3D-CFD Analysis of innovative diesel combustion systems

Diesel engine design is a very complex challenge, which aims to find the best compromise between different hardware and calibration parameters. The guidelines that lead the diesel engine combustion system design are the need to comply with pollutant emissions regulations and to reduce fuel consumption and consequently to lower CO2 emissions levels. Moreover, to improve customer acceptance, noise, harshness and vibration have to be minimized as well, and vehicles performance has to be enhanced as much as possible. Although the current emissions standards for light-duty vehicles can be met only with an optimized aftertreatment system design, the “in-cylinder” control of pollutant emissions remains nevertheless of crucial importance. Therefore, the combustion process has to be carefully optimized in order to minimize engine-out pollutant emissions, that for a diesel engine are mainly represented by NOx and particulate matter (PM). In a diesel engine the undermixed zone within the combustion chamber can be widespread, leading to a lower air utilization, which translates in to lower thermal efficiency and increased PM emissions. An accurate combustion chamber design can provide an effective solution to the above mentioned issues, generating a proper turbulent intensity within the piston bowl and enhancing the flow motion in the squish region. Past research activities have confirmed how different piston bowl geometries can improve the engine behaviour in terms of emissions reduction and fuel economy. Generally, for light-duty diesel engine applications, a “re-entrant” bowl shape is used. Hence, the present work started to consider this geometry as the baseline concept. Then, a computational fluid dynamics analysis of other bowl shapes was carried out. The final goal was to identify, for a next generation light-duty diesel engine, new geometries capable to provide significant benefits both in terms of soot formation and fuel consumption, in comparison with the baseline concept. First of all, a “TWIN VORTEX” design was investigated. In this geometry, the presence of stepped-lip around the piston rim is able to split the fuel jet between the piston bowl and the squish region, forming a twin toroidal vortex. Then, an increase of effective air utilization can be obtained, leading to a higher thermal efficiency during the combustion mixing controlled phase. Moreover, being the fuel more evenly distributed, the soot emissions can be significantly reduced. Another interesting solution is represented by the so called “WAVE” concept. In this case the outer bowl rim presents a series of radial bumps. The fuel jet momentum is therefore redirected toward the central region where available oxigen is placed. Moreover, the radial bumps avoid flame to flame interaction and reduce rich zones formation. Then, a significant overall soot formation reduction can be achieved, while the better air utilization leads to a higher heat release rate in the late combustion phase. Therefore the fuel consumption can be remarkably lower than the baseline.