Investigation Into Partially Premixed Combustion in a Light-Duty Diesel Engine Based on Different Injection Strategies

The performance of Partially Premixed Combustion (PPC) relies heavily on the proper mixing between the injected fuel and the in-cylinder gas mixture. This pre-mixing aims to eliminate over-rich regions where the mixture forms soot, and at the same time to avoid the NO_X formation region by lowering the combustion temperature by introduction of a large amount of EGR The main effort of this paper focuses on investigating the characteristic of PPC combustion and a suitable injection strategy for achieving the PPC combustion mode. Two injection strategies (i.e. double and single injection) were investigated on a four-cylinder heavy-duty diesel engine operating at low, medium and high load conditions. Injection timing, injection pressure, the pilot-main interval, the pilot injected fuel mass, the ratio between the two pilot injection pulses for the double pilot injection as well as the comparison of single injection mode and multi-injection mode in acquiring PPC was swept to study the combustion behavior in terms of combustion heat release, combustion phase, emissions and different efficiencies. Finally a comparison between single and multi-injection strategies was performed. The experimental results indicated that the premixed and low temperature combustion are the key factors to reduce particulate matter (PM) and nitric oxides (NO_X) simultaneously. The early and late injection timing and optimized double injection strategy should be preferred for PPC as it offers good combustion controllability and combustion performance (i.e. low emissions and high efficiency) while keeping the combustion noise at very low levels. Single injection strategy aims at achieving PCI combustion by increasing ignition retardation and for the same objective multi-injection strategy fulfills that by changing pilot-main interval and pilot fuel mass in order to control main combustion. Compared to conventional diesel combustion, in low temperature premixed combustion mode, the peak cylinder pressure and peak pressure rise rate significantly decreased and NO_X and smoke emissions can be substantially improved but CO emission became worse. Compared with single injection strategy, multi-injection strategy targets at PCI combustion with lower cylinder pressure, peak pressure rise rate and considerably low NO_X emission by improving mixing of fuel and air.