Nowadays, environmental concerns are posing a great challenge to DI Diesel engines. Increasingly tightening emission limits require a higher attention on combustion efficiency. In this scenario, computational fluid-dynamics can prove its power guaranteeing a deeper understanding of mixture formation process and combustion. A high efficiency Diesel engine can be developed only mastering all the parameters that can affect the combustion and, therefore, NOx and soot emissions. In this work, the development of an engine in order to fulfill Tier 4i emission standard will be presented. Originally, the engine was a two-valve engine supplied with a DPF. Since no SCR aftertreatment is supplied, NOx emission target are achieved through external exhaust gas recirculation and retarding the start of injection. In order to fulfill Tier 4i emissions, the main concern is on soot emission and, thus, the combustion chamber has been re-designed, through CFD simulations, leading to a better interaction between the flow field, the fuel spray and the piston bowl geometry.
Particularly, through intake phase simulations, performed with the CFD code Fire v2009 v3, different intake ducts, with different swirl ratio, have been simulated in order to provide a flow field as realistic as possible for the combustion simulations. Through combustion process simulations, performed with the CFD code Kiva, by varying different parameters the interaction between the swirl flow field, generated by the intake duct, the reverse squish motion, and motions aerodynamically generated by spray has been investigated leading to the definition of a new engine lay-out. The study shows how, given the need of retarded injection for limiting NOx emission, the decrease of swirl ratio, when combined with a proper piston bowl design, allows a significant decrease of soot emissions and the achievement of Tier 4i emission standard.
Combustion of a diesel engine with the M-combustion system. Effects of swirl ratio and surface temperature of combustion chamber.