Influence of Pilot Pre-Injection Mass on Partially Premixed Dual Fuel (PPDF) Engine Combustion

Abstract

-The development of advanced techniques of diesel engines brings many novel ideas to dual fuel engines, where the key to combustion mode change is essentially the optimization of the pilot injection strategy. Splitting the diesel pilot into a main injection preceded by a pre-injection could bring about a change in the combustion mode from the conventional diesel dual fuel (DDF) combustion to the novel partially premixed dual fuel (PPDF) combustion; if the pre-injection part of the pilot is optimized for timing and quantity. In diesel engines with split injection, the pre-injection part is set to a certain percentage of the total diesel fuel supplied to the engine at the particular operating condition. This could be challenging to PPDF combustion, where the relative proportions of gaseous fuel and diesel could change at the same power level produced. With high levels of gaseous fuel substitution, the mass of diesel pre-injection could drop substantially and potentially lose its effect. The present work aims at investigating the influence of pilot pre-injection mass on PPDF combustion and performance at part load conditions. A comparative study was conducted on a modern automotive direct injection (DI) diesel engine working on PPDF with methane and diesel, between two pre-injection strategies; namely the fixed ratio (FR) and the fixed mass (FM). The tests were performed at different engine speeds ranging from 1400 to 2000 rpm at 25% of the engine load at the particular speed, with different substitution ratios of methane for diesel fuel (from 20 to 80% on energy basis). Cylinder pressure, rate of pressure rise (ROPR), coefficient of variation of gross IMEP (COVIMEPgross), heat release rate (HRR), combustion phasing (CA50), brake thermal efficiency, total equivalence ratio, and total diesel fuel injected; are all studied and compared for both pilot pre-injection strategies.