Kiyomi Nakakita

Achieving Lower Exhaust Emissions and Better Performance in an HSDI Diesel Engine with Multiple Injection

The effects of multiple-injection on exhaust emissions and performance in a small HSDI (High Speed Direct Injection) Diesel engine were examined. The causes for the improvement were investigated using both in-cylinder observation and three-dimensional numerical analysis methods. It is possible to increase the maximum torque, which is limited by the exhaust smoke number, while decreasing the combustion noise under low speed and full load conditions by advancing the timing of the pilot injection. Dividing this early-timed pilot injection into two with a small fuel amount is effective for further decreasing the noise while suppressing the increase in HC emission and fuel consumption. This is realized by the reduced amount of adhered fuel to the cylinder wall. At light loads, the amount of pilot injection fuel must be reduced, and the injection must be timed just prior to the main injection in order to suppress a possible increase in smoke and HC. After-injection, which injects a small amount of fuel immediately after the end of the main injection, reduces smoke, HC and fuel consumption. This is because the jet flame of the after-injection carries the remaining soot of the main injection to the squish area, and then oxidizes that together with the soot which previously existed in the squish area by promoted atmospheric temperature and enhanced mixing with fresh air.

Effect of the hydrocarbon molecular structure in diesel fuel on the in-cylinder soot formation and exhaust emissions

Abstract Evaluations of diesel fuel effects on combustion and exhaust emissions in single-cylinder direct-injection diesel engines led to the unexpected result that a Swedish ‘class 1’ fuel generated more particulate matter (PM) than a fuel denoted ‘improved’, even though ‘class 1’ fuel had much lower distillation temperatures, aromatic concentration, sulphur level, and density than the ‘improved’ fuel. Little differences were observed in the combustion characteristics between these fuels, but detailed compositional analyses showed that ‘class 1’ fuel contains higher levels of cyclic and/or branched paraffins. Subsequent investigations in a laboratory flow reactor showed that ‘class 1’ fuel produces more soot precursors such as benzene and acetylene than the ‘improved’ fuel. In addition, experiments in a low-pressure laminar flame apparatus and shock tube with model (single-molecule) paraffin fuels showed that isoparaffins and cycloparaffins generate more soot precursors and soot than n-paraffins do. These results strongly suggested that the effect of molecular structure on exhaust PM formation should be more carefully examined. Therefore, a new series of investigations were performed to examine exhaust emissions and combustion characteristics in single-cylinder engines, with well-characterized test fuels having carefully controlled molecular composition and conventional distillation characteristics and cetane numbers (CNs). These investigations revealed the following. Firstly, under low and medium loads, cycloparaffins (naphthenes) have a higher PM formation tendency than isoparaffins and n-paraffins. Under high-load conditions, however, the paraffin molecular structure has a very small effect. Secondly, a highly n-paraffinic fuel does not yield PM reductions as high as expected, due to its high CN and corresponding shorter ignition lag, which initiates combustion under a state of insufficient fuel-air mixing. This finding was corroborated by laser-induced incandescence analyses. Thirdly, the lowest PM emissions were observed with a paraffinic fuel containing 55 per cent isoparaffins and 39 per cent n-paraffins. Fourthly, aromatics give higher soot and PM levels than paraffins do at high and medium load conditions. Smaller differences are observed at lower speeds and loads. Fifthly, the best fit to the PM emissions was obtained with an equation containing the regression variables CN, aromatic rings, and naphthene rings. This expression of the fuel effects in chemical terms allows well-to-wheel analyses of refining and vehicle impacts resulting from molecularly based fuel changes.

