Reduce Soot Emissions Research Articles

133 ディーゼル機関における燃料消費率及び煤排出低減に関する研究

133 ディーゼル機関における燃料消費率及び煤排出低減に関する研究

Regulated and unregulated emissions from a compression ignition engine under low temperature combustion fuelled with gasoline and n-butanol/gasoline blends

In this paper, the study on the combined effects of EGR and n-butanol additive on the regulated and unregulated emissions from a CI engine has been conducted at a load of 0.9MPa IMEP with the speed of 1500rpm. Mixtures of 0%, 10%, 20% and 30% by mass fraction of n-butanol with gasoline fuel were evaluated, referred as B0, B10, B20 and B30. The results show that n-butanol additive can effectively reduce soot emissions, while the EGR rate that corresponds to the peak value of soot is also decreased. With lower EGR, CO and HC emissions are low and not affected by fuel properties; in the LTC region with higher EGR, n-butanol additive improves the oxidation of CO and HC emissions slightly, whereas still has little impact on NOX emissions. In the unregulated emission respects, NO proportion in total NOX decreases while NO2 shows an opposite trend as EGR increases, and NO2 proportion reaches up to 25% with EGR of 45%. n-Butanol additive has almost no effect on the proportions of NO and NO2, which indicates that the local equivalence ratio plays a more important role in influencing NO and NO2 proportions compared with the global oxygen concentration. Under LTC condition, the primary individual hydrocarbons include C2H4, C3H6, NC5, IC5 and AHC, and their proportions in total HC increase with higher n-butanol fractions except AHC’s. CH4 has not been a major issue yet in Gasoline LTC due to less EGR used, while AHC proportions decrease with the increase of n-butanol fraction. HCHO has a good linear correlation with total exhaust HC, and the ratio of MeCHO to total HC exhibits decrease first and then increase with the increasing EGR, while the formation of both HCHO and MeCHO is promoted by the addition of n-butanol in the blends.

Oxygen Extended Sooting Index of FAME Blends with Aviation Kerosene

The use of biofuels in the aviation sector has economic and environmental benefits. Among the options for the production of renewable jet fuels, hydroprocessed esters and fatty acids (HEFA) have received predominant attention in comparison with fatty acid methyl esters (FAME), which are not approved as additives for jet fuels. However, the presence of oxygen in methyl esters tends to reduce soot emissions and therefore particulate matter emissions. This sooting tendency is quantified in this work with an oxygen-extended sooting index, based on smoke point measurements. Results have shown considerable reduction in the sooting tendency for all biokerosenes (produced by transesterification and eventually distillation) with respect to fossil kerosenes. Among the tested biokerosenes, that made from palm kernel oil was the most effective one, and nondistilled methyl esters (from camelina and linseed oils) showed lower effectiveness than distilled biokerosenes to reduce the sooting tendency. These results may co...

Ru-doped ceria–zirconia mixed oxides catalyze soot combustion

We used ceria-based catalysts to reduce the soot combustion temperature from 600 °C to values included in the operation range of diesel exhausts (270–400 °C). We synthesized CeO2–ZrO2 by the sol–gel method and effectively deposited the mixed oxide on cordierite. We confirmed the structural and morphological properties of the material and the formation of mixed oxides by N2 adsorption/desorption isotherms, temperature-programmed reduction, oxygen storage capacity, and scanning and transmission electron microscopies. We investigated how the addition of Ru nanoparticles (0.5 wt%) affected the catalytic activity of Ce0.4Zr0.6O2 in terms of soot oxidation. Thermal analyses of soot oxidation in the presence of the catalyst revealed that the soot combustion temperature decreased 250 °C on average. This result indicates that oxygen species originated at low temperatures, accounting for high reactivity toward soot combustion. We assessed the catalytic activity of the fresh and recycled supported catalysts in a stationary engine motor and conducted a comparative analysis of the final soot impregnated on paper filters by diffuse reflectance spectroscopy. We were able to use the catalyst in several cycles (three times of 40 min); we did not detect activity loss after the recycling, and the material retained its structural properties. Our data demonstrated that we can reuse the catalyst to efficiently oxidize soot, as attested by the thermal analysis. Moreover, the use of this catalyst reduced soot emissions by more than 84 % according to dynamic tests in a stationary engine.

