Knock Properties of Oxygenated Blends in Strongly Charged and Variable Compression Ratio Engines

Replacing the conventional fossil fuel totally or partially with alcohols or ethers in spark-ignition (SI) engine is a promising way to reduce pollutant emissions. A large number of studies on alcohol-containing blends in SI engines could be found in the literature. Nonetheless, investigations of ether-containing blends are by far much less numerous, especially for modern boosted engines. Blending with ether compounds might change the burning rate at high pressure, which consequently changes the anti-knock properties of these fuels and leads to a deterioration in the vehicle drivability. This work reports experiments carried out in two one-cylinder engines: one is a naturally aspirated, variable compression ratio engine, and the other is a strongly charged optical engine. Three fuels with different RON and MON numbers were tested: Iso-octane, a blend Ethyl Tert Butyl Ether (ETBE) with a primary reference fuel, and a commercial gasoline fuel containing 5% by volume of ethanol (E05). The experimental results show a significant difference of knock boundaries of three fuels in the boosted engine at the initial, i.e. equivalent of the intake manifold, pressure of 1.6bar, and almost similar knock boundaries under different compression ratios in the naturally aspirated engine. The fuel sensitivity upon the knock boundary of oxygenated blends was identified in order to compare the fuels' performance in different engines. The burning rate was determined at the same compression ratio for the two engines from the high speed flame imaging and a reverse-thermodynamic analysis, in order to clarify the effects of the burning rate on the anti-knock behaviour.