The effects of residual gas trapping on part load performance and emissions of a spark ignition direct injection engine fuelled with wet ethanol

Biofuels, such as ethanol, have been introduced as a solution to decrease total CO2 emissions from transport sector as well as an alternative to increase the domestic energy security against international fuel price fluctuations. The use of high water content ethanol, the so-called wet ethanol (ethanol with higher than 5% water content v/v), has been proposed to reduce ethanol production cost. This work presents the application of wet ethanol on a naturally aspirated direct injection single cylinder research engine equipped with a fully variable electro-hydraulic valve train running on stoichiometric air/fuel ratio. The negative valve overlap (NVO) strategy was used to retain high residual gas fraction (RGF) at the part load operation of 3.1 bar IMEP and 1500 rpm. Anhydrous ethanol and different wet ethanol compositions (10% and 20% water-in-ethanol content v/v) were tested for several NVO durations, as well as European RON 95 gasoline. A one-dimensional engine model was built and validated against experimental data to estimate the RGF for each operating point. It was possible to achieve stable stoichiometric operation with more than 35% RGF for anhydrous ethanol and RON 95 gasoline. On the other hand, the maximum supported RGF for stable operation decreased as the water-in-ethanol content increased. The increase in water content reduced the tolerance to hot residuals due to lower combustion temperatures, which lengthened the flame initiation and main combustion phases. Even then, the increase in NVO period resulted in net indicated efficiency gains for all fuels due to less pumping losses, lower combustion temperature, and the possibility to maintain combustion efficiency at acceptable levels even with the maximum achievable RGF of each fuel. Anhydrous ethanol presented the highest net indicated efficiencies, while 10% water-in-ethanol mixture presented slightly higher indicated efficiency compared to gasoline. 20% water-in-ethanol mixture provided the lowest indicated efficiencies over the whole range of tested RGF.