Air entrainment and combustion process of high-pressure gas jet in gas direct injection engines

Gas engines, or piston engines burning natural gas become more popular not only in automobiles, but also in the fields of power generation and marine propulsion thanks to their lower fuel cost, more environment-friendly nature than their diesel counter parts. Although both premixed and diffusive combustion modes are used in gas engines, a gas direct-injection engine so called GI engine seems to be a favorable choice for large ocean-going ships, since the premixed combustion has fatal knocking problems and severe methane slip in larger engine dimensions. As for the combustion chamber configuration, GI engines have so high compatibility with large marine diesels that their injection strategy tends to follow them. GI engines, however, are said to have no efficiency improvement and longer after-burning period when compared with conventional diesels. It is necessary to investigate air entrainment behavior of a high-pressure gas jet and to optimize the injection conditions of GI engines. In the study, jet properties such as penetration length, jet cone angle, jet profile, and ambient velocity field around the jet were investigated in detail using a pressured constant volume chamber. Dynamic PIV system was applied to get the velocity field and the other properties were determined through shadowgraph jet images simultaneously taken with the PIV. The estimated air entertainment mass clarified the difference clear between a diesel spray and a high-pressure gas jet in their air entrainment process. Availability of the momentum theory for a liquid spray was examined in order to characterize the gas jet developing behavior. Brief firing experiments were additionally done with a Rapid Compression-Expansion Machine separately equipped with an original gas jet injector and a conventional diesel injector. The measured combustion processes successfully backed up the estimation results of air entrainments and implied the way to improve the diffusive gas jet combustion.