The Effect of Operating Parameters on Soot

Downloaded from SAE International by Rolf Reitz, Wednesday, March 04, 2015 2015-01-1071 Published 04/14/2015 Copyright © 2015 SAE International doi:10.4271/2015-01-1071 saeeng.saejournals.org

The Effect of Operating Parameters on Soot Emissions in GDI Engines Qi Jiao and Rolf D. Reitz Univ. of Wisconsin

ABSTRACT Due to the upcoming regulations for particulate matter (PM) emissions from GDI engines, a computational fluid dynamic (CFD) modeling study to predict soot emissions (both mass and solid particle number) from gasoline direct injection (GDI) engines was undertaken to provide insights on how and why soot emissions are formed from GDI engines. In this way, better methods may be developed to control or reduce PM emissions from GDI engines. In this paper, the influence of engine operating parameters was examined for a side-mounted fuel injector configuration in a direct-injection spark-ignition (DISI) engine. The present models are able to reasonably predict the influences of the variables of interest compared to available experimental data or literature. For a late injection strategy, effects of the fuel composition, and spray cone angle were investigated with a single-hole injector. For an early injection strategy, the effects of multi-component fuel surrogates for gasoline, SOI timings and wall temperatures were studied with a six-hole injector. The investigations confirmed the necessity to consider the multi-component fuel composition and also demonstrate how and why wall films significantly contribute to soot emissions from DISI engines.

CITATION: Jiao, Q. and Reitz, R., "The Effect of Operating Parameters on Soot Emissions in GDI Engines," SAE Int. J. Engines 8(3):2015, doi:10.4271/2015-01-1071.

INTRODUCTION The percentage of gasoline vehicles with DISI engines is expected to continue increasing significantly with a projection of 60% of all new vehicles by 2016 in the United States[1]. However, more stringent regulations for DISI engines have also been proposed recently, and the number-based regulation has become a new challenge for DISI engines, which historically only needed to meet a mass-based regulation. Therefore, DISI engines are still under scrutiny because of the particulate or smoke emissions they can produce. Particulate matter emission from internal combustion engines usually can be divided into solid (ash, carbon) and volatile components (liquid phase). The solid particles mainly consist of agglomerated carbonaceous primary particles, which are usually described as soot, and are usually formed during combustion in locally rich regions. Both organic compounds (i.e., unburnt hydrocarbons and oxygenated hydrocarbons) and inorganic compounds (i.e., sulfur dioxide, nitrogen dioxide, and sulfuric acid) can be absorbed upon those solid particles [2, 3, 4, 5]. A small fraction of the fuel and some atomized and evaporated lube oil can escape oxidation and also appear as volatiles in the exhaust. The formation of volatile nanoparticles strongly depends on the dilution rate, temperature, the cooling process of the exhaust, residence times, surface area of pre-existing particles, and humidity [6, 7, 8]. The composition of the exhaust particles also strongly depends upon where and how they are collected. For instance, nucleation, condensation and adsorption transform volatile components to solid and liquid PM as the exhaust is diluted and cooled, while most of the volatile materials are in the gas phase in the tailpipe, where temperatures are high enough.

Therefore, only the carbonaceous portion of engine particulate emissions is modeled in this work (i. e., smoke, dry particulate, or elemental carbon (EC)). For gasoline fuel the light-end components tend to play significant roles in the spray, ignition and combustion processes, while the heavy-end components vaporize later, and exert important roles in soot emissions, especially in the presence of wall films [9, 10, 11]. The aromatic structure of toluene addition was likely to be responsible for higher soot emissions in Ref.[12]. The influence of multi-component fuel surrogates for both fuel physical properties and chemistry reactions will be investigated in this work. For a prototype DI engine with a side-mounted injector, a strong correlation between smoke emissions and spray cone angle has been observed that has been directly attributed to variations in the fuel film thickness on the piston[13]. Similar experimental observations have also been presented from parametric studies of the effect of spray cone angle on soot emissions for a GDI combustion system with a side-mounted injector[14]. The smoke amount reduces with increased the spray cone angle due to less spray impingement on the piston cavity. In a DISI optical engine with a stratified-charge combustion system, the liquid fuel is concentrated inside the bowl in the region opposite the injector side, and carbon deposits have been observed by Han et al.[15] to build up inside the piston bowl due to the rich mixture burning near the liquid fuel film regions. A decrease of wall films was predicted with a wider spray cone angle, in agreement with the decreased smoke in the experiments.

