Abstract: Abstract: Nowadays, the performances of diesel engines under transient conditions become the research focus. Because of the frequent start and stop of the series hybrid diesel engine, the diesel engine is always in the steady state or idle speed switching state. In order to study the transient performance of diesel engine during the idle transient conditions, 3 indices are firstly adopted: The lag coefficient, which is an evaluation index for performance of diesel engine transient lag, the deterioration coefficient, which is an evaluation index for performance of diesel engine transient deterioration, and the transient mean, which is an evaluation index for comprehensive performance of diesel engine transient process. And then an analysis is carried out from the 3 dimensions of time, peak and mean value, and the influences of idle transient transition time and injection parameters on the maximum cylinder pressure, fuel flow and emissions are investigated. The test results show that the cylinder pressure, fuel consumption and emission performance of the diesel engine are worse than the steady state during the transient conditions. The deterioration of the engine can be reduced in the idle speed transient process by adjusting the transition time and smoke emission limit. With the increase of the transition time, the transient mean and lag coefficient of the maximum cylinder pressure decrease, and the deterioration coefficient increases; with the smoke limit increasing, the lag coefficient of the maximum cylinder pressure increases, and the deterioration coefficient and transient mean decrease. When the transition time is 10 s and the smoke emission limit is reduced by 10%, the lag coefficient of the maximum cylinder pressure gets the minimum value of 0.7. When the transition time is 3 s, and the smoke emission limit increases by 10%, the deterioration coefficient obtains the minimum value of 0.69. When the transition time is 10 s, and the smoke emission limit increases by 10%, the transient mean of the maximum cylinder pressure takes the minimum value of 5.79 MPa. With the increase of the transition time, the lag coefficient, deterioration coefficient and transient mean of fuel flow first increase and then decrease; with the increase of smoke emission limit, the lag coefficient and deterioration coefficient of fuel flow increase, and the transient mean decreases. When the transition time is 10 s, and the smoke emission limit is reduced by 10%, the lag coefficient of fuel flow gets the minimum value of 1.1. When the transition time is 3 s, and the smoke emission limit increases by 10%, the deterioration coefficient obtains the minimum value of 1.01. When the transition time is 10 s, and the smoke emission limit is increased by 10%, the fuel flow gets the minimum value of 12.49 kg/h. With the increase of the transition time, the lag coefficient and the transient mean of NOx emission decrease, and the deterioration coefficient increases; with the smoke emission limit increasing, the lag coefficient and the deterioration coefficient of NOx emission increase, and the transient mean decreases. When the transition time is 10 s, and the smoke emission limit is reduced by 10%, the lag coefficient of NOx emission gets the minimum value of 1.1. When the transition time is 5 s, and the smoke emission limit is reduced by 10%, the deterioration coefficient obtains the minimum value of 1.046. When the transition time is 10 s, and the smoke emission limit is invariable, the transient mean of NOx emission takes the minimum value of 7.23×10-4. With the increase of the transition time, the lag coefficient of smoke emission first increases and then decreases, and the deterioration coefficient and transient mean increase; with the smoke emission limit increasing, the lag coefficient of smoke emission increases, and the deterioration coefficient and transient mean decrease. When the transit time is 10 s, and the smoke emission limit is reduced by 10%, the lag factor of smoke emission obtains the minimum value of 0.7. When the transition time is 3 s, and the smoke emission limit increases by 10%, the deterioration coefficient obtains the minimum value of 5.2. When the transition time is 3 s, and the smoke emission limit is increased by 10%, the transient mean of smoke emission takes the minimum value of 0.025 m-1. These 3 indices can be used to evaluate the performance of the diesel engine in the transient process, which can provide the basis for further optimization.