A Computational Investigation of the Effects of Swirl Ratio and Injection Pressure on Mixture Preparation and Wall Heat Transfer in a Light Duty Diesel Engine

摘要

In a recent study, quantitative measurements were presented of in-cylinder spatial distributions of mixture equivalence ratio in a single-cylinder light-duty optical diesel engine, operated with a non-reactive mixture at conditions similar to an early injection low-temperature combustion mode. In the experiments a planar laser-induced fluorescence (PLIF) methodology was used to obtain local mixture equivalence ratio values based on a diesel fuel surrogate (75% n-heptane, 25% iso-octane), with a small fraction of toluene as fluorescing tracer (0.5% by mass). Significant changes in the mixture's structure and composition at the walls were observed due to increased charge motion at high swirl and injection pressure levels. This suggested a non-negligible impact on wall heat transfer and, ultimately, on efficiency and engine-out emissions. In this work, the extensive and quantitative local information provided by the PLIF experiments was used as the reference for assessing the accuracy of the CFD modeling of the engine. The KIVA3V-ERC code was used, with a sector mesh featuring high spatial resolution (about 0.1 cm). A compressible model for the extended piston and connecting rod assembly was introduced, and observed to significantly improve modeling of motored engine operation. The validation was then further extended by comparison with measured in-cylinder equivalence ratio distributions over a broad parameter range, and with measured average pressure and apparent heat release rate traces. Finally, an analysis of the effects of varying fuel injection pressures (500 - 2000 bar) and nominal swirl ratios (1.55 - 4.5) on the heat losses caused by different flow fields at the liner and piston bowl walls was conducted. The results showed the sensitivity of the combustion timing to swirl- or injection-induced wall heat transfer, and its interaction with equivalence ratio stratification.