Measurement And Comparative Analysis Of Gasoline Engine Transient Performances In The States Of Cold Start Vs.Warmed Up

Due to the complex and changeable driving conditions,the comparative study on the transient performance of the gasoline engine in the states of cold start and warmed up has certain guiding significance to improve the vehicle performance.Based on the steady state test,the further perfect transient testing method was used on a gasoline direct injection(GDI)engine under cold start and warmed up “New European Driving Cycle(NEDC)” tests respectively and the engine transient performance in two different states were compared and analyzed.The results show that the differences of engine performance under cold start and warmed up conditions is mainly in the early stage of NEDC test when the differences of engine temperature are great.There are slight differences in dynamic indicators such as torque and indicated mean effective pressure(IMEP)while the economic indicators are significantly different such as instantaneous fuel consumption and indicator thermal efficiency.To analyze the internal causes of the differences,the influence of operating parameters was revealed.1)The oil and coolant temperature of the engine under cold start condition are lower than that of warmed up condition and the lubricating viscosity of the oil is large.As a result,the friction is higher,which leads to a higher engine torque and IMEP requirements and bigger fuel consumption.2)In the early stage of cold start,the engine speed is higher,the intake temperature is lower and the intake density is bigger.The intake pressure and excess air coefficient in the corresponding time under cold start condition is higher than that of warmed up condition,thus the mixture gas is too thin and burned slowly.Besides this,the cyclic injection quantity is greater than that of warmed up condition,which leads to that the fuel consumption is higher and the indicated thermal efficiency is lower.3)Because the sparking time is retarded and the excess air coefficient is bigger under cold start condition.50% combustion position is retarded,the equivalence ratio is lower and 10~90% combustion duration is longer,which result in the lower effective expansion ratio(EER)and heat-power conversion efficiency.The indicated thermal efficiency is lower.Based on the test data and analysis results,optimization suggestions for the prototype are proposed.The more detailed calibration on the inlet pressure and cyclic injection quantity under idle condition can control the excess air coefficient more accurately and reduce the fuel consumption or unnecessary power output.Optimizing the control strategy of variable valve timing(VVT)can improve the dynamic and thermal efficiency.Under cold start condition,the fuel consumption can be reduced by increasing the ignition advanced angle appropriately when exhaust temperature is no problem.Proper oil pressure and current is helpful to reduce fuel consumption.This study modifies and shows a method of continuous test,analysis and evaluation for engine performance under a vehicle condition,which provides an effective tool for the calibration and improvement of vehicle performance under actual conditions.The research results provide a large amount of data for the subsequent study,which is of gr eat significance to improve the vehicle performance,especially the optimization of vehicle calibration strategy.