For this wall-guided GDI engine, the SOI should be controlled around 270 dBTDC at low load to avoid liquid fuel impingement on piston bowl. When fuel liquid impingement on the piston bowl occurs, the

of injection (SOI), fuel injection pressure, spark timing, exhaust cam phasing, intake cam phasing, and AFR. Start of Injection. Previous studies using the production LNF-series engine showed strong correlation between PN emissions and SOI. At idle and 1000-3 conditions, significantly higher PN emissions were observed.10,11 It was hypothesized that this was primarily due to the very early fuel injection timing at these conditions that caused more liquid fuel impingement on the piston bowl. This may lead to liquid fuel that is not totally vaporized and well mixed with the intake air at the start of combustion. As a consequence, local fuel-rich combustion or even pool-fires could occur near the piston, generating high PN emissions. Figures 3 and 4 show the effect of SOI on PN emissions at 1000-3-cold conditions and 1500-13-hot conditions, respectively. For both conditions, PN emissions were dominated by accumulation mode particulates at most SOIs. For the 1000-3cold conditions (Figure 3), the production engine calibration SOI is 307 degrees before top dead center (dBTDC), referenced to the top of the combustion stroke. A greater than 80% reduction in total PN emissions was observed by retarding the SOI from 309 dBTDC to 249 dBTDC. The major difference in PN emission was observed for the accumulation mode particles. Advancing SOI not only increases total PN emissions but also shifts the peak accumulation mode particles to larger sizes. Increases in PN emission were observed if SOI 2018


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Figure 7. Impact of spark timing on PN emissions at 1000-3-cold.

Figure 8. Impact of spark timing on PN emissions at 1500-13-hot.

necessarily decrease linearly with increasing injection pressure. Rather, the rate of PN reduction appears to diminish with fuel injection pressure increases at this engine condition. At 150013-hot, a linear reduction of about 20% in total PN emissions was observed by increasing injection pressure from 48 to 88 bar. Spark Timing. Advancing spark timing reduces the fuel−air mixing time but increases the soot residence time in the combustion chamber. Advancing spark timing also increases the combustion temperature, which could have significant impacts on soot formation and oxidation during combustion. The final PN emission is a result of two competing factors: soot formation and soot oxidation. In the spark timing study, engine NMEP was held constant for each test condition by adjusting the air and fuel flow accordingly. Figures 7 and 8 show the results of spark timing sweeps at 1000-3-cold conditions and 1500-13-hot conditions, respectively. At 1000-3-cold, advancing spark timing tends to produce higher accumulation mode particles. The difference in total PN emission is relatively small; on average, about a 20% change in PN emissions was observed. At 1500-13-hot, the impact of spark timing on accumulation mode particles is very small; however, major differences were observed in nucleation mode particles. The highest nucleation mode particle emission was observed when spark timing was 11 dBTDC. Of the three spark

engine tends to produce large particles, which will also have significant impact on the mass portion of PM emissions. Fuel Injection Pressure. Fuel injection pressure is the other key parameter that affects fuel−air mixing. Higher injection pressure reduces droplet size, which achieves better fuel atomization.21 Higher injection pressure also increases fuel jet momentum, which improves fuel−air mixing. Note that increasing injection pressure does not necessarily increase fuel liquid penetration length, which means it does not increase the likelihood of liquid fuel impingement on the piston bowl.22 In diesel engines, increasing fuel injection pressure is a very efficient approach in reducing PM emissions.23 Similar to the SOI sweeps, fuel injection pressure sweeps were performed at 1000-3-cold conditions and 1500-13-hot conditions. SOIs at 269 dBTDC and 279 dBTDC were used for 1000-3-cold and 1500-13-hot, respectively. As seen in Figures 5 and 6, both conditions show a clear trend of total PN emissions reduction with increasing fuel injection pressure (the production engine calibration fuel injection pressures are also referenced in the figures). The injection pressure exhibits stronger effects on accumulation mode particles than nucleation mode particles at these speed-load conditions. At 1000-3-cold, about a 30% reduction in total PN emission was observed by increasing injection pressure from 23 to 33 bar. An additional 8% reduction in total PN emission was observed by increasing injection pressure to 43 bar; so, PN emissions do not 2019