Effects of altitude and fuel oxygen content on the performance of a high pressure common rail diesel engine

The change of intake oxygen content caused by altitude variation and the change of fuel oxygen content both affect the performance of diesel engines. In this paper, comparative experiments were performed on a high pressure common rail diesel engine fueled with pure diesel and biodiesel–ethanol–diesel (abbreviated as BED) blends with oxygen content of 2%, 2.5%, and 3.2% in mass percentage at different atmospheric pressures of 81kPa, 90kPa, and 100kPa. Moreover, in order to study the effect of different fuel blends with the same oxygen content on the performance of the diesel engine, tests were conducted on the diesel engine fueled with the BED blend and a biodiesel–diesel (abbreviated as BD) blend at 81kPa ambient pressure.The experimental results indicate that the influence of altitude variation on the full-load engine brake torque is not significant when the pure diesel fuel is used. With the increase of BED fuel oxygen content, the engine brake torque reduces. When the pure diesel fuel is used, with the increase of atmospheric pressure, the brake specific fuel consumption (BSFC) decreases. As the fuel oxygen content increases, there is no significant difference in brake specific fuel consumption of the BED blends. And the values of brake specific energy consumption (BSEC) gradually decrease. Soot emissions of the diesel engine decrease with the increase of atmospheric pressure and fuel oxygen content. The effect of soot emission reduction by increasing the oxygen content of the fuel is more significant than the effect of increasing atmospheric pressure. The effects of BD and BED fuels with basically the same oxygen content on the full-load performance, fuel economy, and soot emissions of the diesel engine are different. The BSFC and soot emissions of the BED fuel are lower than those of the BD fuel.

Study on the Mechanism of Soot Reduction by Multi-Injection Coupled with EGR

The effects of multiple injections coupled with medium EGR (25% - 30%) on heavy duty diesel engine were investigated. Injection timing and mass were adjusted with different injection strategies (main-post and pilot-main) to study the influence of these parameters on combustion and emissions. The mechanism of soot emission reduction was discussed. Results indicate that, at fixed total injection quantity and EGR rate, NOx is reduced, while soot is decreased followed by an increasing with increasing post injection quantity; NOx nearly kept constant and soot declined before rising with lager main-post interval. Optimum post injection could accelerate soot oxidation rate. Pilot injection has no positive impact on NOx, while soot decreases with less pilot fuel mass and lager pilot-main interval. Optimum pilot injection could be beneficial for a better mixture property. The acceleration of soot oxidation rate is the basic reason of soot emission reducing by multiple injections.

Effects of exhaust gas recirculation and load on soot in a heavy-duty optical diesel engine with close-coupled post injections for high-efficiency combustion phasing

In-cylinder strategies to reduce soot emissions have demonstrated the potential to lessen the burden on, and likely the size and cost of, exhaust aftertreatment systems for diesel engines. One in-cylinder strategy for soot abatement is the use of close-coupled post injections. These short injections closely following the end of the main injection can alter soot-formation and/or oxidation characteristics enough to significantly reduce engine-out soot. Despite the large body of literature on post injections for soot reduction, a clear consensus has not yet been achieved regarding either the detailed mechanisms that affect the soot reduction, or even the sensitivity of the post-injection efficacy to several important engine operating parameters. We report that post injections reduce soot at a range of close-coupled post-injection durations, intake-oxygen levels, and loads in an optical, heavy-duty diesel research engine. Maximum soot reductions by post injections at the loads and conditions tested range from 40% at 21% intake oxygen (by volume) to 62% at 12.6% intake oxygen. From a more fundamental fluid-mechanical perspective, adding a post injection to a constant main-injection for conditions with low dilution (21% and 18% intake oxygen) decreases soot relative to the original main injection, even though the load is increased by the post injection. High-speed visualization of natural combustion luminosity and laser-induced incandescence of soot suggest that as the post-injection duration increases and the post injection becomes more effective at reducing soot, it interacts more strongly with soot remaining from the main injection.

