Determination of Heat Transfer Augmentation Due to Fuel Spray Impingement in a High-Speed Diesel Engine

As the incentive to produce cleaner and more efficient engines increases, diesel engines will become a primary, worldwide solution. Producing diesel engines with higher efficiency and lower emissions requires a fundamental understanding of the interaction of the injected fuel with air as well as with the surfaces inside the combustion chamber. One aspect of this interaction is spray impingement on the piston surface. Impingement on the piston can lead to decreased combustion efficiency, higher emissions, and piston damage due to thermal loading. Modern high-speed diesel engines utilize high pressure common-rail direct-injection systems to primarily improve efficiency and reduce emissions. However, the high injection pressures of these systems increase the likelihood that the injected fuel will impinge on the surface of the piston. This research focuses on identifying impingement in a high-speed direct-injection diesel engine as well as characterizing its effects on heat transfer to the surface of a piston. This is achieved by measuring the instantaneous surface temperature of one of the pistons in the engine using eight, fast-response surface thermocouples and a wireless microwave telemetry system for data transfer. By reorienting the fuel injector, and therefore repositioning the fuel spray plumes in the combustion chamber, the effects of the fuel spray impingement on the heat transfer to the surface of the piston have been identified at various locations on the piston. The extent to which the spray plume augments the heat transfer was quantified by calculating the instantaneous surface heat flux with the injector (i.e. spray plumes) in various positions. The minimum heat flux occurred between spray plumes where no impingement signature was found. The maximum heat flux occurred when the spray plume was impinging directly upon the thermocouples on the piston bowl lip. The difference in surface heat flux was determined to vary by a factor of three, between the two injector locations.