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hot, however, PN emission was insensitive to the exhaust cam position. There are two major effects of retarding exhaust valve timing. First, it increases the expansion work due to the late exhaust valve opening. Late exhaust blow-down increases the time available for soot oxidation. Second, it increases the internal EGR quantity. At the 0 CAD exhaust cam phaser position, the exhaust valve is closed near TDC, which provides the lowest amount of internal EGR. As exhaust valve close timing is retarded toward the intake stroke, more internal EGR will flow back from the exhaust runner into the cylinder. Retarding the exhaust valve opening also improves fuel economy by increasing the expansion work and reducing pumping losses (due to higher internal EGR). Since the engine load (NMEP) was maintained constant during the exhaust cam sweep, less fuel was needed when retarding the exhaust cam phasing. This could be another reason for lower PN emissions. Note that changing the exhaust valve timing had significant impacts on NOx emissions due to the change in internal EGR, as shown in Figures 10b and 11b. At 1000-3-cold, an order of magnitude reduction in NOx emission was observed by retarding the exhaust cam phasing to 29 CAD. At 1500-13hot, a reduction of about 10% NOx emission was observed. Considering the relatively small impact of exhaust cam phasing on PN emission and large effect on NOx emission, these results suggest that exhaust cam phasing should be optimized to minimize NOx emissions, but not primarily for PN emission control. Intake Cam Phasing. Figure 12 shows the timing of the intake valve opening and closing. Similar to the exhaust cam phaser, the intake cam phaser allows a maximum of 45 CAD phase change in intake valve timing. In Figure 12, 0 CAD intake cam phasing represents the default intake cam park position, which corresponds to the latest (most retarded) intake valve open and close timing. Both intake valve open and close timings are advanced when the cam phasing changes from 0 to 45 CAD. Changing intake cam phasing complicates the gas exchange process during the intake stroke; two competing factors are at play. The first factor is the change in intake valve closing. In a gasoline engine, late intake valve closing (LIVC) can be used to reduce pumping losses. LIVC can also reduce compression temperature, which benefits NOx reduction.24 The second factor is the change in the intake valve opening, which can also impact pumping losses, internal EGR and NOx

timings conducted at 1500-13-hot, the difference in total PN emission is less than 20%. It should be noted that spark timing also has significant impacts on engine efficiency and other pollutant emissions (especially NOx). Considering the relatively small impact of spark timing on PN emission, these results suggest it should not be used for PN emission control. Exhaust Cam Phasing. Figure 9 shows the timing of exhaust valve opening and closing for this SCE. The “0” exhaust cam

Figure 9. Exhaust cam phasing and exhaust valve timing.

phasing represents the default (most advanced) exhaust cam phaser parked position. The exhaust cam phaser allows a maximum of 45 CAD phase change in exhaust valve timing. As the cam phasing changes from 0 to 45 CAD, the exhaust valve open and close events are retarded accordingly. Figures 10 and 11 show the impact of exhaust cam phasing on PN emission at 1000-3-cold conditions and 1500-13-hot conditions, respectively. When performing exhaust cam phasing sweeps, the engine load was maintained constant. At 1000-3cold, exhaust cam phasing has a significant impact on combustion stability due to the changes in internal exhaust gas recirculation (EGR). Therefore, spark timing was adjusted accordingly to maintain the same combustion phasing. At 10003-cold, about a 22% reduction in total PN emission was observed by retarding the cam phaser to 29 CAD. At 1500-13-

Figure 10. Impact of exhaust cam phasing on PN and NOx emissions at 1000-3-cold. 2020


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Figure 11. Impact of exhaust cam phasing on PN and NOx emissions at 1500-13-hot.

−341 degrees after top dead center ((dATDC), referenced to the top of the combustion stroke). With 0 CAD intake cam phasing, the intake and exhaust valves are in negative valve overlap. When the intake cam phasing was advanced by 15 CAD, lower PN emissions were observed. Advancing intake cam by an additional 15 CAD further increases internal EGR, which significantly retarded combustion phasing. Although spark timing was advanced to maintain combustion phasing, higher engine-out CO and THC emissions were observed with longer combustion duration, which reduced overall engine thermal efficiency. Both air and fuel must be increased to maintain the same engine load; consequently, this is probably the major cause of the slight increase in PN emissions when intake cam phasing is advanced to 30 CAD. At 1500-13-hot, intake cam position showed no significant impact on total PN emissions. At most particle size ranges, the differences are within the measurement uncertainty limits. Similar to exhaust cam position, intake cam phasing also has significant impacts on NOx emissions, as shown in Figures 13b and 14b. At 1000-3-cold, more than an 80% reduction in NOx emissions was observed by advancing the intake cam phasing by 30 CAD. At 1500-13-hot, the reduction is relatively small; about a 20% reduction was observed. Considering the relatively small impact of intake cam phasing on PN emission but significant impact on NOx emission, intake cam phasing should not be used as a primary means for PN control.

