Diesel Engine Low Temperature Combustion Mode Shift Study Based On Transient EGR Control

The dual combustion mode diesel engine has performance deterioration problem in the combustion mode switching process because the response time of the control parameters of the EGR system and the fuel injection system are different.This paper studies the problem as follows:Firstly,the stable low temperature combustion is realized by two-stage injection strategy and large EGR rate and the load boundaries at certain engine speed are determined.Secondly,the relationship among EGR rate,EGR valve opening degree and engine speed is studied.The research shows that during the valve opening process,the change time of the EGR rate decreases with speed of the same opening degree,and the EGR rate change time decreases with the opening of the valve at the same speed.The EGR rate takes nearly 0.4s to decrease to zero and both the EGR valve position and engine speed have little influence on it.Secondly,the dual dimension combustion mode shifts at these load boundaries are conduct as the research foundation,the opening time of EGR valve is moved up and the injection strategy is modified in order to smooth the mode shift process.The result shows that COV_IMEP decreases by 73.6%and NOx emission spike is avoided by advancing the EGR valve opening time by 5 combustion cycles when conventional combustion mode shifts to low temperature mode at the upper load boundary.The COV_IMEP deteriorates by advancing the EGR valve open timing when conventional combustion mode shifts to low temperature mode at the lower load boundary,but it decreases by 28.2%by adding the1:4 injection transitional strategy.As for the shift from low temperature mode to conventional combustion mode,the COV_IMEP are 3.35%（at the upper load boundary）and 3.21%（at the lower load boundary）,which are acceptable levels for single-cylinder diesel engine.The coupling effect of EGR advancement strategy and injection strategy is studied in the end.The result shows that the coupling strategy with 1:2 injection strategy’s COV_IMEP is 4.78%at the upper load boundary from conventional combustion to low temperature combustion mode,which decreases by 33.6%compared with the direct shift’s COV_IMEP.