Analysis of Advanced Multiple Injection Strategies in a Heavy-Duty Diesel Engine Using Optical Measurements and CFD-Simulations

In order to meet future emissions legislation for Diesel engines and reduce their CO₂ emissions it is necessary to improve diesel combustion by reducing the emissions it generates, while maintaining high efficiency and low fuel consumption. Advanced injection strategies offer possible ways to improve the trade-offs between NOx, PM and fuel consumption. In particular, use of high EGR levels (⥸ 40%) together with multiple injection strategies provides possibilities to reduce both engine-out NOx and soot emissions. Comparisons of optical engine measurements with CFD simulations enable detailed analysis of such combustion concepts. Thus, CFD simulations are important aids to understanding combustion phenomena, but the models used need to be able to model cases with advanced injection strategies. Thus, in the study presented here, engine tests were performed with settings selected to simplify CFD simulation, with long dwell times between the injections and only injection changes between engine settings in test cases presented in this paper. The key to reducing both soot and NOx emissions by applying pilot injections is that the pilot injected fuel should not ignite before sufficient mixing/lean-out has occurred. Hence, substantial heat releases prior to the main injection must be prevented. Thus, high EGR levels are needed to increase the bulk gas mass and reduce the temperature so that there is sufficient time for the pilot injections to mix and become locally lean before ignition.