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.

通常而言，在传统的火花点火发动机中，在高负荷条件下，均质混合燃烧更为优选。然而，均质混合燃烧在低负荷条件下可能导致高碳氢化合物（HC）排放。因此，在低负荷条件下，采用整体偏瘦混合物的分层混合燃烧更为适宜，这可以显著降低HC排放，但氮氧化物（NOx）和烟尘排放将相应增加。如今，由于汽油直喷（GDI）发动机在所有负荷条件下的热效率更高且排放更低，其应用日益普及。这些发动机在低负荷条件下以分层混合物工作，在高负荷条件下则以均质混合物工作。但这些发动机在低负荷条件下的氮氧化物和烟尘排放问题依然存在。因此，当前在这些发动机中尝试采用部分分层概念，即将分层混合和均质混合的燃烧相结合，并使用GDI和歧管喷射（PFI）技术。在部分分层混合燃烧中，预计HC、NOx和烟尘排放将得到降低。此外，使用激光点火替代火花点火也有助于降低NOx和HC排放。因此，本研究涉及对单缸发动机在GDI-PFI模式下，以部分分层混合工作时的燃烧、性能和排放特性的计算流体动力学（CFD）分析。分析中考虑了三种整体当量比（OER）分别为0.5、0.7和0.9。对点火火花和激光点火在点火点湍流动能（TKE）形成、指示平均有效压力（IMEP）和排放方面的效应进行了分析。为了量化火焰速度，使用了相对燃烧相位（RCP）这一参数。分析通过调整火花点火开始时间（SOS）来保持恒定的CA50（总热释放量中50%发生的位置，以曲轴转角度[CAD]表示）进行。结果分析表明，激光点火下的燃烧速度比火花点火快。当量比为0.7的激光点火与火花点火相比，HC和烟尘排放分别降低了5.8%和2.2倍。激光点火相对于相应的火花点火，RCP降低了约34.5%，IMEP提高了约10.4%，而NOx排放增加了约3.2%。