This study aims to obtain a strategy for optimizing the combination of diesel-fuel injection conditions and combustion chamber geometry to achieve high thermal efficiency with low exhaust emissions in a natural-gas/diesel dual fuel engine. Experiments were performed using a single-cylinder test engine with a common-rail injection system varying injection timings of two-stage pilot injection and piston bowl diameter. The pilot spray development was calculated using CFD simulation to understand the mechanism of experimental results. The experimental results showed that a smaller bowl diameter reduces THC and NOx emissions under a condition of high equivalence ratio, and the CFD results showed that the smaller bowl diameter provides a richer mixture of 1st pilot injection and a leaner mixture of 2nd pilot injection than a larger bowl diameter.
