A Reduced Thermokinetic Model for the Autoignition of Low-Heating-Value Gases with Variable Cetane Ratings

Abstract

The autoignition of low-heating-value gaseous fuels for direct injection into diesel engines was studied in the context of providing a direct correlation and a methodology for rating their ignition quality. A computer model which includes a detailed chemistry and incorporates the effects of fluid mixing through the Damkohler number is presented. The computer model demonstrates the relative importance of the chemical kinetic characteristic time as compared to the characteristic flow time. It also demonstrates high sensitivity to the cylinder temperature and methane and propane concentrations in these types of low-quality fuels. Pressure was identified as a second-order variable. The deficiencies of the current cetane and methane number rating scales are discussed in terms of their inability to rate these fuels because of the range of the scales and their delay time fixed nature for variable compression ratios. These results show that the ignition characteristics of these fuels cannot be accurately modeled by the cetane scale, by the reference fuels, and for the whole range of cold start to warmed-up, ignition-assisted probable engines. An ignition delay correlation is presented, for a limited range of the engine parameters, in terms of the classical Arrhenius type of expressions as a function of the Damkohler number, the compression ratio, engine speed, and mixture composition. A rating scale corresponding to the range of valid parameters for this correlation is introduced as a linear scale for the delay time and a newly defined low-Btu number.