Development of a new reduced hydrogen combustion mechanism with NOx and parametric study of hydrogen HCCI combustion using stochastic reactor model

Abstract

Hydrogen is a potential alternative and renewable fuel for homogenous charge compression ignition (HCCI) engine to achieve higher efficiency and zero emissions of CO, unburned hydrocarbons as well as other greenhouse gases such as CO2 and CH4. In this study, a detailed hydrogen oxidation mechanism with NOx was developed by incorporating additional species and NOx reactions to the existing hydrogen combustion mechanism (10 species and 40 reactions). The detailed hydrogen combustion mechanism used in this study consists of 39 species and 311 reactions. A reduced mechanism consisting 30 species and 253 reactions was also developed by using directed relation graph (DRG) method from detailed mechanism. Developed mechanisms were validated with experimental data by HCCI engine simulation using stochastic reactor model. Sensitivity analysis was performed to identify the most important reactions in hydrogen combustion and NOx formation in HCCI engine. Pathway analysis was also performed to analyze the important reaction pathways at different temperatures. Results revealed that H2+HO2 [=] H+H2O2 and O2+NNH [=] N2+HO2 are the most significant reactions in the hydrogen HCCI combustion and NOx formation respectively. Detailed parametric study of HCCI combustion was conducted using developed chemical kinetic model. Numerical simulations are performed at different engine operating condition by varying engine speed (1000–3000rpm), intake air temperature (380–460K), and compression ratio (16–18) at different relative air fuel ratios (λ). The HCCI operating range was determined for different compression ratios and results show that operating range expands with increase in compression ratio. The effect of intake temperature, engine speed and equivalence ratio on cylinder pressure and heat release rate were investigated. Maximum thermal efficiency of 46% and maximum combustion efficiency of 98% was observed among all the test conditions. Parametric study of NOx emissions was also conducted and it was found that NOx emissions decrease exponentially from higher to lower engine loads.