Research Octane Number (RON) and Motor Octane Number (MON) have traditionally been used to describe fuel anti-knock quality. The test conditions for MON are harsher than those for RON, causing the RON for a particular fuel to be higher than the MON. Previous researchers have proposed the anti-knock performance of a fuel can be described at other operating conditions using the Octane Index (OI), defined as OI=RON-K (RON-MON), where âKâ is a weighing factor between RON and MON, and is a function of engine operating condition. The K-factor indicates that at a particular operating condition, knock tolerance is better described by RON as K approaches a value of 0, and MON as K approaches a value of 1. Previous studies claim that K-factor is dependent only on the engine combustion system and the speed-load point, and that it is independent of fuel chemistry. In most of these studies, K was determined experimentally using linear regression. In this particular study, K was determined using the PRF method for two test fuels; EPA certification tier 2 and tier 3 fuel. K was calculated for these fuels at multiple test points and the results showed that the K was different for the two fuels and thus it did depend on fuel chemistry. For a majority of the test points, the fuel with the lower RON and MON values (tier 3 cert fuel) had a lower K-factor as compared to the tier 2 cert fuel. A parameter was developed to relate engine speed, combustion phasing and cylinder trapped mass to predict K-factor, independent of the fuel. The correlation of K with end gas conditions was also investigated as a part of this project.