Mixture condition, combustion and sooting characteristics of ethanol–diesel blends in diffusion flames under various injection and ambient conditions

Blend of oxygenated alcohol fuels to diesel has drawn great attention as a way to promote the use of bio-derived alternative fuels and for reduction of soot emissions in direct-injection diesel engines. This study compares the mixture condition, combustion and sooting characteristics of ethanol–diesel blends (E30 and E50: 30% and 50% ethanol blend to diesel in volume respectively) to those of a diesel in diffusion flames under various injection and ambient conditions. The comparisons were made in terms of liquid-phase penetration, flame structure, ignition delay, flame lift-off length, mixture condition at flame lift-off location, and soot concentration (KL factor) that obtained using Mie-scattering, natural flame luminosity imaging and two-color method.The liquid-phase penetration of the ethanol–diesel blends was almost equivalent to that of diesel under diesel-like injection and ambient conditions. The ethanol–diesel blend (E30) showed longer ignition delay than diesel, but this difference became insignificant at high ambient temperatures over 1000K. The flame lift-off length of the E30 was longer than that of diesel that formed less fuel-rich mixture of E30 at the flame lift-off location. The difference in mixture condition between E30 and diesel became larger with increase in injection pressure, but smaller with increase in ambient temperature. The E30 suppressed the soot formation near the flame lift-off location, and also promoted the soot oxidation process. This soot reduction potential of E30 became larger with increase in injection pressure, but insignificant at high ambient temperatures. Increasing ethanol blend ratio decreased the flame temperature slightly, but further enhanced the soot reduction potential.

The Low Load Limit of Gasoline Partially Premixed Combustion Using Negative Valve Overlap

Partially premixed combustion has the potential of high efficiency and simultaneous low soot and NOx emissions. Running the engine in partially premixed combustion mode with high octane number fuels has the advantage of a longer premix period of fuel and air which reduces soot emissions, even at higher loads. The problem is the ignitability at low load and idle operating conditions. The objective is to investigate different multiple-injection strategies in order to further expand the low load limit and reduce the dependency on negative valve overlap in order to increase efficiency. The question is, what is the minimum attainable load for a given setting of negative valve overlap and fuel injection strategy. The experimental engine is a light duty diesel engine equipped with a fully flexible valve train system. The engine is run without boost at engine speed 800 rpm. The fuel is 87 RON gasoline. A turbocharger is typically used to increase the boost pressure, but at low engine speed and load the available boost is expected to be limited. The in-cylinder pressure and temperature around top-dead-center will then be too low to ignite high octane number fuels. A negative valve overlap can be used to extend the low engine speed and load operating region. But one of the problems with negative valve overlap is the decrease in gas-exchange efficiency due to heat-losses from recompression of the residual gases. Also, the potential temperature increase from the trapped hot residual gases is limited at low load due to the low exhaust gas temperature. In order to expand the low load operating region further, more advanced injection strategies are investigated.

Combustion mode switching control in a HCCI diesel engine

HCCI (Homogeneous Charge Compression Ignition) combustion has the potential to significantly reduce NOx and PM emission of diesel engines due to its nature of even mixture and low temperature combustion. This combustion mode is only applicable in part load and low speed area so far. This range limitation results in frequently combustion mode switching from HCCI to CI and vice versa in real applications. HCCI realized by late injection with heavy EGR rate is investigated, which is also named PCCI (Premixed Charged Compression Ignition) sometimes. By changing control parameters (including injection timing, injection pressure and EGR rate) individually, the influences of these parameters on HCCI combustion and emissions are identified. Based on the evaluation of the influences, one pre-defined step-by-step air–fuel coordination strategy is presented. Additionally one hysteresis combustion mode determination method is proposed to avoid target mode fluctuation due to variation of engine speed or engine load in a small range. Torque deficiency occurs in acceleration transients as a result of fuel limitation to reduce soot emission, especially during HCCI to CI transitions. One ISG (Integrated Starter Generator) motor is applied to perform dynamic torque compensations in transient operations. Engine friction torque is estimated by means of cylinder pressure information from in-cylinder pressure sensors. On the basis of torque estimation, the difference between desired engine brake torque and actual engine brake torque is obtained. The torque output of ISG motor accounts for this torque gap and the torque pre distributed to ISG motor. The strategy is verified by engine tests. Experimental results indicate that, with the finely defined air–fuel coordination, less unstable combustion occurs and the target modes are established more quickly during transitions both in steady states and in transients. The torque deficiency during acceleration operations could be well compensated by the ISG motor.

