A Method of Defining Ignition Quality of Fuels in HCCI Engines

A homogeneous charge compression ignition (HCCI) engine has been run at different operating conditions with fuels of different RON and MON and different chemistries. The ignition quality of the fuel at a given operating condition is characterized by CA50, the crank angle at which the cumulative heat released reaches 50% of the maximum value for the cycle. It is found that CA50 might show no correlation with either RON or MON but correlates very well with the Octane Index, OI defined as OI = (1-K)RON + KMON = RON - KS, where K is a constant depending on the engine operating condition and S is the fuel sensitivity, (RON - MON). The higher the OI, the more the resistance to autoignition and the later is the heat release in the HCCI engine at a given condition. When the engine is run with a boost pressure of 1 bar and with the intake air temperature maintained at 40° C, K is highly negative and fuels of low MON, such as those containing aromatics, olefins or ethanol, have a higher OI and ignite later than paraffinic fuels of comparable RON. When the engine is run with no intake boost and the intake air is heated to 120° C, the value of K becomes slightly greater than zero so that the autoignition quality of the fuel is determined mostly by its RON. Thus K decreases as the engine is run relatively cooler and at higher pressures and MON contributes less to the measure of resistance to autoignition. This trend is in line with earlier studies of autoignition in knocking engines.