Numerical investigation is carried out to explore various strategies of combustion mode switching in a diesel engine operating at high power. Numerical results are compared with high power single cylinder (CAT 3401E) experiments for combustion phasing and emission characteristics. In this study CFD calculations are carried out using the KIVA CFD code with Large Eddy Simulation turbulence model and Direct Chemistry Solver sub-models. The advanced turbulence and combustion sub-models enabled more realistic visualization of the effects of single-cycle mode switching on in-cylinder flow structures, fuel-air mixing behavior and combustion phasing.
Two circumstances of mode switch are presented in this study. Mode switches are performed from traditional High Temperature Combustion to early injection PCCI combustion and vice versa. In this investigation several aspects of combustion control are investigated. The effects of fast response controls such as fuel injection timing on in-cylinder combustion are studied. Slow response controls, such as EGR and intake boost are not realized in single cycle mode switching. The effects of such transients on CA501 and emission characteristics are presented.
It is found that in-cylinder mixture charge preparation and gas exchange processes plays a vital role in the combustion control during mode transition. By moderating the injection schedule and employing internal EGR using VVT, mode transition can be safely achieved. Further studies shows, by actively controlling the in-cylinder fuel-air reactivity, an intermediate mode between traditional diesel and LTC can be obtained. This intermediate mode can then be used for an improved mode transition.