Numerical Study On The Effects Of Injection Strategy And Piston Shape On The Performance Of GDI Engine

The gasoline direct injection (GDI) has become the main technology for futurevehicle engines due to its excellent performances in fuel economy, power and emissions.By combining with other technologies such as EGR, turbo-charge and VVT, GDI getswider applied. It is the key point for GDI engines to form stable and reliablecombustible mixture near spark plug at spark time, which needs accuracy match ofmany parameters such as in-cylinder flow, combustion-chamber geometry, injectiontime, spray characteristics, spark time and so on. In this paper, the operations of GDIengines were simulated by CFD code. Some conclusions were summarized which mayprovide materials for optimization of GDI engines.The effects of injection strategy, including injection proportions and injection time,on mixture formation and combustion for GDI engines were studied via control variablemethod. The results show that both injection proportions and injection time haveimportant influences on formation and combustion of mixture. Finally, the injectionproportion was defined as25:75and the second injection time was defined as635oCA,which was demonstrated to be beneficial for formation and combustion of mixture,while producing less wallfilm and emissions.Then the effects of piston shape on GDI engines operations were studied and theinfluence of four pistons on mixture preparation and combustion performances werecompared and analyzed. The results show that, piston shape has little influence on airinflow. For GDI engines with forward tumble and multi-holes high pressure injector,piston’s flat top surface with pit is benefit to maintain in-cylinder turbulence intensityand reduce wallfilm on piston. It helps form stable and reliable combustible mixturearound spark plug at spark time. Smoother piston’s top surface makes turbulence kineticenergy stronger at spark time, which accelerates combustion process and the peakvalues of in-cylinder temperature and pressure are higher. The emission of NOxisincreased because of high in-cylinder temperature and pressure, while the emission ofsoot is decreased due to later oxidation.The depth and width of piston pit were optimized after confirming scheme ofpiston and the performances of GDI engines were improved. The system’s sensitivity tospark time was also studied. Either too early or too late spark time is pernicious for GDIengines. More compression negative work will be produced if the spark time is too early, and the pressure rise rate is increased which may lead to knock. However, too lateignition will decrease the in-cylinder pressure, giving rise to decreasing power output ofengines. Furthermore, the fuel combustion may be deteriorated due to too late ignitionand worsen the emissions of engines. Finally, the appropriate spark time is proved to belocated between698°CA and700°CA.