Dual Fuel Combustion of N-heptane/methanol-air-EGR Mixtures

Abstract

Numerical simulations are performed to study the combustion processes of n-heptane and methanol/air/EGR under high-temperature and high-pressure conditions relevant to a dual-fuel compression-ignition engine. Detailed chemical kinetic mechanism and transport properties are considered in the simulation. The simulations are carried out by performing three-dimensional (3D) direct numerical simulation (DNS) in a cuboid constant volume enclosure and two-dimensional (2D) DNS in a constant volume domain corresponding to the 3D domain. The results reveal three combustion modes involved in dual fuel engines, the ignition of the n-heptane jet, the propagation of thin reaction fronts in the methanol/air/EGR mixture, and finally the onset of ignition of the entire mixture. In dual-fuel combustion under high initial temperature conditions (1000K or higher) the three different combustion modes occur rather quickly with two distinct peaks of heat release corresponding to the two ignition modes. At low temperatures (800K or lower) the ignition delay time of the mixture is longer and more complete mixing is achieved before the onset of a single ignition of the n-heptane/methanol mixture.