The critical role of swirl in high-specific-output diesel engines

Abstract

In-cylinder flows are critical for the understanding of the combustion process in internal combustion engines. Although swirl flow has been extensively employed as the primary in-cylinder flow motion in diesel engines, it is usually accompanied with reduced intake mass, and the necessity and the role of swirl flow in high-specific-output diesel engines still remain inadequately understood. In this work, the combustion process of a high-specific-output diesel engine is numerically investigated under different swirl ratios (SR). Results show that the indicated work exhibits a nonmonotonic variation of “increase–decrease” with increasing SR, peaking at SR = 1 with the shortest combustion duration yet slightly higher heat loss compared to the non-swirl condition. The introduction of swirl is capable of reducing high-equivalence-ratio regions through enhanced transport and turbulent diffusion, while the spray-spray interaction may be too strong to yield a rich region in the central cylinder when the swirl is excessively high. The chemical reaction pathway analysis indicates that a moderate swirl ratio could promote mixing, yielding an improved utilization of fresh O2 that further affect the reactions. Specifically, the highly reactive mixture enlarges the radical pool and yields a higher reaction rate during the mixing-controlled combustion stage; While an overlarge swirl ratio may cause excessive mixing, leading to reduced local availability of fresh O2 in the mixing-controlled combustion stage, thereby suppressing the oxidation of small molecule hydrocarbon and hence reducing the global heat release rate. Compared to the spray-induced turbulent mixing for the non-swirl case, the turbulence intensity is significantly increased through swirl-spray interaction under a moderate SR, which could preserve throughout late-cycle combustion and yield improved soot oxidation rate, while an excessive swirl would trap some soot within the central cylinder region and suppress late-cycle soot oxidation. According to the present results, solely high-pressure injection is not sufficient to control the combustion, particularly due to the poor late-cycle mixing and combustion, and an adequate swirl flow is still indispensable in high-specific-output diesel engines.