Figure 12. Intake cam phasing and intake valve timing.

emission. Due to these two competing factors, different trends of intake cam phasing on PN emissions were observed. Figures 13 and 14 show the impact of intake cam phasing on PN emissions at 1000-3-cold conditions and 1500-13-hot conditions, respectively. At 1000-3-cold, PN emissions first decrease with advancing intake valve timing, and then increase. At this operating condition, the exhaust valve close timing was

Figure 13. Impact of intake cam phasing on PN and NOx emissions at 1000-3-cold. 2021


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Figure 14. Impact of intake cam phasing on PN and NOx emissions at 1500-13-hot.

Figure 15. Impact of AFR on PN emissions at 1000-3-cold.

Figure 16. Impact of AFR on PN emissions at 1500-13-hot.

Air−Fuel Ratio. Under normal operation, gasoline engines run at stoichiometric condition to take the advantage of threeway catalysts for pollutant emission control.25 However, of necessity engines periodically operate fuel rich under some circumstances. For example, during cold start, fuel rich combustion is used to achieve better combustion stability. At high load, fuel-rich combustion is adopted to maximize engine power and to reduce combustion and exhaust gas temperatures

for piston and catalyst protection. In addition, it is difficult to precisely control combustion near stoichiometric during sharp vehicle accelerations due to the lag in air flow.26 Figures 15 and 16 show the sensitivity of PN emissions to AFR at 1000-3-cold conditions and 1500-13-hot conditions, respectively. It is not surprising to find higher PN emissions when the engine was running fuel rich. With E0, the stoichiometric AFR is 14.6. At 1000-3-cold, the PN emission 2022


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Figure 17. Summary of the impacts of engine operating parameters on PN emissions.

Figure 18. Comparison of total PN emission between cold engine and hot engine (SOI Sweep): (a) 1000-3, (b) 1500-6.

There are relatively few solid particles in sizes smaller than 23 nm.8 SOI is the only parameter that significantly impacts accumulation mode PN emissions. All other engine operating parameters change PN emissions by less than 20%. Cold Engine vs Hot Engine Conditions. Previous studies showed that a major amount of particulate mass and number is emitted in the early phase of the FTP when the engine is cold.14,27 In this work, comparison of cold and hot engine conditions on engine-out PN emission was conducted by sweeping SOI. The engine coolant and intake air temperatures were controlled at 30 °C to simulate engine operation during cold-start. For hot engine conditions, engine coolant temperature was maintained at 88 °C; other engine operating parameters are listed in Table 3. Figure 18 shows the total PN emissions from the cold engine and hot engine for E0 at 1000-3 and 1500-6. When performing cold and hot engine experiments, the spark timing was adjusted to maintain the same combustion phasing. It is not surprising that a hot engine produces less PN emission than a cold engine. Higher intake air temperature, fuel temperature, and piston surface temperature promote fuel vaporization and thus better fuel−air mixing. For cold engine conditions, sharp increases in total PN emission were observed when SOI was advanced from 270 dBTDC to 290 dBTDC; nearly an order of magnitude increase in PN emissions could be observed in the case of 10003-cold. However, for a hot engine, SOI can be advanced to 290

doubled when the AFR was reduced from near stoichiometric to 13.1. At 1500-13-hot, about a 20% increase in PN was observed when the AFR was reduced from near stoichiometric to 13.4. Summary of Engine Operating Parameter Effects. Figure 17 compares the sensitivities of six engine operating parameters (SOI, fuel injection pressure, spark timing, exhaust cam phasing, intake cam phasing, and AFR) on engine-out PN emissions. The engine operating parameters for the baseline case are partially optimized and thus exhibit the lowest PN emissions. All other cases show the maximum impact of each operating parameter on PN emissions compared to the baseline. Note that y-axis is presented in log scale for better evaluation of the increasing magnitude. At both 1000-3-cold and 1500-13-hot conditions, SOI is the dominant parameter that could significantly increase PN emissions. At low load, AFR increased PN emissions by a factor of 2. All other parameters exhibit relatively small impact with less than a 50% difference in total PN emissions in the ranges tested. The impact of engine operating parameters on PN emissions has the following order: SOI > AFR > intake cam phasing > injection pressure > exhaust cam phasing > spark timing. At high load, random variations in nucleation mode particles are observed. However, the total number of the nucleation mode particles is less than 20% of total PN emissions. Also note that the current PMP protocol includes only solid particles. 2023


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Figure 19. Comparison of PN size distribution between cold engine and hot engine conditions.

