Effects of Lambda on Knocking Characteristics and RON Rating 2019-01-0627
The knock resistance of fuels is measured through the ASTM RON and MON methods, but it has been well-documented that there has been increasing discrepancies between the fuel octane number and knock-limited performance in modern spark-ignited (SI) engines. Operating conditions during RON rating, such as engine speed, intake air temperature, spark timing, and compression ratio are defined by the ASTM D2699 method, but differ significantly from how conventional engine knock calibrations are performed, such as knock-limited spark advance sweeps. While octane ratings are performed for each fuel at the lambda that gives the highest knock intensity (based on the CFR knockmeter), modern SI engines are calibrated at stoichiometry and based on the high frequency cylinder pressure oscillations associated with knock (which the CFR knockmeter filters out). Subsequently, fuels with the same RON can vary significantly in their knock-limited combustion phasing based on engine operating conditions and fuel chemical composition. Other researchers have proposed new RON test conditions at stoichiometry which evaluate the knock performance of a fuel based on the high frequency cylinder pressure variations used for calibration with modern SI engines. In the present work, an instrumented CFR engine (still compliant to the standard ASTM setup) was used to analyze lambda effects on both the conventional RON rating method and cylinder pressure transducer based knocking characteristics. Multiple RON calculation methods were explored and contradictory trends were found based on the used reference parameter and the test fuel chemical composition. All fuels were formulated to a RON of 98, but varied in chemical composition so that the effects of isoparaffinic, aromatic, and ethanol content could be examined.
Alexander Hoth, Jorge Pulpeiro Gonzalez, Christopher P. Kolodziej, Toby Rockstroh