Development and Validation of a Reduced Chemical Kinetic Mechanism for Computational Fluid Dynamics Simulations of Natural Gas/Diesel Dual-Fuel Engines

A reduced chemical kinetic mechanism consisting of 141 species and 709 reactions has been constructed to simulate the combustion of both natural gas and diesel fuels in a dual-fuel engine. Natural gas is modeled as a mixture of methane, ethane, and propane, while the diesel fuel is modeled as n-heptane. The new reduced mechanism combines reduced versions of a detailed n-heptane mechanism and a detailed methane through n-pentane mechanism, each of which was reduced using a direct relation graph method. The reduced dual-fuel mechanism is validated against ignition delay computations with full detailed mechanisms, adiabatic homogeneous charge compression ignition simulations with full detailed mechanisms, experimental premixed laminar flame speeds of CH4/O-2/He mixtures at 40 and 60 atm, ignition delay and lift-off length from a diesel spray experiment in a constant-volume chamber, and finally against dual-fuel engine experiments using multidimensional computational fluid dynamics simulations. The engine simulations were performed for direct comparison against natural gas/diesel dual-fuel engine experiments at varying injection timings, engine loads, substitution percentages, and natural gas compositions. An engine experiment with additional propane was used to induce engine knock for the purpose of validating the reduced mechanism's ability to predict natural gas autoignition. The results show that the newly reduced mechanism accurately reproduces the chemical kinetic behavior of the detailed mechanism, including the laminar flame speed at high pressure, the ignition delay and lift-off length in the diesel spray experiment, and the pressure and heat release rate in the engine experiments. In the experiment where engine knock occurred, the model predicts the phasing and magnitude of a sudden acceleration in the combustion rate and reproduces the observed high-frequency pressure oscillations.