Effects of engine operating parameters on gaseous and particulate emissions in direct-injection spark-ignition engine

Abstract

The characteristics of particulate matter emissions from the direct-injection spark ignition (DSI) gasoline engine was investigated under various engine operating conditions using particle number (PN) emissions, gaseous emissions, cylinder pressure and in-cylinder visualization. A single cylinder gasoline direct injection engine equipped with an optically accessible system was used to perform combustion experiments and to capture in-cylinder images during fuel injection and combustion. Injection timing, injection pressure, engine load and injector pattern were selected as the engine operating parameters. To verify the effects of piston position and stage of the intake process, the injection timings were varied from before top dead center (BTDC) 360 deg, when the piston is positioned at the top death center, to BTDC 210 deg at the end of the intake process. Time-averaged spray images and spray variations between cycles were used to analyze the spray behavior and factors that affect the spray variations. The spray injected at the early stage of the intake process is deflected by the strong intake flow, and the magnitudes of spray deflection and spray variations are increased owing to the smaller droplets formed by higher injection pressure. The emission of a large number of particles was observed at early injection timings before BTDC 330 deg owing to fuel impingement on the piston top causing pool fire. At the late injection timing of BTDC 210 deg, insufficient time for fuel evaporation and mixture formation leads to high PN emissions at locally rich regions with mixture of a low homogeneity. The increase in injection pressure at early injection timing elevates the droplet momentum and reduces the fuel film with increasing rebounded droplets, thus mitigating PN emissions. The enhanced fuel atomization due to the increase in injection pressure promotes mixture formation and improves mixture homogeneity, reducing PN emission at retarded injection timing after BTDC 210 deg. The high in-cylinder pressure corresponding to a high engine load expands the cone angle and increases the interaction between spray plumes, stimulating the collision between droplets. In addition, high density in the cylinder rapidly withdraws the momentum from the droplets and fuel impinging on the piston forms a thick wall film at early injection timings, increasing PN emission. Further increase in injection pressure was applied at high engine load, reducing PN emissions at the early injection timing of BTDC 330 with a thin fuel film on the piston. Fuel injected at end of the intake process travels in the injection direction, and high injection pressure increases the travel distance. Thus, with higher injection pressure, over 35 MPa, and under high engine load, a greater amount of fuel collides with the cylinder wall opposite to the injector and, consequently, at late injection timing, more particles are emitted by fuel film on the cylinder wall. Downward adjustments of the injector pattern at the upper, lower, and whole parts reduce the PN emissions at injection timing of BTDC 330 deg owing to the narrow impingement area and large collision angle. In addition, at injection timing of BTDC 210 deg, a decrease in PN emissions is achieved by a reduction in the amount of fuel reaching the wall opposite to the injector owing to weakened spray collapse and mitigation of momentum concentration on spray center. The effects of exhaust load corresponding to the utilization of gasoline particulate filters (GPFs) were tested with increasing exhaust pressure from ambient pressure to 40kPa. The residual gas (i.e., internal exhaust gas recirculation (iEGR)) and amount of mixture were increased 3.91% and 4.68% respectively, and this was determined by a one-dimensional simulation (i.e., GT-Power) using the cylinder pressure. The mixture of high temperature with a large proportion of internal EGR promotes fuel evaporation and enhances mixture homogeneity, reducing PN emissions. However, the engine performance is deteriorated owing to the large pumping loss. Bioethanol was used to suppress soot formation and reduce particulate matter emissions at low injection pressure. PN emissions with injection pressures over 35 MPa were barely affected by the ethanol addition and contents at the entire injection timing. The environmental effect of bioethanol was determined as production energy and greenhouse gas emission calculated by a well-to-wheel analysis. In total, 2.03% of greenhouse gas emission is reduced during the tank-to-wheel process by increasing the bioethanol contents in blended fuel from 10 vol% to 50 vol% at injection timing of BTDC 330 deg.