Downloaded from SAE International by Rolf Reitz, Wednesday, March 04, 2015

Jiao et al / SAE Int. J. Engines / Volume 8, Issue 3 (June 2015) Injection timing is also known to be important since it influences the amount of fuel-wall impingement, and optimization of injection timing is necessary to avoid spray over-penetration and the associated unintended wetting of chamber surfaces [11, 16, 17, 18, 19, 20]. The influence of injection timing on fuel wall wetting for a combustion system with an intake-side-mounted injector was also seen in CFD modeling works [21, 22]. Injection timing is also a key factor for soot emissions. Yang et al.[23] reported that retarding the injection timing led to more stable combustion for an overall lean air/fuel mixture due to enhanced local charge stratification. However, extreme retardation of the injection timing resulted in a significant increase of smoke emissions, indicated by a sharp increase of the AVL filter smoke number (FSN). Moreover, the injection timing is also influenced by the in-cylinder pressure, which influences spray atomization. A correlation of the measured smoke emissions with the amount of calculated fuel wall wetting and the wall film thickness is also observed in CFD modeling for a range of injection timings in engine tests[21]. The correlation suggests that a small amount of fuel impinging on the piston crown can produce a considerable amount of smoke if the resultant wall film is relatively thick. The temperature of wall surfaces is also found to have a significant effect on film vaporization. Generally, slow wall film vaporization is observed for a cold wall, leading to degraded spatial uniformity of the fuel-air mixture, while enhanced wall film vaporization exhibits when the surface temperature ranges from 90 °C to 130 °C. Recent experiments[24] report that a seemingly small change of wall temperature causes a significant change of heat flux, leading to the changes in the fuel depositing on the surfaces. The objective of this work was to investigate the influence of fuel composition, spray cone angle, injection timing, and wall temperature for early- and late-injection strategies on soot emissions in a DISI engine using a newly formulated version of the KIVA CFD code[25], which can predict in-cylinder distributions of spray, wall films, spark-ignition and soot emissions (both mass and solid particle number) to provide insight on how and why soot emissions are formed from GDI engines. In this way, better methods may be developed to control or reduce PM emissions from GDI engines

CFD MODEL The multi-dimensional KIVA3V Release 2 code, coupled with a list of improved submodels (see Table 1), was used in this work. The model was formulated to predict soot emissions (mass and solid particle number) using multi-component fuel surrogates to represent gasoline in full-cylinder DISI engines. The performance of the models in predicting particle size distributions is investigated for an SI engine at premixed operating conditions[26].

Table 1. Submodels in KIVA.

MODEL APPLICATION Geometry and Operating Conditions of a DISI Engine The engine's geometric specifications are given in Table 2. The computational mesh contains around 79,000 cells, including the intake and exhaust manifolds and the cylinder, as shown in Figure 1. The computations were usually started near intake TDC and terminated at 80 ° after compression TDC. Therefore, intake, compression, fuel injection, spark ignition and combustion processes were simulated. Table 2. DISI Engine Specifications.


Jiao et al / SAE Int. J. Engines / Volume 8, Issue 3 (June 2015) In the following sections the influence of fuel compositions, spray cone angle will be investigated using the late injection strategy, while the influence of wall temperature and start of injection (SOI) timing were studied for early injection operating conditions. Table 3 illustrates the operating conditions for late injection study, and the early injection operating conditions are presented in Table 4.

Fuel Modeling As introduced in Table 1, the current CFD code uses a discrete multi-component approach to model the physical properties of the fuel components. This approach has been successfully applied to diesel and gasoline engines and represents great advantages in modeling the characteristics of real transportation fuels [30, 42, 43, 44, 45]. Figure 1. Computational mesh of the spark-ignition direct-injection engine with pent-roof and bowl in piston, 79,000 cells at BDC including intake and exhaust valves. Table 3. DISI engine specification of late injection strategy.

