Effects of mixture inhomogeneity and combustion temperature on soot surface activity and soot formation in diesel engines

The soot surface growth plays significant role on the soot mass accumulation, which starts with H (hydrogen) atom abstraction forming activated soot surface sites, and is followed by the acetylene addition process. In this study, the effect of the mixture inhomogeneity and combustion temperature on the soot surface activity and soot formation was investigated by developing a new multi-step phenomenological (MSP) soot model of diesel engines. A new detailed soot surface growth mechanism was proposed by correlation analysis of combustion parameters with soot formation. The inhomogeneity coefficient of soot surface activity alpha (CH) and the specific rate of soot surface growth R (CH) were derived to highlight the effect of inhomogeneity of mixture and combustion temperature on soot formation. The predicted diesel engine-out soot agreed well with experimental findings in wide ranges of combustion conditions. In the case of lower engine load with single fuel injection and higher EGR (exhaust gas recirculation) rate, it had quiet homogeneous mixtures before ignition when the combustion temperature dominated the soot surface activity. At medium engine load with multi-pulse fuel injections, it got mixture slightly stratified before ignition and revealed that the mixture inhomogeneity became more dominated on soot surface activity than the combustion temperature. An increased soot surface activity led to increased soot emission. Under the full engine loads with single fuel injection but quite high boost pressure over 0.4 MPa, it led to the combustion conditions of higher mixture density and higher mixture heat capacity, which benefits the mixture homogeneity. The decay rate of soot surface activity became lower due to the decreased combustion temperature and the soot surface activity decreased due to improved mixture homogeneity. In addition, the lowered intake oxygen concentration due to usage of EGR played a role to lower the specific rate of soot surface growth R (CH), but to increase the soot surface activity alpha (CH).