Kinetic Modeling of the Influence of Cyclohexane on the Homogeneous Ignition of a Gasoline Surrogate Fuel

The importance of adding a naphthene in the form of cyclohexane in surrogate mixtures to emulate homogeneous ignition of fully blended research gasoline has been examined with kinetic modeling. On the basis of a nonlinear by volume octane blending model and a correlation of anti-knock index to simulated ignition delay time, a quinary surrogate mixture, including cyclohexane, primary reference fuel (PRF), toluene, and diisobutylene (DIB-1), has been formulated that matches the research octane number, motor octane number, and H/C ratio of the target fuel. Simulated ignition delay times for the quinary mixture and a quaternary mixture without cyclohexane have been compared to measured data for the target fuel in a shock tube and rapid compression machine. Kinetic analysis shows that there is an increased production of HO₂ during the induction period for the quinary mixture. This leads to an increased OH production/consumption ratio for PRF, toluene, and DIB-1 in the quinary mixture. Simulated homogeneous charge compression ignition experiments at naturally aspirated conditions show that predictions are sensitive to operating conditions. Predictions of intake temperatures needed for phase combustion at top dead center using the quinary mixture are closer to measured data than those using the quaternary mixture when the engine speed is increased from 600 to 1200 rpm and ϕ is >0.2. This is explained by the fact that the cool flame present for the quaternary mixture at 600 rpm disappears when the engine speed is increased to 1200 rpm because the time needed for combustion is not long enough to sustain the low-temperature reactions.