Transient Characteristics of Multi-Hole Diesel Sprays 900480

The spatial and temporal characteristics of a diesel spray injected into the atmosphere through a multi-hole nozzle used in small DI Diesel engines have been investigated by laser techniques as a function of pump speed and load. The results showed that spray tip penetration and velocity depend on injection frequency rather than injected volume and the spray is asymmetric during the early and main part of the injection period. In the time/space domain different structures have been identified within the injection period, with the early injection period characterized by a well atomized cloud of droplets, the main period by the spray head and a dense core and the late injection period by the disintegrating dense core and the spray tail.

IN DIRECT-INJECTION DIESEL ENGINES for passenger cars, fuel is injected through multi-hole nozzles at high pressure to promote mixing with the rapidly swirling air inside the combustion chamber. The key to improved fuel/air mixing is good atomization of the liquid fuel sprays within the millisecond injection period and this depends on nozzle geometry, injection conditions and fuel properties. For a given fuel and nozzle, the momentum of the injected fuel and the pre-injection flowfield control to a large extent the rate of fuel burning. Since the flowfield at the time of fuel injection is determined by chamber geometry, control of the spray development and positioning of the sprays in regions of high turbulence represent methods of improving combustion in the cylinder of small, high-speed direct-injection diesel engines.

Knowledge of the transient and spatial characteristics of diesel sprays is essential for further refinement of the injection system and improvement of combustion, but there are practical difficulties to obtaining data under normal operating conditions during and after fuel injection. Although laser techniques are capable of providing most of the required information, optical access to the near nozzle region between piston crown and cylinder head is not easily available near TDC. To over-come some of these difficulties and obtain data with temporal and spatial resolution adequate for the formulation and validation of predictive computer methods, fuel sprays have been examined in constant-volume combustion chambers at realistic pressures and temperatures, e.g. [1, 2, 3 and 4]* and at atmospheric conditions, e.g. [5, 6]. Although quantitative differences in spray droplet sizes and velocities are expected between the production engine and the simulated environment, essential physical insight can be obtained.

This paper presents the results of the second stage of a research programme aimed at characterization of transient single- and multi-hole diesel sprays under idealized and engine operating conditions and at providing boundary conditions and validation data for spray computer models. In the first stage [7], the structure of a diesel spray injected through a pintle nozzle, as used in many IDI diesel engines, into the atmosphere has been characterized by a range of complementary laser techniques as a function of pump speed and load. Here, the spray from a five-hole nozzle, to be used in a new generation of small high-speed DI diesel engines, is examined under similar conditions and conclusions are drawn about its transient development and spatial structure.