A Reduced Reaction Mechanism for Predicting Knock in Dual-Fuel Engines

The present study extends our previous methane flame chemistry to methane autoignition based on most recent shock-tube experiments. It results in a detailed mechanism that consists of 128 elementary reactions among 31 species and that can be applied to predicting methane autoinginition times for temperatures between 1000 K and 2000 K, pressures between 1 bar and 250 bar and equivalence ratios between 0.4 and 3. A 9-step short mechanism is derived from this detailed mechanism with the objective of predicting knock in dual-fuel engines for equivalence ratio between 0.5 and 1.5 with temperature ranging 800 to 1200 K and pressure from 50 to 150 bar. To further simplify the computation, a systematically reduced chemistry, which retains essential features of the 9-step mechanism, is developed and consists of the following six steps:

Results of experiments and numerical computations demonstrate that the dominant factors influencing knock occurrence for specified compression ratio and equivalence ratios are θ_ƒs, the crank angle at which the premixed flame starts to propagate, and T_ic, the temperature of the fuel-air mixture when the intake valve is just closed. Good agreement is found between numerical predictions and experimental results for one particular engine. The present results can facilitate practical attempts at increasing knock resistance in dual-fuel engines.