A numerical study on the ignition of lean CH4/air mixture by a pre-chamber-initiated turbulent jet 2020-01-0820
Ultra-lean combustion in a spark-ignited (SI) engine is one of the most promising methods explored to reduce the nitrogen oxides (NOx) emissions and increases engine efficiency by decreasing the peak combustion temperature. However, this leads to an increase in combustion instabilities, hence emissions of unburned hydrocarbons (UHC). A spark ignited stratified pre-chamber combustion system has been demonstrated to overcome the obstacles related to ultra-lean combustion. However, the ignition mechanism of the ultra-lean mixture is not well studied. For that, the turbulent jet forced out of the pre-chamber through the nozzles needs to be well understood. The turbulent jet ignition can be either flame ignition (hot active radicals) or jet ignition (hot combustion products), depending on the pressure gradient and nozzle diameter. This hot jet determines the combustion characteristics of the lean main-chamber mixture and engine performance. Converge CFD code was used in this study, and the model used were validated against the measurements performed in-house. To elucidate the main-chamber ignition mechanism, the spark plug location and the spark timing were varied, resulting in different pressure gradients during the turbulent jet formation. It is observed that the high-pressure gradient during turbulent jet formation causes flame ignition whereas low-pressure gradient results in jet ignition in the main chamber.
Sangeeth Sanal, Mickael Silva, Ponnya Hlaing, Emre Cenker, Bengt Johansson, Hong G. Im
King Abdullah University of Science & Technology, Saudi Aramco