Visualization and Modeling of Pilot Injection and Combustion in Diesel Engines

An endoscope-based image acquisition-and-processing camera system was used for diagnostics of pilot injection combustion in a single-cylinder heavy duty diesel engine. A study of the pilot injection or light load is of interest because the spray breakup, mixing and vaporization processes are less influenced by heat feedback from the flame than in full injection cases. This allows the spray process to be decoupled from the combustion process. The experimental cases were modeled using a version of the KIVA-II code that includes improvements in the turbulence, wall heat transfer, spray, ignition and combustion models. Pilot injections of three different amounts (10, 15 and 20% of the fuel injected at 75% load and 1600 RPM) at different start-of-injection timings were studied. The imaging system included an endoscope, an intensified CID camera, a frame grabber and the control circuitry. The combustion of the pilot injections was characterized by ignition sites located below the point where the spray impinges on the piston bowl surface for the injector configuration used in this study. Multiple ignition sites were observed and the majority of the combustion occurred at the, bottom of the piston bowl, spreading along the bowl edge. As the start-of-injection timing was retarded some evidence of ignition above the impingement point was observed. However, these ignition sites did not develop into major combustion zones. Comparisons of the experimental results, which included pressure traces, heat release rates and the luminous flame images, and the numerical computations were made to assess the performance of current models in the KIVA-II code. Good agreement was obtained for the timing and location of ignition. The penetration of the flame observed in the luminous flame images was also captured in the predictions. A feature of the pilot injection that was not captured by the prediction is the spread of the flame along the edge of the piston bowl. The comparisons indicate that recent improvements in KIVA-II allow better prediction of the combustion of pilot injections.