Numerical Investigation of In-Cylinder Stratification with Different CO ₂ Introduction Strategies in Diesel Engines

In order to improve the performance of low temperature combustion of diesel engines to achieve ultra-low emissions and load condition expansions, exhaust gas recirculation (EGR) stratification in the cylinder was proposed to further intensify local EGR concentration and reduce the amount of EGR to acquire high average oxygen concentration within cylinder. In this study, the intake/exhaust port and combustion chamber models were explored by CFD software on a four-valve HD diesel engine, and fresh air and EGR respectively replaced by O₂ and CO₂ were introduced with division and timing intake strategies during the intake process for stratification optimization. Computation results show that: divided intake strategy that CO₂ and O₂ respectively introduced into the helical and directional port has no obvious stratification of gas components within cylinder, but timing intake strategy that O₂ and CO₂ orderly introduced to both helical and directional port has much better O₂ mass concentration gradient at end of compression process, and unevenness characteristic of gas components has been better improved. Simulation data from this time-based intake strategy also shows greater effect on deceasing emissions of NOx and soot with a drop of 51% and 13% as compared with traditional even intake strategy; a longer ignition delay and higher peak heat release rate also can effectively help to improve thermal efficiency of the diesel engine, and the possibility of further LTC optimization for higher heat efficiency and emission reduction has been well demonstrated.