Comparative study of pilot fuel property and intake air boost on combustion and performance in the CNG dual-fuel engine

Abstract

As the pilot fuel property and equivalence ratio of the mixture exhibit significant effect on the combustion and emission characteristics in the dual-fuel operation mode with compressed natural gas (CNG), in this work, bio-fuel of n-butanol as the additive in diesel in conjunction with boost air intake pressure (Pin) was studied at indicated mean effective pressure (IMEP) of 10 bar. Various n-butanol blending ratios in the pilot fuel of B0 (pure diesel as the pilot fuel), B10 (90% diesel/10% n-butanol by volume basis as the pilot fuel), B20 and B30 were compared at Pin from 1.1 to 1.5 bar under two CNG substitution rates of 50% (CNG50) and 70% (CNG70). The experimental results revealed that for the increasing n-butanol content and decreasing Pin, the ignition delay is prolonged, and combustion duration becomes shorter. While CA50 is advanced with diesel/n-butanol as the pilot fuel at relatively higher Pin, e.g. 1.4 and 1.5 bar. Compared to CNG70, CNG50 can enhanced the indicated thermal efficiency (ITE), but also inducing higher maximum pressure rising rate (MPRR). Adding n-butanol can further improve ITE at 50% CNG relatively to pure diesel. For the emission characteristics, the THC and CO emissions can be reduced by adding n-butanol in the pilot fuel due to more homogeneous distribution of pilot fuel. However, much higher n-butanol content, such as B20 and B30, could lead to higher NOx emission. In addition, it is interesting to find that B10CNG50 can improve ITE, NOx, THC and CO simultaneously compared to B0CNG50 at relatively higher Pin.