A phenomenological relationship between Octane Number and Cetane Number and the impact of alcohols in transportation fuels

The Octane Number and Cetane Number metrics have long been used to characterize relative reactivities of fuels used in traditional, liquid-fueled spark ignited and direct injection compression ignition engines, respectively. The apparent inverse relationship between these two metrics has previously been reported, but a fundamental relationship between the two metrics has yet to be developed. In this study, a phenomenological relationship between these two traditional fuel reactivity scales is presented. The relationship is then compared against experimental results from a Waukesha Fuel Ignition Tester (FIT), a device typically used to measure Cetane Number. Fuel blends of primary reference fuels (PRFs and toluene/n-heptane) and n-butanol/n-heptane are examined and reveal very different ignition trends for alcohol fuels when compared to hydrocarbon fuels. Lastly, single-zone CHEMKIN® modeling is used to accompany the experimental results and suggests that much of the differences in the observed alcohol ignition trends are due to the relatively higher low temperature H abstraction coefficients of alcohols compared to traditional fuels.