Table 4. DISI engine specifications of early injection strategy

The optimum physical compositions for real transportation fuels were obtained by a novel methodology based on local optimization and sensitivity analysis in terms of matching the distillation curve, mass fraction of family groups (saturates, aromatics and olefins), hydrogen to carbon (H/C) ratio, Cetane Number (CN) and the Research Octane Number (RON)[44]. The surrogates are introduced as ‘fuel physical surrogates’. However, reaction chemistry mechanisms are not available for all the individual components proposed in modeling the fuel physical surrogates. Therefore the group chemistry representation (GCR) approach proposed by Ra and Reitz[42] was utilized to represent the fuels chemical reactions. These surrogates are ‘fuel chemical surrogates’, which are used to model fuel oxidation processes. The fuel physical surrogates are grouped based on their chemical classes and the combustion characteristics of each group are represented by representative chemical surrogates, which do have available fuel chemistry mechanisms. In this work, the chemical classes were classified into groups based on their molecular structure: viz., n-paraffins, iso-paraffins and aromatics. They are therefore represented using available chemical mechanisms for n-heptane, iso-octane and toluene, respectively, as suggested by Ra and Reitz[42]. Table 5 presents the proposed fuel physical surrogates and the corresponding fuel chemical surrogates for 91 RON gasoline. The fuel physical surrogates were found to represent experimental fuel distillation curve reasonably well, and it is of great importance to model the boiling range of the fuel to be able to predict its vaporization characteristics for engine applications. The simulated distillation curve of the fuel surrogates was compared with the experimental distillation curve of 91 RON gasoline, as shown in Figure 2, although minor discrepancies still exist for the heavy-end and light-end of the distillation curve, the current 10-component fuel surrogate was considered to be satisfactory for the present study.


Jiao et al / SAE Int. J. Engines / Volume 8, Issue 3 (June 2015) Table 5. Experiment and modeling surrogates for 91 RON gasoline [41].

temperatures (i.e., toluene and m-cymene), respectively, which are correspondingly used as the fuel physical surrogates, while toluene is used as the chemical surrogate for both toluene and m-cymene, as shown in Table 5. The predicted mass of fuel film on the different wall surfaces is shown in Figure 3 for the three cases at the start of ignition and at 80 °ATDC, respectively. As can be seen, the mass of total wall films from case 3 is approximately 3 times from case 1, and the film mass from case 2 is almost twice that of case 1. This is mainly due to the different boiling temperatures of iso-octane, toluene and m-cymene, as seen in Table 5. By 80 °ATDC, no films are left for case 1, while the highest film mass is observed for case 3. Table 6. Fuel compositions investigated in late-injection DISI engine.

Figure 2. Measured and predicted distillation curves of a gasoline fuel (91RON). Experimental distillation curves from Ref.[41].

Fuel Composition The fuel composition investigated is represented in Table 6, where case 1 used only iso-octane as the fuel surrogate, as has been done in earlier studies [25, 46]. For cases 2 and 3, 30% of the iso-octane mass fraction is substituted by two aromatics with different boiling


Jiao et al / SAE Int. J. Engines / Volume 8, Issue 3 (June 2015) This study of fuel composition effects shows that the present models are able to capture the expected influence of light and heavy components on wall films and soot emissions, and confirm the importance of considering fuel surrogates when modeling gasoline fuel. In summary, the heavy components in the fuel tend to be left as wall films before the start of ignition and then dramatically contribute to soot emissions during the combustion process. Wall films can still be observed at the end of the expansion stroke for mixtures including heavy components, which is likely to further contribute to soot emissions in the next cycle depending on the engine operating conditions.

Spray Cone Angle The influence of spray cone angle on soot emissions was also investigated, and the spray cone angles of interest are represented in Table 7. The predicted fuel film mass on the different combustion chambers walls are illustrated at the start of ignition in Figure 5. The wall film mass decreases with the increase of spray cone angle, and case 1 gives the highest total film mass, including nearly 1/3 of the film mass on cylinder head, while no wall film is left for case 5 at the start of ignition. The film mass at 80 °ATDC is not shown because it is completely vaporized during the combustion process for all cases. Correspondingly, in Figure 6, no soot is observed for case 5, while the highest soot amount is produced in case 1, probably because of the locally rich regions near the wall films. To summarize, soot emissions tend to be reduced with a wider spray cone angle and the predicted trend agrees with findings from literature [13, 14, 47], suggesting that the present models are able to reasonably represent the influence of spray cone angle on soot emissions. Figure 3. Predicted fuel film mass on different wall surfaces (blue: cylinder head, red: cylinder liner, black: piston) for the three cases. (a) at the start of ignition (b) 80 °ATDC.

Table 7. Spray cone angles investigated in the late injection DISI engine.

Figure 4. Soot mass history from the three fuel compositions.

