Investigation to charge cooling effect and combustion characteristics of ethanol direct injection in a gasoline port injection engine

Ethanol direct injection has the potentials to increase the engine compression ratio and thermal efficiency by taking advantages of ethanol fuel such as the high octane number and latent heat. In this study, CFD modelling and experiments were carried out to investigate the charge cooling effect and combustion characteristics of ethanol direct injection in a gasoline port injection (EDI + GPI) engine. Experiments were conducted on a single-cylinder spark ignition engine equipped with EDI + GPI over a full range of ethanol ratio from 0% (GPI only) to 100% (EDI only). Multidimensional CFD simulations to the partially premixed dual-fuel spray combustion were performed to understand the experimental results. The simulations were verified by comparing with the experimental results. Simulation results showed that the overall cooling effect of EDI was enhanced with the increase of ethanol ratio from 0% to 58%, but was not enhanced with further increase of ethanol ratio. When the ethanol ratio was greater than 58%, a large number of liquid ethanol droplets were left in the combustion chamber during combustion and fuel impingement on the cylinder wall became significant, leading to local overcooling in the near-wall region and over-lean mixture at the spark plug gap. As a consequence, the CO and HC emissions increased due to incomplete combustion. Compared with GPI only, the faster flame speed of ethanol fuel contributed to the greater peak cylinder pressure of EDI + GPI condition, which resulted in higher power output and thermal efficiency. Meanwhile, the mixture became leaner with the increase of ethanol ratio. As a result, the IMEP was increased, combustion initiation duration and major combustion duration were decreased when ethanol ratio was in 0-58%. The combustion performance was deteriorated when ethanol ratio was greater than 58%. Experimental and numerical results showed that the IMEP, thermal efficiency and emissions of this EDI + GPI engine can be optimised in the range of ethanol ratio of 40-60%.