The Experimental Research On Low-speed Pre-ignition In Boosted Direct Injection Spark Ignition Engine

The development and application of boosting direct injection downsized SI engine has been recognized as an effective way to solve the oil resource over-consumed problem,meet the increasingly stringent fuel regulation and exhaust regulation.With the increase of BMEP,low-speed pre-ignition,which self-ignites before the spark,has taken place.It can bring damage to the engine and is the main obstruction of downsized SI engine.Therefore,it is necessary to focus on the research on the mechanism,impact factor and control strategy.Firstly,based on a boosting direct injection SI engine,five modes of low-speed pre-ignition(LSPI)has been given by the constant acquisition of in-cylinder pressure.The abnormal combustion brought by LSPI are divided into light knock,heavy knock and super knock.Meanwhile,the oscillation pressure of LSPI has been discussed compared with normal combustion.What’s more,the impact factor of engine on LSPI has been researched,the results indicated that delay ignition time cannot restrain the frequencies of LSPI and super knock.Compared with the air-fuel ratio 1.0,when air-fuel ratio 0.9 has been taken,the frequencies of LSPI and super knock have decreased 49.7% and 98.2%.When air-fuel ratio 1.1 has been taken,frequencies of LSPI and super knock have decreased 76.9% and 98.2%.The increase of boosting pressure can promote the frequencies of LSPI and super knock,as well as the high or low temperature of cooling.Compared with the 70°C cooling temperature,when 75°C taken,the frequencies of LSPI and super knock have decreased 3.7% and 40%.While 85°C taken,the frequencies of LSPI and super knock have increased 20.5% and 200%.Lastly,the mechanism and control strategies of LSPI have been researched,the results indicated that as the oil percentage in fuel improved from 0% to 10%,the frequencies of LSPI and super knock have increasingly changed.When the exhaust valve close time were delayed from-3°aTDC to 37°aTDC,the frequencies of LSPI and super knock had decreased 46.6% and 60%.At the same time,the engine torque increased 8.0%,fuel consumption improved 3.4% and the exhaust temperature decreased 22°C.Compared to start of injection on 300°bTDC,the frequencies of LSPI and super knock on 240°bTDC respectively decline 96.0% and 88.2%.While the engine torque increases 2.5%,fuel consumption improved 1.2% and the exhaust temperature decreased 13°C.With the improvement of the injection pressure,the frequencies of LSPI and super knock firstly improved and then declined.Improving injection pressure from 12 MPa to 14 MPa,the frequencies of LSPI and super knock respectively declined 72.6% and 63.3%.The engine torque increased 1.1% while fuel consumption decreased 3.4°C and exhaust temperature declined 2°C.The intensity of LSPI is random and the control strategies are supposed to apply in restraining the frequencies of LSPI in order to avoid the damage of LSPI.