The corresponding soot mass histories for the three cases are represented in Figure 4. As is expected, the least soot is predicted for the pure iso-octane case, while the soot predicted from the binary fuel mixtures including aromatics is almost 4 times that from iso-octane. Interestingly, almost identical total soot is observed for cases 3 and 4, probably due to the fact that the same fuel chemical surrogates are used, though the different fuel physical surrogates do lead to slight difference in vaporization and spray breakup.

Figure 5. Fuel film mass at different walls for various spray cone angles at the start of ignition.


Jiao et al / SAE Int. J. Engines / Volume 8, Issue 3 (June 2015) approximately 40 °ATDC. The solid lines present the present simulated pressure traces for three cases with sweeps of SOI timings. (Faster pressure rise rates are observed compared to experiments due to the higher in-cylinder temperatures resulting from the higher compression ratio of the modeled engine in the simulations, as indicated in Table 4). Comparisons of CA50 from the experiments and predictions are also summarized in Table 8, and only a quantitative attempt was made to model the CA50 with retard of SOI timings. Table 8. CA50 for sweeps of SOI timings in measurements[41] and simulations.

Figure 6. Soot histories for various spray cone angles.

Start of Injection Timing An early injection strategy was also investigated to study the influence of injection timing on soot emissions using the present improved models with multi-component fuel surrogates for gasoline fuel (Table 5). The simulations were also compared qualitatively with experiments in a lower compression ratio engine, as described in Table 4.

Figure 8. Predicted soot mass at the exhaust valve open (EVO) and the measured FSN for a sweep of SOI timings.

Figure 7. Pressure traces for cases with different injection timings. Solid line: simulation (CR=12). Dotted line: similar CA50 timing experiments (CR=10) [41].

The experiments were conducted on a similar engine with a lower compression ratio by VanDerWege et al.[41]. The ignition timings were kept near TDC (± 2 °CA) and gave decreasing soot with retard of SOI timing (SOI = −330, −320, −290 °ATDC) while maintaining similar pressure traces and CA50. In the simulations, similar conditions were investigated while maintaining similar pressure traces and CA50 for the same sweeps of SOI timings, as presented in Figure 7. Two representative measured pressure traces are shown by the dashed lines for reference, and their peak pressures occur at

In spite of the differences between the modeled and the experimental engines, the general trend of decreasing soot emissions with retarding SOI timing (FSN in experiments from Table 4, similar trend also reported in Ref.[48]) was successfully captured by the model, as seen in Figure 8. However, quantitatively, the exhaust soot was underpredicted compared to the soot from the measured FSN. Differences in soot emissions between the experiments and simulations reflect several factors, namely: (1) the different bore and stroke lead to different spray/wall impingement and fuel splash, and the resultant fuel/air mixing at the same SOI timings and injection pressures; (2) the different engine size influences the time for the flame surface to reach wall films to form soot; (3) the predicted particulate does not account for the presence of volatile compounds; (4) the boiling temperature of the heavy-end of gasoline surrogate components is under-predicted compared to that of the gasoline used in experiment. In spite of these differences, it is interesting that the measured trend is still recovered in the predictions.


Jiao et al / SAE Int. J. Engines / Volume 8, Issue 3 (June 2015)

Figure 9. Distributions of in-cylinder spray parcels at different crank angles for varied start of injection (SOI) timings. Red dots: spray parcels.

(a). At the start of ignition

(b). At 80° ATDC

Figure 10. Fuel film mass at different surfaces: cylinder head, cylinder wall, and piston.

Figure 9 illustrates the in-cylinder distributions of the spray at CA = 10 °CA after SOI, 20 °CA after SOI, −10 °ATDC for the three SOI timings. By 20 °CA after SOI = −330 °ATDC, a large amount of fuel impinges on the piston bowl and cylinder wall on the opposite side of the fuel injector. Part of fuel hitting the piston bowl stays on the piston bowl, while the rest splashes, and some becomes airborne spray and hits the cylinder head again, forming wall film. Fuel hitting the cylinder wall also contributes to the cylinder head wall film if

splash occurs. Thus, wall films are observed on cylinder head, cylinder liner and the piston. Later at −10 °ATDC for the case with SOI = −330 °ATDC, films that were part of the cylinder wall film are scraped up and assigned to the piston surface in the code. Thus, an exchange of film mass between the piston and cylinder wall occurs, and fuel film mass on both the cylinder wall and piston surfaces is summed in order to give an overall view of where the fuel films are located.



