Numerical Study of the Effect of Injection Strategy and Compression Ratio on Gasoline/Diesel Fueled RCCI Engine 2018-32-0017

RCCI engine is proven to have better combustion control and to produce very low NOx and soot emissions. However, its operations is limited by HRR and PPRR as well as weak combustion efficiency which results in high levels of HC and CO emissions. Engine geometry and operation parameter such as injection strategy and compression ratio can affect the reactivity of fuels in cylinders as well as the gas temperature increase rate which are the important factors in controlling RCCI combustion. Injection strategies such as single and double injections have been previously studied but the effects are still unpredictable and the effects of compression ratio towards combustion characteristic and emissions require further analysis. This work deploys a 3D computational fluid dynamic (CFD) combustion model to study the effects on combustion characteristic and emissions with respect to single injection, double injection strategy and compression ratio. The model is validated by comparing the simulation result with a previous experimental work. The results show that double injections can produce more extensive combustion propagation than single injection. This is because the second injection of diesel fuel which is more reactive creates steeper reactivity gradient and equivalence ratio gradient throughout the combustion chamber. This also results in lower NOx, soot, HC and CO compare to single injection strategy. Single injection strategy combustion depends on the reactivity gradient and equivalence ratio gradient of injected diesel fuel which need to be at the right levels or else the combustion propagation will be retarded. The study then shifted to study the effect of compression ratio on the best parameter of double injection strategy. The results show that the increase of compression ratio can further reduce soot, HC and CO to a very low level while NOx is observed to increase. Lowering compression ratio results in worsening HC and CO emission due to incomplete combustion of gasoline, The incomplete combustion is likely to be caused by low gas temperature increase rate.