Experimental study on combustion and emission characteristics of a diesel engine fueled with 2,5-dimethylfuran–diesel, n-butanol–diesel and gasoline–diesel blends

In the paper, combustion and emissions of a multi-cylinder CI (compression-ignition) engine fueled with DMF–diesel, n-butanol–diesel and gasoline–diesel blends were experimentally investigated, and fuel characteristics of DMF, n-butanol and gasoline were compared. Diesel was used as the base fuel. And 30% of DMF, n-butanol and gasoline were blended with the base fuel by volume respectively, referred to as D30, B30 and G30. Results show that compared to B30 and G30, D30 has longer ignition delay because of lower cetane number, which leads to faster burning rate and higher pressure rise rate. With increasing EGR (exhaust gas recirculation) rate, D30 gets the lowest soot emissions, and extended ignition delay and fuel oxygen are two key factors reducing soot emissions, and ignition delay has greater effects than fuel oxygen on soot reduction. In addition, D30 and B30 improve the trade-off of NOx-soot remarkably and extend low-emission region without deteriorating fuel efficiency by utilizing medium EGR rates (<40%). As diesel additive, DMF is superior to n-butanol and gasoline for reducing soot emissions due to its prolonged ignition delay and atomic oxygen. Using DMF–diesel blends combined with medium EGR may be a better choice for diesel engine to meet future emissions regulations with high fuel efficiency.

Comparison of Negative Valve Overlap (NVO) and Rebreathing Valve Strategies on a Gasoline PPC Engine at Low Load and Idle Operating Conditions

Gasoline partially premixed combustion (PPC) has the potential of high efficiency and simultaneous low soot and NOx emissions. Running the engine in PPC mode with high octane number fuels has the advantage of a longer premix period of fuel and air which reduces soot emissions. The problem is the ignitability at low load and idle operating conditions. In a previous study it was shown that it is possible to use NVO to improve combustion stability and combustion efficiency at operating conditions where available boosted air is assumed to be limited. NVO has the disadvantage of low net indicated efficiency due to heat losses from recompressions of the hot residual gases. An alternative to NVO is the rebreathing valve strategy where the exhaust valves are reopened during the intake stroke. The net indicated efficiency is expected to be higher with the rebreathing strategy but the question is if similar improvements in combustion stability can be achieved with rebreathing as with NVO. The results show that the rebreathing valve strategy has similar improvements on combustion stability as NVO when the same fuel injection strategy is used. This work also includes results with the NVO valve strategy where a fuel injection is added during the NVO. When a fuel injection is added during the NVO, an additional improvement on combustion stability can be seen which is unmatched by the rebreathing valve strategy. (Less)

Experimental investigation into the effects of two-stage injection on fuel injection quantity, combustion and emissions in a high-speed optical common rail diesel engine

Diesel combustion and the formation of pollutants are directly influenced by the spatial and temporal distributions of the fuel within the combustion chamber of an internal combustion engine. The requirements for more efficient and responsive diesel engines have led to the introduction and implementation of multiple injection strategies. However, the effects of such injection modes on the hydraulic systems, such as the high pressure pipes and fuel injectors, must be thoroughly examined and compensated for. The objective of this study was to investigate the effects of fuel injection equipment characterisation and optimisation on diesel combustion and emissions with two-stage fuel injection. The fuel injection system was characterised and optimised through the measurement of the fuel injection rate and quantity, in particular, the interaction between the two injection events was quantified and compensated for. The effects of twin and variable split two-stage injection and dwell angle on diesel combustion and emissions were investigated in a high-speed direct injection single-cylinder optical diesel engine using heat release analysis and high-speed fuel spray and combustion visualisation technique. The results indicated that two-stage injection has the potential for simultaneous reduction of NOx and soot emissions. Nevertheless, the studied two-stage strategies resulted in higher soot emission, mainly due to the interaction between two consecutive fuel injection events, whereby the fuel sprays during the second injection were injected into burning regions, generating fuel-rich combustion. In addition, the variable two-stage strategies produced high levels of uHC emission in comparison to single and twin split injection cases. This was mainly attributed to firstly greater fuel quantity injected during the second injection and secondly poor mixing and air utilisation during the second fuel injection event.

