Soot emission prediction in pilot ignited direct injection natural gas engine based on n-heptane/toluene/methane/PAH mechanism

A dual fuel mechanism for pilot ignited direct injection natural gas engine was developed. Fuel blend of n-heptane and toluene was selected as the surrogate fuel for diesel, methane was selected as the surrogate fuel for natural gas. A reduced mechanism was adopted for methane oxidation while three skeletal mechanisms were used for n-heptane oxidation, toluene oxidation as well as PAH formation and growth respectively. The sub-mechanisms were then integrated into a complete mechanism with 84 species and 336 steps. Based on the mechanism, a 3D simulation model was built for pilot ignited direct injection natural gas engine and was then used to evaluate the effects of different natural gas post injection strategies and EGR addition. The simulation results indicated that CO and soot emissions can be effectively reduced with the adoption of natural gas post injection strategy, however, sacrifices in NOx emissions will be accompanied. With the addition of EGR, the higher NOx emissions resulted from the adoption of the post injection strategies could be avoided. Therefore, it can be concluded that the combined use of the natural gas post injection strategy and EGR addition is an effective way for the achievement of the engine optimized performance.