Investigation On The Effects Of The Scavenged Pre-chamber Turbulent Jet Ignition On The Performance Of Gasoline Engine With EGR Dilution

EGR dilution combustion in gasoline engines can achieve high thermal efficiency and low emissions,but there remains a problem of unstable combustion under high EGR rate.Turbulent jet ignition(TJI),as an approach of using the hot jet from the prechamber to ignite the main combustion chamber mixture in a distributed manner,can improve the lean-burn stability of the engine and expand the diluted combustion limit.Therefore,in this paper,stable combustion of a gasoline engine under large EGR rate is achieved by the way of adopting TJI.This study is of great significance for improving thermal efficiency and reducing emissions of gasoline engines.In this paper,the experimental research is carried out based on the visualized constant volume combustion bomb(CVCB)and the single-cylinder engine platform,adoping the selfdesigned scavenged pre-chamber device.First,based on the visualization of CVCB,the jet and flame development at different EGR rates under three ignition methods,i.e.spark ignition(SI),passive TJI and scavenged TJI are studied.The results show that TJI ignites the main chamber through hot jets,which can form strong turbulent combustion.Compared with SI,TJI has a faster combustion rate.As the EGR rate increases,the flame propagation speed decreases,jet appearance from the pre-chamber is largely delayed,and the jet intensity is gradually reduced,which lead a lower combustion rate of the main chamber.By actively scavenging the pre-chamber with air,reducing the residual exhaust gas within the pre-chamber,the state of the mixture in the pre-chamber can be improved,so that the combustion rate within the pre-chamber is accelerated after ignition,and the jet intensity is not largely affected by the EGR rate,thereby accelerating the combustion in the main chamber and extending the EGR limit.Under the scavenged TJI ignition mode,the twice scavenging strategy can achieve a more thorough removal of the exhaust gas remained in the pre-chamber.Compared with the once scavenging,the jet intensity under twice scavenging strategy can be improved,thereby accelerating the combustion in the main chamber.Secondly,the combustion characteristics,knock characteristics and combustion stability of a real thermal single-cylinder engine under four ignition modes of SI,passive TJI,fuel-injected TJI and scavenged TJI are studied.The results show that the combustion duration of TJI is significantly shorter than SI,indicating that TJI has a higher combustion rate.And as the EGR rate increases,the SI engine is more prone to be unstable than the TJI engine.At a high EGR rate,scavenged TJI can fully improve the state of the mixture in the prechamber,increasing the jet intensity and accelerating the combustion in the main chamber,so it can achieve the highest combustion stability and the lowest fuel consumption,and it can further expand the EGR rate limit.Finally,the effects of scavenging strategy,jet aperture and compression ratio on engine performance are studied for scavenged TJI.The results show that the twice scavenging strategy can fully improve the combustion state of the prechamber and the main chamber,thus reducing the fuel consumption compared with the once scavenging strategy.Moreover,under the twice scavenging strategy,the combustion stability is not greatly affected by the scavenging timing.At a high EGR rate,a smaller jet aperture(3mm)is easy to cause jet quenching,and a larger jet aperture(6mm)is not conducive to reducing the residual gas within the prechamber,affecting the scavenging effect,both of which will hinder the stable operation of the engine under high EGR rates.The engine used in this experiment can achieve the best performance under the pre-chamber with a 4 mm jet aperture and 3.6 m L pre-chamber volume.In addition,increasing the compression ratio can improve combustion stability and fuel economy at high EGR rates.