In-cylinder stratification of external exhaust gas recirculation for controlling diesel combustion

Abstract A technique for achieving the in-cylinder stratification of external exhaust gas recirculation (EGR) gas in direct-injection (DI) diesel engines has been developed to reduce toxic exhaust emissions. The external EGR gas is supplied from one of the two intake ports which can create a swirl flow in either the upper or lower portion of the cylinder during the intake stroke. In the final stage of the compression stroke, a squish flow conveys the vertically stratified EGR gas into the piston cavity, generating a radially stratified EGR gas in the piston cavity at the end of the compression stroke. This strategy for achieving EGR gas stratification in the piston cavity was developed by using an unsteady computational fluid dynamics (CFD) code. Prior to the exhaust emission tests, the accuracy of the simulation was evaluated by planer laser-induced fluorescence (LIF) imaging. The exhaust emission tests showed that there was less smoke emission under medium load conditions when the EGR gas was delivered to the inner part of the piston cavity. The mechanism of this smoke reduction was investigated using CFD simulation, which is based on a series of calculations related to the internal flow of the injector nozzle, the in-cylinder fuel spray, and mixture formation and combustion. It has been shown that, at the beginning of the combustion, the higher concentration of EGR gas in the inner part of the cavity lowers the combustion temperature and reduces the soot formation rate. Air, which exists in the outer part of the cavity at the start of fuel injection, enhances the oxidation of the soot cloud in the piston cavity periphery in the latter half of the combustion period.

Effect of Hydrocarbon Molecular Structure on Diesel Exhaust Emissions Part 2: Effect of Branched and Ring Structures of Paraffins on Benzene and Soot Formation

In order to determine diesel fuel characteristics that might influence particulate matter (PM) emission, we have conducted a detailed investigation that combines combustion/exhaust emission measurements, in-cylinder observations, fuel analyses and chemical reactor experiments. A comparison between three representative diesel fuels, viz., “Base” (Japanese market fuel), “Improved”(lighter fuel with lower aromatics) and Swedish “Class-1” yielded the following results: (1) The amount of PM emission decreases in the order of “Base” > “Class-1” > “Improved”. Unexpectedly enough, “Class-1” produces more PM than “Improved” despite its significantly lower distillation temperature, and lower aromatics and sulfur content. (2) There is little difference in the combustion characteristics of the three fuels. (3) Only “Class-1” contains significant quantities of iso and naphthenic structures. (4) Flow reactor pyrolysis shows that “Class-1” produces the largest amount of PM precursors, such as benzene and toluene. These results suggest that the presence of branched and ring structures can increase exhaust PM emissions. This finding was confirmed by flowreactor and shock tube experiments using hexanes, which revealed that isoand cycloparaffins produce more benzene and soot than n-paraffins do. The results obtained in this study indicate that the specific molecular structure of the paraffinic components needs to be considered as one of the diesel fuel properties closely related to PM formation.

Combustion Improvement for Reducing Exhaust Emissions in IDI Diesel Engine

Means for reducing PM from a swirl chamber type diesel engine were examined and the mechanisms of the PM reduction were investigated using both a multi-cylinder and an optically accessible single-cylinder engine. The following points were clarified. (1) At light load, suppression of initial injection rate reduces PM, because of both the change of ignition point to reduce SOF, and the retarded flowout of dense soot from the swirl chamber to reduce smoke. (2) Over medium load, the main cause of the exhaust smoke is hard spray-wall impingement which leads to both fuel adhesion on the wall and the stagnation of rich fuel-air mixture.

Effect of hydrocarbon molecular structure in diesel fuel on in-cylinder soot formation and exhaust emissions

Exhaust emissions and combustion characteristics from well-characterized diesel test fuels have been measured using two types of single-cylinder HSDI diesel engines. Data were collected at several fixed speed/load conditions representative of typical light-duty operating conditions and full-load performance (smoke-limited maximum torque) points. In addition, in-cylinder soot formation processes of these fuels were investigated via Laser Induced Incandescence (LII) using an optically accessible single-cylinder engine. The test fuels used in this study have been formulated with a sophisticated blending algorithm that systematically varies the hydrocarbon molecular structure in the fuels while maintaining the distillation characteristics of market diesel fuels. The following results have been obtained from this study. (1) The lowest PM emissions were observed with a fuel containing approximately 55% iso-paraffins and 39% n-paraffins with CN=52.5. Compared with the base fuel (corresponding to average market fuel in Japan), this fuel yields a 40 - 70% PM reduction and an increase in the maximum torque of approximately 8%. (2) A highly n-paraffinic fuel representative of a Fischer-Tropsch liquid did not yield PM reductions as high as expected. This is due to its very high cetane number (CN=80.5), resulting in a decreased ignition delay which initiates combustion before sufficient fuel-air mixing has occurred. This conclusion is corroborated by LII analyses of highly n-paraffinic fuels which show regions of high soot concentration in the burning fuel spray jet near the injector. (3) Under low and medium loads, cyclo-paraffins (naphthenes) have a higher PM formation tendency than iso- or n-paraffins. Under high load conditions, however, paraffin molecular structure has a very small effect on PM formation. (4) Aromatics have a higher soot/PM formation tendency than paraffins under all speed/load combinations investigated. A correlation of PM formation with fuel chemical composition has been developed from a statistical analysis of the data. Expressing the fuel effects in chemical terms allows well-to-wheel analyses of refining and vehicle impacts resulting from molecularly based fuel changes.

