Modeling and Experiments on Mixture Formation in a Hydrogen Direct-Injection Research Engine

Direct injection offers a large number of degrees of freedom, as it strongly influences the mixture stratification process. Experiments on a single cylinder research engine fuelled by H2, carried out at Argonne National Laboratory, showed the influence of injection parameters (timing and geometry) on engine efficiency and combustion stability. At low load, when a late injection strategy was performed, an unstable engine behavior was detected varying the injection direction. In order to optimize the mixture stratification process in DI H-2 engines, it is important to understand the physics underlying the experimental results. A spatially resolved representation of the in-cylinder processes is a useful tool to properly set the injection parameters. Also, the knowledge of the pre-injection flow field is of added value in optimizing the injection process. This paper describes the initial development of a 3DCFD tool for direct-injection hydrogen engines, including numeric approach, simulation validation, and application to the mixture-formation process for a specific injection strategy. Simulations were performed by the commercial solver Fluent and the numerical results were compared to laser-based measurements of the fuel distribution in an optically accessible engine. The calculations were found to be reasonably accurate, especially for late injection timings. The simulation results show that intake-induced tumble combines with the jet momentum when the jet is pointed away from the intake valves, which can improve engine operation for early and slightly retarded injection. For late injection timings the engine can fire stably if the injector is pointed towards the spark plug.