Using gasoline and diesel simultaneously in a dual-fuel combustion system has shown effective benefits in terms of both brake thermal efficiency and exhaust emissions. In this study, the dual-fuel approach is applied to a light-duty spark ignition (SI) gasoline direct injection (GDI) engine. Three combustion modes are proposed based on the engine load, diesel micro-pilot (DMP) combustion at high load, SI combustion at low load, and diesel assisted spark-ignition (DASI) combustion in the transition zone. Major focus is put on the DMP mode, where the diesel fuel acts as an enhancer for ignition and combustion of the mixture of gasoline, air, and recirculated exhaust gas.
Computational fluid dynamics (CFD) is used to simulate the dual-fuel combustion with the final goal of supporting the comprehensive optimization of the main engine parameters. In the proposed automated procedure, the 1-D code GT-Power is used to provide the initial and boundary conditions for detailed 3-D simulations, carried out by the multi-dimensional CFD code CONVERGE. A dual-fuel chemical kinetic mechanism consisting of 43 species and 78 reactions is initially used in 0-D combustion simulations and the results are validated against experimental data of ignition delay. The dual-fuel mechanism is then implemented in the 3-D combustion model and results from numerical simulations of the entire engine cycle were validated against experimental data (pressure and rate of heat release traces) on a large variety of operating conditions consisting of 6 DMP modes and 1 SI mode.
Results show accurate prediction of combustion in all the examined cases. Two-stage heat release was observed in the DMP mode due to a gradient in fuel reactivity. Combustion is sensitive to several parameters, such as injection pressure and timing for gasoline and diesel, intake temperature, EGR rate, etc. The effect of such parameters on the dual-fuel combustion process is therefore evaluated.