Effects of exhaust gas recirculation on low temperature combustion using wide distillation range diesel

The experiments were performed using a modified six-cylinder turbocharged direct injection diesel engine with diesel of different distillation ranges to investigate the effects of (exhaust gas recirculation) EGR on engine performance and emission characteristics of low-temperature combustion engine. The results showed that for the same fuel, with the increase of the EGR rate, the cylinder pressure remained almost unchanged. Increasing the EGR rate slowed the combustion, the peak value of heat release rate increased and the indicated thermal efficiency decreased. The peak value of heat release rate increased with the decline of the initial boiling point at the same EGR rate. The ignition delay of combustion increased with the decrease of initial boiling point. When EGR rate was low, the level of HC and CO emission was low, while the HC and CO emission increased rapidly with the increase of EGR. The NOx emission was reduced with the increase of the EGR rate. The study also showed that the wide distillation range diesel can reduce emission over diesel in low temperature combustion. Wide distillation range fuel had potential to reduce soot emission at different EGR for low temperature combustion engine.

Improvement of emissions and performance by using of air jet, exhaust gas re-circulation and insulation methods in a direct injection diesel engine

This article investigates the improvement of operation characteristics and emissions reduction by means of creating an air-cell inside the piston body, exhaust gases recirculating and insulating combustion chamber in a direct injection diesel engine simultaneously. The engine considered is a caterpillar 3401 which was modeled with an air-cell included as part of the piston geometry. This air-cell demonstrates that air injection in late combustion period can be effective in a significant reduction of Soot emission while cold EGR can be effective in reduction of NOx emission. Also for increasing of performance parameters, combustion chamber with air-cell is insulated. The analyses are carried out at part (75% of full load) and full load conditions at the same engine speed 1600 rpm. The obtained results indicate that creating the air-cell has a slight effect on improvement of performance parameters and it has significantly effect on Soot reduction. The air-cell decreases the Soot pollutant as a factor of two at both part and full load conditions. Also, the adding 5% of cold EGR in inlet air decreases NOx by about half and insulating the engine increases the power and IMEP by about 7.7% and 8.5% and decreases the ISFC by about 7.5% at part load and increases power and IMEP by 8.5%, 8.5% and decreases ISFC by 8% at full load condition, respectively. Using this method, it was possible to control emissions formation and increase performance parameters simultaneously. The predicted results for mean in-cylinder pressure and emissions are compared to the corresponding experimental results and show good agreements.

Effect of diluents on soot precursor formation and temperature in ethylene laminar diffusion flames

Soot precursor species concentrations and flame temperature were measured in a diluted laminar co-flow jet diffusion flame at pressures up to eight atmospheres while varying diluent type. The objective of this study was to gain a better understanding of soot production and oxidation mechanisms, which could potentially lead to a reduction in soot emissions from practical combustion devices. Gaseous samples were extracted from the centerline of an ethylene–air laminar diffusion flame, which was diluted individually with four diluents (argon, helium, nitrogen, and carbon dioxide) to manipulate flame temperature and transport properties. The diluted fuel and co-flow exit velocities (top-hat profiles) were matched at all pressures to minimize shear-layer effects, and the mass fluxes were fixed over the pressure range to maintain constant Reynolds number. The flame temperature was measured using a fine gauge R-type thermocouple at pressures up to four atmospheres. Centerline concentration profiles of major non-fuel hydrocarbons collected via extractive sampling with a quartz microprobe and quantification using GC/MS+FID are reported within. The measured hydrocarbon species concentrations are vary dramatically with pressure and diluent, with the helium and carbon dioxide diluted flames yielding the largest and smallest concentrations of soot precursors, respectively. In the case of C2H2 and C6H6, two key soot precursors, helium diluted flames had concentrations more than three times higher compared with the carbon dioxide diluted flame. The peak flame temperature vary with diluents tested, as expected, with carbon dioxide diluted flame being the coolest, with a peak temperature of 1760K at 1atm, and the helium diluted flame being the hottest, with a peak temperature of 2140K. At four atmospheres, the helium diluted flame increased to 2240K, but the CO2 flame temperature increased more, decreasing the difference to approximately 250K.

