Abstract
Keywords hydrogen, spark-ignition engine, simulation, laminar burning velocity, residuals
Abstract In view of the recent commitments towards CO2 emission reductions, hydrogen is receiving increasing attention as a fuel eliminating any greenhouse gas emission and of which there is an unlimited resource. The authors aim to develop a computer model for the combustion of hydrogen in spark-ignition engines to optimise the performances of these engines (power output, fuel consumption, exhaust emissions).
A quasi-dimensional simulation programme was chosen as a compromise between accuracy and calculation time. During its development, the lack of a suitable laminar flame speed formula for hydrogen/air mixtures became apparent. A literature survey shows that none of the existing correlations covers the entire temperature, pressure and mixture composition range as encountered in spark-ignition engines. Moreover, there is ambiguity concerning the pressure dependence of the laminar burning velocity of hydrogen/air mixtures. Finally, no data exists on the influence of residual gases. A promising operating strategy for a hydrogen engine is load variation by using considerable exhaust gas recirculation (EGR) at the stoichiometric mixture and thus minimise NOx emissions while maximising power output. It is therefore important to be able to calculate the effect of residuals on the burning velocity. This paper looks at several reaction mechanisms found in the literature for the kinetics of hydrogen/oxygen mixtures. An extensive set of simulations with a one-dimensional chemical kinetics code is performed to evaluate these mechanisms. Their performance is judged by comparing with experimental data for varying equivalence ratio, temperature, pressure, and residual gas content. The calculated effect of residuals on the burning velocity is shown to be fairly accurate, and an extension to existing correlations that do not implement residual gas effect is proposed. The extension is valid for residual gas volume fractions up to 30%. The performance of the mechanisms to predict temperature and pressure effects is shown to be quite poor.
Finally, attention is given to the appearance of cellular flames. The occurrence of cellular instability for hydrogen flames at engine conditions is shown and implications for turbulent combustion models are discussed.