Multidimensional Simulation of the Influence of Fuel Mixture Composition and Injection Timing in Gasoline-Diesel Dual-Fuel Applications

Homogeneous charge compression ignition (HCCI) combustion is considered to be an attractive alternative to traditional internal combustion engine operation because of its extremely low levels of pollutant emissions. However, there are several difficulties that must be overcome for HCCI practical use, such as difficult ignition timing controllability. Indeed, too early or too late ignition can occur with obvious drawbacks. In addition, the increase in cyclic variation caused by the ignition timing uncertainty can lead to uneven engine operation. As a way to solve the combustion phasing control problem, dual-fuel combustion has been proposed. It consists of a diesel pilot injection used to ignite a pre-mixture of gasoline (or other high octane fuel) and air. Although dual-fuel combustion is an attractive way to achieve controllable HCCI operation, few studies are available to help the understanding of its in-cylinder combustion behavior. In this paper, numerical simulations of dual-fuel combustion processes are presented. An implemented version of the multi-dimensional CFD KIVA3V code was used for the study. This version incorporates various advanced sub-models, including the G-equation flame propagation model coupled with CHEMKIN. The influence of injection timing and mixture composition on emission production, as well as on combustion pressure and on heat release rate has been analyzed with two different multi-hole injectors (six and eight holes). Four combustion strategies were compared, and pressure, heat release rate, temperature and emissions results are provided. As expected, decreasing the amount of diesel fuel injected, combustion behavior becomes closer to HCCI combustion, especially with the eight-hole injector. An effective ignition controllability has been obtained with very low percentages of injected diesel fuel for the whole set of considered injection timings.