Experimental Investigation of the Effects of Air Injection and Injection Timing in a Natural Aspirated DI Diesel Engine

In this experimental study, the effect of creating an air jet inside the combustion chamber by establishing an air-cell inside of the piston body at different injection timings in a DI diesel engine has been studied at 2000 rpm and full load operation. The performed tests include studying the Soot and NOx emissions, combustion and performance parameters. The obtained experimental results show that using an air-cell inside the piston causes in simultaneous reduction of Soot and NOx emissions, also advanced injection timing results in increasing of brake power and reduction of brake specific fuel consumption(bsfc) with increasing of maximum pressure in the combustion chamber and maximum value of heat release rate. Also, the results showed that the best time injection for air cell engine is 13 BTDC, in which the NOx and soot emissions decrease by 15% and 50%, respectively, while BSFC and power remains relatively unchanged.

Soot Reduction Strategy: A Review

The aim of study is soot reduction strategy by using microwave radiation. As a first step toward the aim is a review. This study provides a review of soot particles generation and their influence on air pollution and illustrates the diesel particulate matter emission regulation, composition, and characterization including size, shape, process, and oxidation. The paper also discusses the effect of temperature, pressure, air/fuel ratio, exhaust flow rate, fuel type on soot process and heat release from soot combustion. The use of filter regeneration strategies for reducing soot emission from vehicles, stationary plants and factories due to the burning of diesel fuel have been presented with advantage and disadvantage of each method. The various strategies include catalyst, fuel injection, electric heating, engine operation condition control, non-thermal plasma, and absorbed microwave energy. The practical feasibility for soot oxidation by microwave heating technique has been explained depending on soot material property. The proposed new technique utilizing the reflected and absorbed energies of microwave for soot reduction strategy is presented to develop a new emission after-treatment system. The new system will be modeled and simulated using appropriate software. The simulation results will be validated experimentally. © 2012 Asian Network for Scientific Information.

Hydrocarbon species concentrations in nitrogen diluted ethylene-air laminar jet diffusion flames at elevated pressures

Hydrocarbon species concentrations are measured in a laminar jet diffusion flame at elevated pressures with the objective of better understanding soot production and oxidation mechanisms, which will ultimately lead to a reduction in soot emissions from practical combustion hardware. Samples were extracted from the centerline of an ethylene flame diluted with nitrogen. The diluted fuel and co-axial air top-hat exit velocities were matched and the mass fluxes were held constant at all pressures. This paper reports centerline concentration profiles of major non-fuel hydrocarbons and 5 different PAH species measured via extractive sampling with a quartz microprobe and quantification using GC/MS+FID. The peak concentration of acetylene decreased with increase in pressure, suggesting rapid conversion to heavier compounds, whereas the concentrations of the other major heavier non-fuel hydrocarbons increase with an increase in pressure. The measured peak species concentration as a function of pressure is seen to closely follow a power law function, Pn, where n varies from less than zero for acetylene, propane and diacetylene to greater than unity for the larger PAH species.

Experimental investigation of hydraulic effects of two-stage fuel injection on fuel-injection systems and diesel combustion in a high-speed optical common-rail diesel engine

In order to meet the ever more stringent emission standards, significant efforts have been devoted to the research and development of internal combustion engines. The requirements for more efficient and responsive diesel engines have led to the introduction and implementation of multiple injection strategies. However, the effects of such injection modes on the hydraulic systems, such as the high-pressure pipes and fuel injectors, must be thoroughly examined and compensated for since the combustion and the formation of pollutants in direct-injection engines are directly influenced by the spatial and temporal distribution of the injected fuel within the combustion chamber. This study investigated the hydraulic effects of two-stage fuel injection on diesel combustion and emissions. The fuel-injection system was characterised for all the tested strategies through the measurement of the fuel-injection rate and quantity. In particular, the interaction between the two injection events was identified. The effects of two-stage injection, dwell angle and the interactions between two consecutive injection events on the combustion process and the emissions were investigated in a high-speed direct-injection single-cylinder optical diesel engine using heat-release analysis and high-speed fuel spray and combustion visualisation techniques. The results indicated that the two-stage injection strategy has the potential for simultaneous reduction of nitrogen oxide, soot and unburned hydrocarbon emissions. The results suggested that an optimum fuel quantity in the first injection exists, 0–30%, with which simultaneous reduction of nitrogen oxide, soot and unburned hydrocarbon emissions can be achieved with the added benefits of improved engine performance, fuel economy and combustion noise. However, higher soot emissions were produced, mainly due to the interaction between the two consecutive fuel-injection events whereby the fuel sprays during the second injection were injected into burning regions, as well as reduced soot oxidation due to the continuation of the combustion into the expansion stroke.