The Influence of Injection Rate Shaping on Diesel Fuel Sprays—An Experimental Study

A current strategy in the development of direct injection (DI) diesel engine combustion systems is the control and limitation of the initial ‘premixed’ combustion heat release ensuing from the auto-ignition of the injected fuel. This requires control of the amount of fuel vaporization and mixing taking place during the ignition delay time. Since the latter is determined by the fuel composition and the in-cylinder gas temperature, development efforts have focused on the injection of well-controlled, portioned fuel quantities prior to the ignition as a means of achieving the desired goal. This practice is becoming known as ‘fuel rate shaping’. Consequently, the fuel spray penetration during this period, fuel evaporation and mixture preparation, as well as the influence of in-cylinder air motion on mixture distribution, are main subjects of interest in affording insight into fuel rate shaping attempts. These have been addressed through a combined experimental and theoretical investigation of the spray characteristics associated with different injection practices.

The experimental investigations have been performed in an optically accessed spray research engine. Basic aspects of fuel spray tip penetration, time and location of auto-ignition and flame propagation have been recorded with a high-speed line-scan camera. The results provide the space and time-scale characteristics for the propagation, ignition and combustion of a selection of diesel fuel sprays.

Investigations have been carried out for a conventional fuel injection system equipped with a set of different single-hole injector nozzles, as well as for a dual-spring injector and an injector with a split injection device. The experimental results provide an insight into the propagation of the fuel spray front, yield qualitative information about its spatial and temporal distribution, and, in the case of split injection, show the interaction of the initial pilot fuel portion with the main injection.