Research On Two Stage Gasoline Direct Injection Combustion System

Gasoline Direct Injection (GDI) engines developed at nineties of the twentieth century can greatly improve the fuel economy with lean burn, accurate electronic control and high compression ratio. But the combustion chamber design and mixture forming control of the engines are very complex compared with Port Fuel Injection (PFI) gasoline engines. A Two Stage Gasoline Direct Injection (TSGDI) combustion system is developed and aimed to solve the problem of the combustion system complexity and mixture formation sensitivity to the combustion chamber. The two-stage fuel injection strategy including flexible injection timings and flexible fuel quantity adjustment is adopted as main means to form reasonable stratified mixture in the cylinder. A simple cylindrical piston combustion chamber and the helical intake port are designed to assist the stable combustion of the mixture, which reduce the difficulties of the combustion system design. A two-cylinder TSGDI prototype engine is modified from a diesel engine. The locations and the inclinations of the injector and the spark plug, the combustion chamber are carefully designed. The gasoline high-pressure common rail system and the electronic uint for cntroling the injection timings and fuel quanties are also developed. On the TSGDI engine test bench, effects of two-stage injection strategies, including the first injection timing, the second injection timing and the ratio of the first injection fuel quantity to the second, on the engine power, fuel economy, emission characteristics, in-cylinder pressure diagram and heat release are studied in detail, and the injection parameters are optimized. The effects of different ignition advances, air/fuel (A/F) ratios and engine speeds on the engine performance are also investigated. Then the engine A/F characteristics and load characteristics are tested and lean burn limit is studied on the bench. After the optimization, the TSGDI engine fuel consumption can be reduced about 13%~24% compared with the common PFI gasoline engines. The mixture formation control is the key to determine directly the combustion efficiency of the GDI engines. A gasoline high-pressure swirl spray model is established on the basis of the original KIVA-3V code. The spray simulation is conducted with changing the injection pressures and ambient pressures. At the same time, the spray behavior is imaged in a constant-volume bomb using CCD camera. The spray development is analyzed using the simulation and experiment results. The results also show that the modified spray models can predict the spray development well. The model of the TSGDI mixture formation in cylinder is set up incorporating the modified gasoline swirl spray model. A three-dimensional mesh of the real configuration of TSGDI combustion chamber including the helical intake port is generated. The experimental results obtained from the steady flow bench are used for boundary conditions. The effects of different injection timings, injection ratios, ignition advance angles, engine speeds and A/F ratios on gasoline spray development and mixture formation processes in cylinder are simulated in detail to provide theoretical explanation to the experimental results.