Effect of Pilot Pre-Injection on Methane-Diesel Dual Fuel Engine Combustion

Abstract

Dual fuelling of diesel engines with natural gas (NG), mainly composed of methane, is considered an attractive approach to reduce the dependence on diesel fuel and provide a more eco-friendly alternative to conventional diesel engines. In this fuelling strategy, most of the engine power output is provided by the NG, while a pilot amount of diesel fuel is used as an ignition source to ignite the NG-air mixture. However, this engine configuration suffers from lower thermal efficiency, slower burning rate, increased engine instability, and higher CO and HC emissions; particularly at part loads. The present work aims at investigating the effect of using pilot pre-injection (i.e. splitting the diesel pilot into a main injection preceded by a pre-injection) on dual fuel engine combustion and performance at part load conditions. The experiments were conducted on a modern automotive direct injection (DI) diesel engine; properly modified to suite dual fuel operation with methane and diesel. 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). The results demonstrated that optimized pilot pre-injection could bring about a change in the combustion mode from the conventional diesel dual fuel (DDF) combustion to a novel two-stage combustion mode; referred to as partially premixed dual fuel (PPDF) combustion. With this strategy, combustion process becomes more intensified yet smoother, with around 17% lower rate of pressure rise (ROPR) and up to 5% improvement in the brake thermal efficiency. PPDF strategy also maintains the engine stability at acceptable levels (even with high substitution ratios) and allows leaner operation at all conditions; indicating lower levels of harmful emissions.