Investigation On The First Cycle Ignition Characteristics During Warm Start In GDI Engine

Because of increasingly attention on the energy shortage and environmental pollution, energy saving and emission reduction technology was caused widely attention. The start-stop technology (the vehicle was flameout in idling period and then could start quickly) which could reduce fuel consuming and emissions in urban conditions was widely accepted by many countries. There would be 2-3 strokes in the period between injection and ignition on port fuel injection engine. As a result, a long time delay existed in restart. However, it only needed one stroke on gasoline direct injection engine which provided favorable support for quick start-stop technology.For achieving quick start-stop technology on gasoline direct injection engine, fuels would be jetted into cylinder which is in the compression stroke or expansion stroke. However, in consider of hot stop and randomness of piston stop position, the mixture quality is uncertain. Moreover, these mixtures are heated by high-temperature cylinder wall and cylinder head. Therefore, the initial boundary conditions of ignition in start were special. In the paper, the research was carried out on the influence of different cooling liquid temperature, piston stopping position and injection parameters on ignition in cylinder of first injection. Additionally, based on the CFD commercial software FIRE, in different piston initial positions, numerical simulation was made on the dynamic distribution characteristics of mixture concentration field, temperature field and velocity field. Moreover the influence of different injection timing and qualities on mixture concentration near spark plug was analyzed. The main conclusions are as follows:a) When the engine started at 60°CA BTDC, the end of spray appeared entrainment phenomenon after injection and clockwise flow existed in cylinder. The temperature in cylinder was relatively low. Ignition was available near TDC when fuel quality was relatively more rarefied or proper. With fuel charge in cylinder being higher and injection timing being earlier, the possibility of misfire was lower. When the engine started at 90°CA BTDC, the influence of injection timing on combustion was little if the fuel charge was proper. When the engine started at 120°CA BTDC or 150°CA BTDC, the influence of injection timing on combustion was not obvious if the fuel charge was relatively more. However, to start at 150°CA BTDC, the possibility of self-ignition increased if the fuel charge was more rarefied. When the engine started at 180°CA BTDC, the later the injection timing was, the higher the temperature in cylinder was. The temperature gradient also increased if the injection timing was earlier. There was nearly no influence of injection timing on combustion when the fuel charge was proper or rarefied. For the fuel charge was relatively more, the possibility of spontaneous combustion increased if the injection timing was too early or too late.b) When the injection timing was at 90°CA BTDC, there were two adverse flows in cylinder which caused jets diffusing to chamber wall. Jets mainly moved to spark plug along the chamber wall if the injection timing was relatively early or late.c) When coolant temperature was 100°C, the initial piston position was less than 90°CA BTDC, it is available for ignition only based on the fuel charge being proper. Otherwise, there would be misfire phenomenon. As the crankshaft angle between the initial piston position and TDC was increasing gradually, the ignition zone expanded to the misfire zone of high fuel charge. When the angle was larger than 120°C, there was spontaneous combustion zone between misfire zone and ignition zone. Additionally, this zone expanded to ignition zone as the angle increased. The proper injection timing (60°CA BTDC --90°CA BTDC) and relatively high fuel charge were good for avoiding spontaneous combustion.d) In the process of hot engine start, the coolant temperature played significant roles on avoiding spontaneous combustion of mixture in cylinder if the injection timing was in compression stroke. The higher coolant temperature was, the larger spontaneous zone was.e) In ignition zone, for certain fuel charge, the influence of different injection timings on combustion was small. The pressure peak value of combustion was similar if the initial combustion positions were the same. In spontaneous combustion zone, the influence of different injection timings on combustion was large. As injection timing was earlier, the initial spontaneous combustion position was earlier, the pressure peak value also increased.