Figure 20. Effect of ethanol on PN emission at 1500-6 (SOI sweep).

Figure 21. Effect of ethanol on PN emission at high loads (SOI sweeps).

slight reduction in total PN emissions was observed when SOI was advanced beyond 310 dBTDC. Figure 19 shows the PN size distributions for two selected SOIs. For the SOI of 270 dBTDC, liquid impingement was avoided for both cold engine and hot engine conditions (Figure 19a). In this case, hot engine conditions reduce particles in all size ranges. Figure 19b shows the differences in the case of fuel liquid impingement. The difference in nucleation mode

dBTDC without causing significant increase in PN emission. This is because higher in-cylinder air and fuel temperatures reduce the fuel liquid length and thus avoid liquid fuel impingement on piston. For cold engine conditions with SOI before 250 dBTDC, PN emission increases monotonically with advancing SOI. For hot engine conditions, however, the highest PN emissions were observed when SOI was at 310 dBTDC. A 2024


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Figure 22. PN size distribution with single-injection and split-injection at 1000-3-cold and 1500-13-hot.

quasi-steady liquid length, which reduces liquid impingement on both piston bowl and cylinder wall. Split injection also helps to distribute the fuel more evenly during the intake stroke, which could achieve more homogeneous fuel air mixture before the start of the combustion. Figure 22 compares the PN size distribution between singleinjection and split-injection at 1000-3-cold and 1500-13-hot. Injector current signals are also presented in Figure 22. The split-injection strategy used two injection pulses of the same duration. The injection separation, defined as the time interval between the end of the first injection and the start of the second injection, is shown in the figure legend. For 1000-3-cold with single fuel injection, the SOI is 289 dBTDC. Based on the result shown in Figure 3, liquid fuel impingement occurred with this SOI. For split-injection, the first SOI is at 289 dBTDC with an injection separation of 8 ms. As shown in Figure 22, splitinjection reduces the PN to the same level as the singleinjection without liquid fuel impingement (SOI = 269 dBTDC). This means liquid impingement on the piston was avoided even though the first SOI is 289 dBTDC. This result confirmed the hypothesis that a short injection duration reduces the fuel liquid length. At 1500-13-hot, two injection separations were tested. For both cases, split-injection reduces particle emissions in all size ranges. Increasing split injection separation from 5 to 7 ms reduced accumulation mode particles with minor increases in nucleation mode particles. Although the data are not presented in this paper, increased injection separation tended to increase CO and HC emissions, which is similar to the phenomena observed with retarded single injection. With 7 ms of injection separation, the end of the second injection is near piston bottom dead center. Further retarding the second fuel injection is not recommended because of the dramatic increase in CO and HC emissions. The examples of split, multi-injection strategy explored in this work demonstrate the potential to reduce PN emissions. At low loads, split injection does not further reduce PN emission if liquid impingement is avoided. At high loads, however, split injection reduces total PN emission by more than 50% compared to the lowest PN emission achievable by singleinjection.

particles is small. However, cold engine conditions produce much higher accumulation mode particles, which means fuelrich combustion near the piston bowl tends to produce large particles. Ethanol Effects (E0 vs E20). The effect of ethanol on PN emissions for both cold and hot engine conditions at 1500-6 is shown in Figure 20. E20 produces less PN emission if SOI is later than 290 dBTDC (without liquid fuel impingement). For SOI earlier than 290 dBTDC (with liquid fuel impingement), E20 tends to produce higher PN emission than E0. This is attributed to two competing factors that influence the final PN emission: heat of vaporization and soot production chemistry. When liquid fuel impinges on the piston, ethanol’s high heat of vaporization (heat of vaporization = 924.2 kJ/kg [ethanol] vs 308 kJ/kg [isooctane]) slows the evaporation process, which leads to more heterogeneous fuel−air mixture near the piston or possibly even pool fires during the combustion process, thus producing higher PN emissions. In the absence of liquid fuel impingement, PN is produced during combustion of a nearly homogeneous fuel−air mixture. In this case, the fuel-bound oxygen in ethanol suppresses soot formation during homogeneous combustion, thus lowering PN emission. The effect of ethanol on PN emission at high engine loads (1500−13 and 2000−14) is shown in Figure 21. For both cases, E20 produces higher PN emission at all SOIs. This may be explained by the long fuel injection durations required for high loads, coupled with E20s higher heat of vaporization; that is, a long injection duration causes liquid fuel impingement on the piston and/or cylinder wall, while higher heat of vaporization inhibits fuel evaporation, producing more heterogeneous combustion. The transient liquid length produced by short fuel injection is shorter than the quasi-steady liquid length produced by long fuel injections. At low loads, injection duration is very short; therefore, fuel injection may end before reaching the quasi-steady period of liquid penetration. This reduces the possibility of liquid fuel impingement on the piston bowl and/or cylinder wall if the SOI is later than 290 dBTDC. However, at high load, fuel injection duration is significantly longer, leading to longer liquid length and thus liquid fuel impingement on the piston and/or cylinder wall. Minimizing PN Emissions by Split Multiple Fuel Injection. Based on the results above, it is clear that one possible approach to reducing PN emissions is to reduce the fuel liquid length, which can be achieved by using multiple fuel injections. By splitting the fuel injection into two or more pulses, fuel injection (per pulse) could end before reaching the

