RON and MON chemical kinetic modeling derived correlations with ignition delay time for gasoline and octane boosting additives

Abstract

For regulatory and certification purposes the indices that characterize the ignition propensity for a fuel, Research Octane Number (RON) and Motor Octane Number (MON), are measured in a Cooperative Fuel Research (CFR) engine. In an effort to reduce the cost and time of CFR engine testing, computer simulation based work capable of predicting the octane numbers for any fuel blend has received increased attention. Notably, the works of Westbrook et al. [1], Badra et al. [2], and Kim et al. [3] simulated the second stage hot ignition delay time (IDT) for fuels and correlated their ignition delay time with experimental RON/MON measurements to determine the relationship between calculated IDT and RON/MON, thereby providing a means to analytically assess the apparent RON/MON and octane sensitivity (OS=RON-MON) values for the fuel. Using a similar methodology, the current study investigated RON-like and MON-like simulated engine compression histories calculated using GT-Power. The current work's calculated IDTs were determined for mixtures of primary reference fuel (PRF) gasoline and major blending components (i.e., iso-octane, n-heptane, ethanol, and toluene), and neat octane boosting additives (e.g., ethyl-benzene, iso-butanol, di-iso-butylene (DIB)) using 0-D closed homogeneous reactor chemical kinetic simulations driven by volume-time and pressure-time compression profiles. The primary goal of the current work was to explore any possible dependence of the fuel's predicted octane number with the compression profile used (i.e., volume-time profile and pressure-time profile). To investigate any possible dependence, the calculated IDTs for both the volume profile and pressure profile driven simulations were used to develop RON and MON correlation curves based on the PRF and neat fuel experimental literature data from McCormick et al. [16]. The model derived RON and MON correlation curves were used to analytically assess the octane numbers of PRFs, toluene PRFs (TPRFs), and both PRFs and TPRFs blended with ethanol for comparison with experimental data from Foong et al. [13]. The simulations indicated a synergistic blending effect on octane number for ethanol blended with PRFs and TPRFs. Overall, the volume profile driven RON/MON/OS assessments had less difference from the experimental PRFs and TPRFs data (i.e., on the order of approximately +/- 2-6 ONs) than the pressure profile driven RON/MON/OS assessments (i.e., greater than approximately +/- 6 ONs). Lastly, the volume and pressure profile driven RON and MON assessments for selected mixtures were compared to the RON and MON assessments completed in [1] which were determined with pressure profile driven simulations.