Simulational Research On Cold Start Of Gasoline Direct-Injection Engine

In recent years, with a huge number of vehicles being produced and sold in China,vehicle emissions become serious threat to the city environment and its residents. In2010, the new State Phase4th emission regulation has been taken out, which increasesthe cold start phase emission limitation standerd. According to the emissionexperiment based on GB18352.1-2001<Emission Limitation Standard andExperimental Methord of Light Vehicles(Ⅰ)>, cold start(120s) emission persess90%CO emission,80%HC emission and60%NOx emission of the whole testcycle(820s).This paper focus on the start-up phase of a GDI engine, use CFD softwareAVL-FIRE as well as the engine experiments to research the fuel-air compacting andignition process, including injection time at60°CA BTDC,90°CA BTDC and120°CABTDC; injection time at90°CA BTDC and ignition time at13°CA BTDC,18°CABTDC and23°CA BTDC, to find the influence of different control regulation to thecold start situation. The conclusions included:1. By the CFD simulation study,it is found that the fuel injection and mixtureformation process of a gasoline direct injection engine is very complicated.The in-cylinder concentration field distribution is not only directly related to theinjection timing, but also has a direct relationship with the in-cylinder gas flow andthe shape of combustion chamber wall.2. The simulation results show that because of the lower the intake air temperatureand wall temperature, the combustion efficiency is very low.The maximum heat release rate peak on the start-up phase is20J/deg, far lessthan the70~90J/deg of heat release rate peak on normal operation condition of the engine.With the advance of the injection timing, the combustion temperature and heatrelease rate peak increase.With the advance of the ignition timing, the heatdissipation rate peak increases, and the peak time is advanced.3. When the injection time is90°CA BTDC, the in-cylinder mixture distribution atthe ignition time is the best, the combustion temperature and the heat dissipationrate are moderate, and the cold start process is completed within2cycles.Too early fuel injection will lead to low combustion temperature and heatdissipation rate,and the delay of fuel injection will leat to a substantial increase inthe emissions of unburned HC and CO;4. CO generated at combustion early stage increases, but with the continuation ofthe combustion process, the in-cylinder CO concentration is rapidly reduced.The in-cylinder combustion product concentration at the combustion later stagedetermines the final CO emissions.With ignition timing retarded, the maximum in-cylinder combustion temperatureis reduced.The mass fraction of NO in the combustion prophase is high, butgradually decreases in the combustion later stage.With the ignition timing retarded, the mass fraction of Soot is reduced.5. Test conclusion consistent with the result of the simulation: with the injectiontime at90°CA BTDC and the ignition time at18°CA BTDC, the gasoline directinjection engine can rapidly complete the cold start within2cycles, and thepollutant emissions are balanced.The ignition timing advance leads to higher NOx emission; retarded ignitiontiming can lead to misfire so that the HC emission is increased.6. Due to the low temperature of inlet air and the cold combustion wall, when theflame transfers to the cold wall, it will appear flame quenching in the combustionprocess. Flame quenching will lead unburned HC increase. Surface quenching effect is th e ma in source of unburned HC emission in enginecold starting condition.