Investigation Of Combustion Process In An Ethanol Direct Injection Plus Gasoline Port Injection Spark Ignition Engine

Ethanol is a widely used alternative fuel to address the issue of sustainability. Compared with gasoline, ethanol has many advantages, including greater latent heat of vaporization, larger octane number, faster laminar flame speed and smaller stoichiometric air/fuel ratio. To make the use of ethanol fuel more effectively and efficiently than the current E10 or E85 for spark ignition (SI) engines, ethanol direct injection plus gasoline port injection (EDI+GPI) has been in development to exploit the ethanol’s potentials in increasing the compression ratio and thermal efficiency and avoiding its drawbacks such as engine cold start issue. Previous experimental results have shown improvement in the performance of a single cylinder SI engine equipped with EDI+GPI. It was inferred that the merits of ethanol fuel played an important role, including the stronger cooling effect of EDI due to ethanol’s larger latent heat, stronger anti-knock ability due to its greater octane number and higher combustion efficiency due to its faster laminar flame speed. This study aimed to understand the fundamental mechanisms behind the experimental results by CFD simulation, constant volume chamber and engine experiments. A thorough understanding of the spray combustion characteristics in the EDI+GPI engine is of great importance to utilise ethanol fuel more effectively and efficiently, and thus facilitate the use of biofuels and protect the environment.Firstly, the spray and evaporation characteristics of ethanol and gasoline fuels injected from a multi-hole injector were investigated by high speed Shadowgraphy imaging technique in a constant volume chamber. Results showed that the spray could be considered as non-evaporating when the vapour pressure was lower than 30 kPa. Ethanol evaporated more slowly than gasoline did in low temperature environment, but they reached the similar evaporation rates when the fuel temperature was higher than 375 K. This suggested that EDI should be only applied in high temperature engine environment. For both ethanol and gasoline sprays, when the excess temperature was smaller than 4 K, the sprays behaved the same as the subcooled sprays did. The sprays collapsed when the excess temperature was 9 K. Flash-boiling did not occur until the excess temperature reached 14 K. The fuel temperature changed not only the spray evaporation modes but also the breakup mechanisms.Then, CFD modelling was carried out to investigate the spray and combustion characteristics of the EDI+GPI engine. The simulation results showed that the combustion process of EDI+GPI dual-fuelled condition was a typical partially premixed combustion because of the low evaporation rate of ethanol spray in the low temperature environment before combustion. 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 severe, 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 condition, 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 cooling effect, thermal efficiency and emissions of this EDI+GPI engine can be optimized in the range of ethanol ratio of 40-60%. Finally, the potential of increasing ethanol fuel temperature on improving the emission performance of the EDI+GPI engine was experimentally investigated. The results showed that the IMEP decreased slightly with the increase of ethanol fuel temperature. The CO and HC emissions decreased significantly and NO increased moderately with the increase ethanol fuel temperature. However even the increased NO emission was still smaller than that of GPI only condition due to the lower combustion temperature and stronger cooling effect of EDI. These indicated that increasing fuel temperature could be effective on reducing the CO and HC emissions while keeping the high thermal efficiency and low NO emission of the EDI+GPI engine.