Mixed-mode homogeneous charge compression ignition—direct injection combustion on common rail diesel engines: An experimental characterization

In recent years, increasing attention has been given to the study of homogeneous charge compression ignition (HCCI) for automotive diesel engines, due to its potential to lower NO x and particulate matter (PM) emissions. However, controlling the combustion process and engineering a mixture preparation method that does not require significant modifications to the engine hardware are still significant barriers to a vehicle implementation. In this scenario, external mixture formation, performed through a diesel fuel atomizer, is a simple and reliable method to obtain a truly homogeneous charge, resulting in ultra-low NO x and negligible PM formation. This technology does not require any modification to the engine compression ratio or to the fuel direct injection (DI) system. The possibility of combining two separate fuelling systems allows for mixed-mode HCCI—DI combustion, where a diesel-type combustion is induced during HCCI operations, increasing the IMEP and the output engine torque. This would allow for a seamless transition from HCCI to conventional DI mode at high load, while retaining the HCCI combustion benefits on emissions at part-load engine operating conditions. Building on preliminary experimental results proving the feasibility and benefits of the HCCI combustion with external mixture preparation on a high-speed common rail diesel engine, the proposed work aims at extending the HCCI combustion characterization, in order to define an engine operating region for its application. Then, mixed-mode HCCI—DI operations were investigated as a strategy to overcome the IMEP limitations of pure HCCI combustion. The analysis of HCCI—DI combustion aims at studying the effect of important control parameters, such as EGR ratio and direct injection timing, on combustion and emissions, showing the effects of partial HCCI combustion at higher engine loads.