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SUMMARY AND CONCLUSIONS In this study, the impacts of engine operating parameters on wall-guided GDI engine-out PN emission were investigated 2025


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using a SCE. The study was conducted by independently sweeping SOI, spark timing fuel injection pressure, intake cam phasing, exhaust cam phasing, and fuel−air ratio. PN size distributions were collected from both hot engine and cold engine conditions. The impacts of ethanol blend at various engine loads were also investigated. The major findings are as follows: 1. SOI is the dominating factor impacting PN emissions, and liquid fuel impingement on the piston must be avoided to prevent high PN emission. 2. Fuel injection pressure is the other factor that significantly changes PN emissions. Higher injection pressure usually reduces PN emissions. At a given engine condition, a threshold rail pressure exists. Further increases in rail pressure beyond the threshold do not significantly change PN emission. 3. PN emissions at high loads are insensitive to spark timing, intake cam phasing, and exhaust cam phasing. Although a 50% reduction in PN emissions could be achieved by optimizing these operating parameters, their use to control PN emission is not recommended because of their significant negative impact on engine efficiency and other pollutant emissions. 4. Ethanol has a mixed impact on PN emission depending on engine operating conditions and SOI. At low and medium loads, E20 usually reduces PN emissions in absence of liquid fuel impingement. However, E20 can produce higher PN emissions than gasoline if liquid impingement on the piston or cylinder wall occurs. This is because the heat of vaporization of ethanol is about three times higher than that of gasoline, requiring much more heat energy to evaporate it from a surface. Also, the boiling point of ethanol is only 78 °C, lower than most other components in gasoline, and its vaporization slows the vaporization of other gasoline components. This increases the possibility of locally fuel-rich combustion near the piston surface, which may explain higher PN. 5. At high engine loads, E20 usually produces higher PN emissions than E0. This may be explained by a long fuel injection duration causing liquid fuel impingement on the piston and/or cylinder wall, coupled with inhibition of fuel evaporation from ethanol. 6. A cold engine can produce an order of magnitude higher PN emissions than a hot engine. However, the difference in PN emissions between cold and warm engine conditions can be reduced by optimizing engine operating parameters to avoid liquid fuel impingement. 7. Fuel-rich combustion should be minimized or avoided to prevent high PN emissions. 8. At high loads, a multiple-injection strategy demonstrated an additional 50% reduction in PN emissions compared to the lowest PN emissions achievable using singleinjection. Considering the impacts of operating parameters on engine performance and other regulated emissions, the authors recommended the following strategy to minimize PN emissions for this type of wall-guided GDI engine. 1. SOI should be later than 270 dBTDC to avoid liquid fuel impingement at any circumstance. At high engine loads, multiple-injection strategy is recommended.

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2. Fuel injection pressure should be set near the maximum if that does not cause a significant fuel economy penalty or pump durability concerns. 3. Although spark timing, exhaust cam phasing, and intake cam phasing exhibited some impacts on PN emissions, they have more significant impacts on engine efficiency and other pollutant emissions such as NOx. Thus, it is not recommended to employ these parameters for PN control. 4. During a cold start, better fuel−air ratio control strategy should be adopted to reach near-ideal λ as soon as possible. Fuel-rich combustion should be avoided at high loads, if possible.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank Kevin Stork of the U.S. Department of Energy for his support of this research through the Vehicle Technologies Program.

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REFERENCES

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