Photographic and Three Dimensional Numerical Studies of Diesel Soot Formation Process

Soot formation process was examined by high speed photographs, using a single combustion diesel engine with a transparent swirl chamber. Fuel-air mixture and flames, and soot clouds were visualized by the schlieren method and the back-illuminated method, respectively. A three dimensional simulation program with soot formation and oxidation models was developed to clarify diesel soot formation processes. The models consist of several models previously proposed and partly improved in this study. Good agreement was obtained between calculated and experimental results. The following points were clarified through observation and numerical studies: The main soot area is considerably smaller than luminous flame area, especially in the initial soot formation process; the main soot cloud first appears in the tip region of fuel-air mixture, downstream of ignition position a few submilliseconds after the ignition. It is the soot carried down from the ignition position by a gas flow; and temperature is more influential in soot formation, rather than fuel vapor concentration.

Quantitative Analysis of Soot Formation and Oxidation Process using Laser-Induced Incandescence

A new technique using Laser-Induced Incandescence (LII) has been developed to quantify the soot concentration in a diesel engine. Characteristic problems in quantitative measurements, such as LII signal attenuation by soot clouds between the camera and the measurement plane, and incident laser attenuation due to soot clouds in the laser path, were corrected by the multi-layer correction method developed in this work. When this LII measurement method is applied to an optically accessible engine, the developing soot clouds in spray combustion can be visualized in detail. The changes in soot formation process caused by increasing fuel injection pressure with reduced hole size of injector, and by altering fuel chemical property, are both clarified quantitatively in this paper.

NOx Reduction Behavior on Catalysts With Non-Thermal Plasma in Simulated Oxidizing Exhaust Gas

NOx reduction activity in an oxidizing exhaust gas was significantly improved by discharging non-thermal plasma and catalysts (plasma assisted catalysis). We investigated effective catalyst for plasma assisted catalysis in view of hydrocarbon-selective catalytic reduction(HC-SCR). Plasma assist was effective for γ-alumina and alkali or alkaline earth metals loaded zeolite and γ-alumina showed the highest NOx conversion among these catalysts. On the other hand, Plasma assist was not effective for Cu-ZSM-5 and Pt loaded catalyst. The NOx conversion for the plasma assisted γ-alumina decreased by formation of a deposit on the catalyst below 400°C. It is shown that indium loading on γ-alumina improved the NOx reduction activity and suppressed the degradation of the NOx reduction activity at 300°C with plasma assist.

State Estimation Algorithm Based on a Disturbance Observer for a Free-Piston Engine Linear Generator

In order to realize a thin, compact, efficient, and fuel-flexible power-generation system, we have been developing and investigating a free-piston engine linear generator (FPEG), which consists of a two-stroke combustion chamber and an adjustable gas spring chamber. This paper proposes a novel observer algorithm that has two advantages. First, the piston position can be interpolated at high accuracy beyond the resolution of the piston position sensor, which improves the accuracy and stability in controlling the combustion parameters, such as ignition timing and compression ratio. Second, the pressure in the gas spring chamber and the coefficient of damping (lubrication condition) are estimated simultaneously and independently. In this paper, the principle of the developed observer algorithm is described and tested through simulation and experimentation using the developed FPEG system. Based on the simulation, the output of the position sensor (resolution: 0.55 mm) was interpolated with an accuracy of ±0.1 mm. Based on the experimental results for the estimation of pressure in the gas spring chamber and the coefficient of damping, the feasibility of sensorless monitoring of the pressure in the gas spring chamber and the coefficient of damping has been verified.