Simulation Studies On The Effect Of Intake Valves And Injection Strategies On Mixture Organization And Combustion Control Of A Gasoline Direct Injection Engine

Gasoline direct injection (GDI) engine is advantageous for its fuel economy,transient response and etc. compared with conventional port fuel injection (PFI)engines. However, when operating at low loads in the stratified combustion mode, theengine might suffer high levels of NOx, UHC and Soot emissions, while theconverting efficiency of the three way catalyst is reduced due to lean burn. As a result,it is still necessary at present that GDI engines run at the equivalence ratio of1inorder to meet the requirement of the environmental legislations.In this study, a hybrid droplet atomization and breakup model, namely theHuh-Gosman-KH-RT model, was developed based on the commercial code Star-CD.The primary breakup was simulated by the competition between the Huh-Gosman andKH mechanisms, and the secondary breakup was simulated by employing the RTmodel as a third competing mechanism beyond spray breakup length. The simulationresults of the model agree well with experiments in terms of the spray tip penetrationand shape. The fuel injection processes of a multi-hole gasoline injector werenumerically studied under different injection and ambient pressures by using thehybrid model. Results indicate that, at fixed injection pressures, elevated ambientpressures result in decreased overall breakup, but relatively intensified local breakupat the spray tips. Intensified breakup is observed when injection pressure is increased,especially at higher background pressures.A3-D simulation platform of a single cylinder GDI engine was established, andthe influence of the intake valve lift and injection timing on the heterogeneity of theair-fuel mixture when the engine was operating at the equivalence ratio of1wasstudied. Results show that the influence of intake valve lift on the mixture formationexceeds that of the injection timing. Lower intake valve lifts coupled with earlier fuelinjection enables lower heterogeneity of the fuel-air mixture.In order to effectively form ignitable mixture around the spark plug in the leanstratified combustion mode, a novel combustion chamber with a re-entrant type mainpiston cavity and an aligned auxillary cavity offset to the intake side was proposed,where the main cavity directs the fuel towards the spark and the auxillary cavity keepsthe late injected fuel within the piston bowl. By using such combustion chambergeometry, lean stratified combustion is realized in the GDI engine at different lateinjection timing. An injection strategy, which uses double injection and elevated second injectionpressure, was proposed to reduce the emissions resulting from the high heterogeneityof the mixture at lean stratified combustion mode while maintaining the ignitioncapability. Results show that the heterogeneity of the mixture equivalence ratio can bereduced by using such strategies, leading to increased burn rate, shortened combustionduration, decreased NOx, UHC and Soot.