Study on auto-ignition characteristics of ethanol and ETBE blended fuels in a gasoline HCCI engine

Bio-fuels such as ethanol (EtOH) and ethyl tert-butyl ether (ETBE) are used as blending components in gasoline for carbon neutralization. These oxygenates are also recognized as high octane number fuels, which are acceptable for spark ignition (SI) combustion, as they inhibit end-gas auto-ignition (partly leading to knocking). However, for homogeneous charge compression ignition (HCCI) combustion, excess delay in ignition timing may occur as an undesirable effect, leading to misfiring or incomplete combustion. In this work, knocking phenomenon and HCCI combustion characteristics of ethanol and ETBE blended gasoline fuels with same octane numbers have been experimentally studied. Chemical reaction simulations were also conducted for further understanding. Experimental work was performed in a single cylinder gasoline engine with compression ratio set to 14.0. Intake air was heated at HCCI combustion operation. SI combustion test results show that when the same octane number fuels are used, ethanol and ETBE have no effect on knocking limit. On the other hand, HCCI combustion results indicate that ethanol retards ignition timing at low intake temperature condition, whereas ignition advancement is observed at high intake temperature condition. ETBE acted as an ignition accelerator for all intake temperatures. Based on these results, an empirical equation is proposed for HCCI timing prediction of the tested blended fuels.