Figure 11. Contour plots for several important parameters for soot at 80 0ATDC in a cut-plane for three SOI timings. Red dots: spray parcels.

For SOI = −320 °ATDC, approximately 2/3 of the total fuel targets the piston bowl, similar to the SOI = −330 °ATDC cases, during the fuel injection process. However, less fuel splashes towards the cylinder head because of the longer distance for the spray to travel before hitting the piston surface. No fuel films are seen on the cylinder head at −10 °ATDC, leaving more film on the piston bowl compared to the SOI = −330 °ATDC case. Interestingly, for SOI = −290 °ATDC, nearly 2/3 of the total fuel targets the cylinder wall as the piston moves downward and no films are splashed back to the cylinder head either. Less splash is expected for fuel parcels hitting the cylinder wall compared to those hitting the piston bowl due to the

longer travel distance. Therefore, slightly lower film mass is observed for the SOI = −290 °ATDC case compared to the SOI = −320 °ATDC case before combustion (Figure 10 (a)). For all cases, preferential vaporization of light-end components in the fuel surrogates greatly influences the vapor distribution near the spray. Fuel vaporization and fuel/air mixture preparation is also influenced by the different levels of fuel spray impingement, especially at low engine speeds and wall temperatures. The heavyend components with low volatility vaporize later and contribute to emissions, as discussed next. Also it is important to note that there is


Jiao et al / SAE Int. J. Engines / Volume 8, Issue 3 (June 2015) a difference of approximately 25 K in the boiling temperature of the heavy-end components between the proposed fuel surrogates and real 91 RON gasoline, as shown in Figure 2. As shown in Figure 10 (a) at the time of spark ignition, wall films on the head are observed only for the earliest SOI, and account for approximately 1/3 of the total film mass. The total film mass keeps decreasing with retard of SOI timing, in agreement with the decreasing engine smoke trend. Thus, it is observed that the amount of remaining liquid fuel adhering on all surfaces at the time of spark correlates with the engine-out smoke level, similar to observations from Han et al.[15]. Most importantly, fuel films are left at 80 °ATDC, as shown in Figure 10 (b), mainly because of the low volatility components in the fuel films, in agreement with results of Drake et al.[11] regarding studies of film vaporization with multicomponent and single component fuel surrogates. Also, for cases with SOI = −330 and −320 °ATDC, the total film mass left at 80 °ATDC accounts for almost half of the film mass at the start of ignition. For the case with SOI = −290 °ATDC, almost 2/3 of the film mass is left. In-cylinder temperature (K), spray parcels, mass fractions of total fuel vapor (-), OH (-), soot (-), soot particle size (nm) and soot particle number density (particles/cm3) are presented at 80 °ATDC for the three SOI timings in Figure 11. Generally, the highest soot tends to be located near the wall films with their enhanced vaporization and continuously enhanced nucleation and surface growth processes (caused by high mass fractions of A4 and C2H2 in the fuel rich regions near wall films) and low oxidation rates (low mass fraction of OH caused by low temperature near wall films). The combination of strong soot formation and weak soot oxidation near wall films finally leads to high levels of soot by 80 °ATDC for all cases. Relatively lower amounts of soot are observed near the wall films on the piston bowl mainly due to lower nucleation and surface growth and strong oxidation caused by high OH concentrations, and the longer residence time for the oxidation process in the center of the cylinder. Overall, the majority of the soot forms near the wall films, and wall films near the piston bowl produce relatively less soot emissions. Particles of large size and large number density tend to reside within the high soot regions. The wall films have been shown to be important sources of soot emission. However, the distributions and vaporization of the wall films are complex processes, which can be influenced by the physical properties of the fuel surrogates, as well as the in-cylinder conditions and the injection strategies. For the present investigations of three different SOI timings, the films form distinctly due to the different locations of spray impingement. As discussed next, the combustion chamber wall temperature was found to have a significant influence on the fuel vaporization. In addition to airborne fuel/air mixing and fuel vapor from wall films, soot oxidation processes also play important roles in determining soot emissions. Generally, strong oxidizers (i.e., OH) are located in high temperature regions and low OH is seen within burnt regions near wall films due to the local cooling process from vaporizing wall films. It is also worth noting that in-cylinder fuel/air mixing caused by fuel splash after wall

impingement plays an important role. This again highlights the necessity to further validate the spray/wall interaction models.

