A Study of the Formation and Break-Up of a Diesel Spray for HSDI Diesel Engine Combustion Systems

The use of in-cylinder computational fluid dynamics (CFD) to model fuel and air interaction is increasingly being used to rapidly design and develop direct injection combustion systems. Use of such CFD techniques has substituted many physical engine testing experiments and hence shortened development times. However, the fundamental propagation of diesel fuel spray is critical in resolving air and fuel mixing characteristics. The lack of realistic measured diesel spray data and inappropriate phenomenological correlation lead the authors to investigate diesel fuel spray at conditions representative of a modern common rail equipped turbocharged and after-cooled HSDI diesel engine. Operating conditions were achieved in an optical rapid compression machine fitted with a common rail fuel injector. The initial stages of these investigations are described within this paper, where both stills and high speed imaging techniques were used. The influences of injector nozzle configuration, injection pressure and air charge conditions on the diesel fuel spray were examined using back-lighting techniques. Qualitative differences in spray structure were observed between tests performed with short and long injection periods. Changes in the flow structure within the nozzle could be the source of this effect. Differences in the fuel spray liquid core were observed between VCO (Valve Covers Orifice) and mini-sac nozzles, with the mini-sac nozzles showing a higher rate of penetration under the same conditions.