Numerical Investigation on Mixture Formation and Combustion Process of Innovative Piston Bowl Geometries in a Swirl-Supported Light-Duty Diesel Engine

In recent years, several innovative diesel combustion systems were developed and optimized in order to enhance the air and injected fuel mixing for engine efficiency improvements and to mitigate the formation of fuel-rich regions for soot emissions reduction. With these aims, a three-dimensional computational fluid dynamics (3D-CFD) numerical study was carried out in order to evaluate the impact of three different piston bowl geometries on a passenger car four-cylinder diesel engine, 1.6 liters. Once the numerical model was validated considering the baseline re-entrant bowl, two innovative bowl geometries were defined: one based on the stepped-lip bowl; the other including a number of radial bumps equal to the nozzle holes number. Firstly, the rated power engine operating condition was investigated under nonreacting conditions to evaluate the piston bowl effects on the in-cylinder mixing. Results highlight for both the innovative piston bowls better air utilization with respect to the re-entrant bowl: the stepped-lip bowl creates a dual toroidal vortex leading to a higher air/fuel mixing, while the radial-bumps bowl significantly affects the jet-to-jet interaction and promotes the recirculation of the fuel jet downstream to the bump, where the available oxygen enhances the mixing rate. After that, the combustion analysis was carried out for both rated power and partial-load engine operating conditions. Results confirmed that thanks to the better air-fuel mixing, the combustion process can be improved thanks to the innovative bowl designs, both increasing the engine efficiency at full-load condition and minimizing the engine-out soot emissions at partial-load operating point.