The effects of injection systems and combustion chamber shapes on the GDI (Gasoline Direct Injection) engine combustion have been studied.
Two types of 2-phase injector (or air-assisted fuel injector), which is one of the most important element for the GDI engines, were developed. Their spray characteristics related with operation and atomization, and combustion performance characteristics were investigated. Each of them produced finely atomized spray characterized with 14.9μm and about 20μm, respectively, in SMD. The structure of spray in terms of mass concentration and SMD distribution was also investigated. Atomization performance was tested as a function of design variables and operating parameters. One of them exhibited stable behavior at various speeds up to 6,000rpm. In the combustion experiment of 2-phase injector using RCEM (Rapid Compression and Expansion Machine) and GDI engine, pressure data, photographs and emission data were taken at several conditions with various ignition, injection, flow, temperature, and engine speeds. Spark plug must be located in the region of spray for stratified combustion. Finding the proper injection timing was important for the lean mixture combustion. Swirl did not give beneficial effect on stratification process. Intake temperature and cylinder wall temperature were not dominant factors on combustion performance. It was hard to achieve stratification at high engine speeds since the intake flow velocity was increased. A deflector installed on the injector nozzle tip could control spray direction. Flame speed of stratified mixture was observed to be faster than that of pre-mixed mixture. Combustion performance of the GDI engine was compared with conventional PFI (Port Fuel Injection) engine in terms of imep and emissions. As a result, the lean limit of GDI engine using 2-phase injector was extended to Φ = 0.65 compared to Φ = 0.75, in PFI engine.
A single-cylinder optical engine was constructed to observe DISC (Direct Injection Stratified Charge) processes. Combustion performance of GDI engine using swirl injector was examined while it was operated with six-shapes of combustion chamber conditions. Two injector positions and three piston shapes were used. When GDI engine was operated at the early injection condition, the evident differences of combustion performance in terms of imep with regard to the combustion chamber shapes could be observed. Additionally, film combustion phenomenon around the end of combustion process was observed in the optical engine. The difference of wetted fuel amount could be regarded as a reason for this imep variation. There was a strong negative correlation between the imep value and the wetted fuel amount. At partial load conditions, the center injection case was better than the case of side injection and stable combustion can be obtained under very lean condition with air fuel ratios exceeding 36.
