Simulation Analysis Of Automotive Diesel Engine Combustion Deterioration And Improving Strategy Under Transient Operations

In order to protect the environment, stringent emissions regulations has been adopted. Therefore, vehicle engine transient performance attracts more researchers attentions. In this paper, based on CA6 DL heavy turbocharged diesel engine, and use STAR-CD simulation, combined with apparent experiment results, the distribution of apparent engine parameters and combustion deterioration laws are revealed under different transient conditions. For the poor thermodynamic atmosphere of small load transient conditions, this study introduced pre-injection strategy to improve the poor thermodynamic atmosphere. For the bad fuel and air mixing effect, this study introduced post-injection strategy to improve the fuel and gas mixing effect. And then the optimal control strategyis formulated to effectively improve the transient performance deterioration under different load conditions.1. Extracted the 50% load corresponding duty cycle from the constant speed and increasing torque transient conditions, and then used the test parameters of this cycle as the boundary conditions to do the simulation and revealed the combustion deterioration laws of transient conditions.The results showed that the combustion deteriorated under transient conditions. The burning rate decreased and combustion phase laged. From the point of in-cylinder physics evolution, the decrease of “Driving energy of air-fuel mixing” resulted in the decrease of evaporation and air-fuel mixing rate, and the increase of rich mixture and the decrease of lean mixture. In addition, the unit cell ratio of in-cylinder temperature between 1800 < K <2500K, the unit cell equivalence ratio of φ>2 and Soot generating area ratio were higher than the values of the steady-state conditions, thus resulting in the deterioration of Soot emissions. But NOx emissions were less than that in steady-state conditions.2. Selected the constant speed and increasing torque transients(1650r / min, 5s, 0-100% load) 25% load conditions to simulate the pre-injection. The results showed that after the introduction of pre-injection, the early combustion pressure increased, even beyond the steady-state conditions value, the early combustion average temperature was lower than that of the steady-state conditions value, but the late combustion average temperature was higher. Evaporation rate values were significantly higher than the value of transient conditions without pre-injection, and higher than the steady-state conditions value. The early combustion burned evaporation fuel ratio increased significantly while residual fuel ratio decreased in the late combustion phase. The greater amount of pre-injection, the reduction was more obvious. At the same amount of pre-injection, the decrease of residual fuel ratio was more obvious when the injection interval was larger. “Oxygen driving energy” was improved when the pre-injection amount was over 10%. At the early combustion phase, “Fuel driving energy” increased and beyond the steady-state conditions value, but decreased at the late combustion phase. “Driving energy of air-fuel mixing” was improved before the main injection when the pre-injection amount was above 10%. The influence of pre-injection on the NOx emissions was not obvious, while the Soot emissions increased, even higher than the none pre-injection conditions. At last, selected 10% pre-injection amount and 5 ° CA pre-injection interval.3. Selected the constant speed and increasing torque transients(1650r / min, 5s, 0-100% load) 50% and 90% load conditions to simulate the post-injection.The results showed that after the introduction of post-injection, the in-cylinder pressure decreased slightly at the early stage, the average combustion temperature wass lower than the steady-state conditions value. While the average combustion temperature began to increased at the late combustion phase, even higher than the steady-state conditions value. Post-injection lead to higher cylinder late average temperature, providing a favorable environment for the Soot oxidation at the combustion phase. Besides, “Fuel driving energy” and “Driving energy of air-fuel mixing” also increased, especially at 90% load conditions. At the main injection stage, the peak ratio of thick mixture and rich mixture continued to decrease, while the lean mixture was rising. At the post-injection stage, thick mixture and rich mixture ratio appeared to rise, while lean maxture showed the opposite trends. It could be concluded that post-injection strategy can effectively control the forming of maxture. Soot emissions reduced,while NOx emissions decreased. At 50% transient load conditions, Soot emissions decreased 33.3% compared to the none post-injection transient conditions when the post-injection amount is 20%. At 90% transient load conditions, Soot emissions decreased 73.3% compared to the none post-injection transient conditions when the post-injection amount was 30%. Given Soot emissions, the indicated thermal efficiency and fuel consumption, 25% post-injection amount was adopted.