High compression ratios are critical to increasing the efficiency of spark ignition engines, but the trend in downsized and down sped configurations has brought attention to the nominally low compression ratios used to avoid knock. Knock is an abnormal combustion event defined by the acoustic sound caused by end-gas auto-ignition ahead of the flame front. In order to avoid engine-damaging levels of knock, low compression ratios and retarded combustion phasing at high loads are used, both of which lower efficiency. Low carbon alternative fuels such as ethanol or water-based alcohol fuels combine strong chemical auto-ignition resistance with large charge cooling characteristics that can suppress knock and enable optimal combustion phasing, thus allowing an increase in the compression ratio.
Of course, these high cooling potential fuels are not immune to knock at high loads at high enough compression ratios and are subject to the same combustion phasing strategies (i.e., spark retard) that diminish efficiency. In this work, an injection strategy is characterized in which the cooling potential of water-based alcohol fuel such as hydrous ethanol (WE92 - 190 proof, 92% ethanol, 8% water by mass) is taken advantage of in order to suppress knock at high load conditions. This work explores the implications of extending the injection window into the compression stroke. The results demonstrated a maximum 1 percentage point increase in net fuel conversion efficiency and 2.0 degree CA50 advance at 17 bar IMEPn with 15% of the total fuel injected in the compression stroke.