Intensity profile of a focusing Infra-Red Radiation
Generally, homogeneous mixture combustion is preferred at high loads in conventional spark-ignition engines. But homogeneous mixture combustion can lead to high hydrocarbon (HC) emissions at low loads. Thereby, stratified mixture combustion with an overall lean mixture is preferred at low loads, which can significantly reduce HC emissions, but NOx and soot emissions will increase. Nowadays, gasoline direct injection (GDI) engines are becoming popular because of better thermal efficiency and low emissions at all loads. These engines work with a stratified mixture at low-load conditions and a homogeneous mixture at high-load conditions. But the problem with these engines is high nitrogen oxides (NOx) and soot emissions at low-load conditions. Therefore, today, the concept of partial stratification is tried in these engines, which is a combination of the combustion of stratified and homogeneous mixtures, using both GDI and port fuel injection (PFI) techniques. With the partial stratified mixture combustion, HC, NOx, and soot emissions are expected to reduce. Also the use of laser ignition instead of spark ignition can reduce NOx and HC emissions. Therefore, this study deals with a computational fluid dynamics (CFD) analysis of the effect of spark and laser ignitions on the combustion, performance, and emission characteristics of a single-cylinder engine operating under GDI-PFI mode operating with a partially stratified mixture. Three overall equivalence ratios (OERs) of 0.5, 0.7, and 0.9 are considered for the analysis. The effects of spark and laser ignitions on turbulent kinetic energy (TKE) formation at the ignition spot, indicated mean effective pressure (IMEP), and emissions are analyzed. To quantify the flame speed, a parameter called relative combustion phasing (RCP) is used. The analysis is performed by maintaining a constant CA50 (crank angle degree [CAD] position where 50% of the total heat release occur in a combustion) by adjusting the start of spark (SOS). Analysis of results showed that the combustion with the laser ignition is faster than that of the spark ignition. The laser ignition with the OER of 0.7 reduced the HC and soot emissions by 5.8% and 2.2 times, respectively, if compared to those of the spark ignitions. The RCP of the laser ignition is about 34.5% lower than that of the corresponding spark ignition. The IMEP for the laser ignition is improved by about 10.4% and the NOx emissions increased by about 3.2% than that of the spark ignition.