Lean combustion has proven to be an effective way to improve the efficiency and emissions of the direct injection spark ignition (DISI) engine. However, one of the main problems at the lean stability limit is the major decrease in flame temperature due to dilution, resulting in a low laminar flame speed, especially under low-speed engine operating conditions. The split injection is a potential technology to realize proper air-fuel mixing and achieve different spray distribution that can help in solving such problems. In this study, split injections with different secondary injection timings were tested to achieve homogeneous and homogeneous-stratified modes in a DISI optical engine under lean-burn mode. The split ratio of each strategy was 1:1. The engine was operated at 800 rpm, and a high-energy ignition system was utilized to realize lean combustion at a lambda of 1.55. Engine combustion performance and emissions were tested while performing high-speed color recording to study the characteristics of flame chemiluminescence through a quartz piston combined with a 45-degree mirror installed below. Flame structure during various combustion phases was compared under different selected conditions based on a digital image processing technique. The results show that the pressure and emissions vary with the second injection timing. Proper control of the split injection timing can improve lean combustion performance, including faster flame speed, increased indicated mean effective pressure (IMEP), and lower harmful emissions. Poor fuel evaporation and soot generation from spatial hot spots in the combustion process of split injection are the major challenges for further improvement.

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