Impact of butanol isomers and EGR on the combustion characteristics and emissions of a SIDI engine at various injection timings

The impact of butanol on a SIDI engine operating with low pressure loop EGR was investigated in this study, with each of the four butanol isomers blended with gasoline by a volume fraction of 30%. Three injection timings during the intake stroke were selected, covering the representative scenarios of severe fuel impingement, adequate fuel-air mixing without obvious fuel impingement, and insufficient fuel-air mixing time prior to combustion. The results show that n-butanol, sec-butanol, and iso-butanol are capable of accelerating flame propagation and hence shorten the combustion duration, while tert-butanol would unfavorably extend the combustion duration. The addition of all the butanol isomers lead to reduced fuel consumption and reduced CO, unburnt HC and particulate number (PN) emissions, with n-butanol and sec-butanol showing more potential in high efficiency and low emission combustion at low load. EGR yields distinct influences on the PN emission at different injection timings: when injection starts at middle or late intake process, PN concentration decreases with EGR rate, which is consistent with previous studies; when injection starts early in the intake stoke, however, PN concentration would otherwise increase with the EGR rate independent of the fuel type, which could be possibly owing to the shift of the locally rich mixture in the Phi-T diagram towards the highly sooty peninsula with the cooling effect of EGR.