A new kinetic model for the closed-cycle diesel engine ignition and combustion characteristics

Abstract

To investigate closed-cycle diesel engine (CCDE) ignition and combustion characteristics, a new kinetic model (Carbon-dioxide Oxygen Environment, COE) is presented, which considered the CO₂ chemical effect based on diesel surrogate fuel (n-heptane, C₇H₁₆). Firstly, the COE model is established by simplifying the LLNL mechanism, adding the new reaction pathways using quantum chemistry and modifying the kinetic parameters. Secondly, a visualization constant volume combustion chamber experimental system is constructed to record the non-premixed ignition and combustion process of C₇H₁₆ under different CO₂/O₂ environments (35% CO₂/65% O₂, 40% CO₂/60% O₂, 50% CO₂/50% O₂, 60% CO₂/40% O₂). Finally, ignition characteristics, combustion characteristics, sensitivity analysis, and chemical effect of CO₂ under different CO₂/O₂ environments are discussed between simulation and experiment. The results show that the COE model is suitable for studying the closed-cycle diesel engine ignition and combustion characteristics, whose maximum errors of ignition delay time and ignition location are 6.1% and 6.7%, respectively. The phenomenon of combustion decay appears before the end of combustion at 35% CO₂ and 40% CO₂. The average flame length decreases significantly by 47.8% due to flame lift-off at 60% CO₂. The cumulative flame intensity decreases by 34.4% at 60% CO₂ compared to 50% CO₂. The sensitivity coefficient of H₂O₂ + M → 2OH + M to temperature increases by 6.2% as CO₂ increases from 40% to 60%. The ratio of the chemical to physical effects of CO₂ rises from 7.6% to 17.3% as the CO₂ volume fraction increases. The third-body effect promoting OH formation more significantly than the inhibitory effect of the direct chemical effect.