Gas Versus Spray Injection: Which Mixes Faster?

Results are presented of 3-D computations of direct injection of gaseous methane and of liquid tetradecane through a multi-hole injector into a Diesel engine. The study focusses on the distribution of fuel/air ratio within the resulting gas and spray jets under typical Diesel conditions prior to ignition. It is shown that for a significant time after start of injection, the fraction of the vapor fuel which is in richer-than-flammable mixtures is greater in gas jets than in sprays. For methane injection, it is also shown that changing some of the flow conditions in the engine or going to a poppet-type injector, does not result in improved mixing. An explanation of these results is provided also through an analysis of the self-similar gas jet and 2-D computations of gas and spray jets into constant pressure gas. A scaling for time and axial distance in the self-similar gas jet also clarifies the results. In all, it is concluded that in general a spray mixes faster than a gas jet in direct-injection Diesel engines.