Wall Temperature The early injection strategy of SOI = − 330 °ATDC discussed earlier was also used for investigating the influence of wall temperature on wall films and soot emissions while maintaining the similar CA50 by slightly changing the ignition timing mainly because of the distinct in-cylinder fuel/air mixture distributions. As shown in Table 9, the cylinder walls from case 6 were set to be colder than those of case 7. The almost identical pressure traces of cases 6 and 7 were presented in Figure 12. However, the predicted EVO soot emissions from case 6 (low wall temperature) can be 2 orders of magnitude higher than that from case 7 (high wall temperature). Figure 14 presents the wall film mass left on the different walls at the start of ignition and at 80 °ATDC, respectively. As expected, the higher wall temperature of case 7 leads to less wall film left before the start of ignition. Interestingly, the wall film mass from case 7 was also slightly reduced during combustion compared to case 6, as shown by the amount of wall film at 80 °ATDC in Figure 14 (b). Most importantly, the trend of wall film left on wall surfaces at the start of ignition agreed with the trend of soot predicted at EVO as well. Therefore, the present investigation of the influence of a slight change of wall temperature showed an important impact on wall films and soot emissions, indicating that uncertainties of wall temperature should also be considered in modeling wall film and soot emissions from DISI engines in the absence of available wall temperature measurements from experiments. Table 9. Varied wall temperature in early injection DISI engine.

Figure 12. Pressure traces for cases 6 and 7.



SUMMARY AND CONCLUSIONS A CFD code with improved submodels was applied to study soot emission processes in DISI engines. A stratified-charge, lean-burn, single-hole, late-injection single-cylinder engine operating in direct-injection mode was modeled. It was found that no wall films were left late in the expansion stroke when a single component fuel surrogate (iso-octane) was used, as in conventional models. The inclusion of multi-component fuels in the CFD models in this work successfully correlated soot formation with wall film fuel, which has also been observed in experiments [11, 49]. The present models were also able to reasonably predict the influence fuel composition and spray cone angles on soot emissions.

Figure 13. The predicted soot mass at EVO for cases 6 and 7.

Figure 14. Fuel film mass at different surfaces for two groups of wall temperature. (a) at the start of ignition. (b) 80 °ATDC.

The study also investigated a 6-hole nozzle with an early-injection strategy to study the influence of injection timings and wall temperatures on soot emissions. The simulations used a 10-component fuel surrogate to represent 91 RON gasoline fuel. Gasoline fuel films were found to remain on the combustion chamber walls late in the expansion stroke due to low volatility components in the fuel, confirming the importance of considering multi-component fuel surrogates for gasoline. The findings also agree with observations from Refs.[9, 10, 50] that light fuel components, which are likely to vaporize more readily, influence ignition and combustion, while the heavy components tend to play significant roles in emissions, especially in the presence of wall films. The models predicted a decrease of soot with retard of SOI timing while maintaining similar pressure traces and CA50. This trend is consistent with experimental results on a similar engine by VanDerWege et al.[41]. Also, it was observed that the amount of remaining liquid fuel adhering on all wall surfaces (cylinder head, cylinder wall, piston surface) at the time of spark correlates with the engine-out smoke level. The distribution of wall films was found to be related to the SOI timing since different injection strategies produce different vaporization and combustion histories. The engine-out soot was also influenced by the local temperatures, residence times in the burnt regions, film distributions, and the film mass and film thickness. In addition to fuel/air mixing and fuel vapor from wall films, soot oxidation processes were also found to play important roles in soot emissions. Strong oxidizer (i.e., OH) concentrations are favored with high in-cylinder temperatures. However, OH concentrations are low near wall films due to local low temperatures caused by wall film vaporization. Therefore, the environment near wall films is quite suitable for soot formation and the formation of a large number of soot particles, especially when the wall films reside in burnt regions of the combustion chamber. Importantly, the wall temperature also plays an important role in wall film and soot emissions, and the soot emissions correlated with the amount of wall film mass remaining at the start of ignition. This also suggests the necessity to use accurate wall temperatures in order to predict wall film and soot emissions. In other words, uncertainties of wall temperature in simulations should also be considered in assessing wall film and soot emission predictions from DISI engines.



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ACKNOWLEDGMENTS Drs. Brad VanDerWege, James Yi of Ford Motor Company are greatly acknowledged for their support and discussions of this project. The authors are also thankful for the support from CEI, Inc. in the in-cylinder visualization made possible by Ensight Software.

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