In-cylinder flow and combustion modeling of a CNG-fuelled stratified charge engine

In-cylinder air-fuel stratification and combustion is evaluated in a CNG-fuelled IC engine using a transient, three-dimensional, computational fluid dynamic (CFD) model. Performance of the stratified mode of operation is compared with the premixed mode. The combustion process is modeled using the extended coherent flame model (ECFM-3Z). For simulating the ignition process, the arc and kernel tracking ignition model (AKTIM) is used. The combustion model is first validated with measured in-cylinder pressure data and other derived quantities such as heat release rate and mass burn fraction for the premixed case. A good agreement is observed between measured and simulated values. It is observed that there is a marginal improvement in terms of overall engine efficiency when the stoichiometric premixed case is compared with the lean stratified condition. However, a major improvement in performance is observed when the lean stratified case is compared with lean premixed condition. The stratified case shows a faster heat release rate which could potentially translate to lower cycle-to-cycle variations in actual engine operation. Also, the stratified cases show as much as 30% lower in cylinder NOx emissions when compared with the premixed case at the same engine load and speed, underscoring the potential of in-cylinder stratification to achieve improved performance and lower NOx emissions