Homogeneous-charge, compression-ignition (HCCI) combustion is triggered by spontaneous ignition in dilute homogeneous mixtures. The combustion rate must be reduced by suitable solutions such as high rates of Exhaust Gas Recirculation (EGR) and/or lean mixtures. HCCI is considered a very effective way to reduce engine pollutant emissions, however only a few HCCI engines have entered into production. HCCI combustion currently cannot be extended to the whole engine operating range, especially to high loads, since the use of EGR displaces air from the cylinder, limiting engine mean effective pressure, thus the engine must be able to operate also in conventional mode.
This paper concerns a study of an innovative concept to control HCCI combustion in diesel-fuelled engines. The concept consists in forming a pre-compressed homogeneous charge outside the cylinder and gradually admitting it into the cylinder during the combustion process. In this way, combustion can be controlled by the transfer flow rate and high pressure rise rates, typical of standard HCCI combustion, can be avoided. This new combustion concept was called Homogenous Charge Progressive Combustion (HCPC).
The paper illustrates a CFD analysis focused on improving efficiency and reducing pollutant emissions at medium and heavy load. Different geometries of the transfer duct were considered. Results show negligible soot emission up to equivalence ratios around 0.85, with indicated efficiency around 46 %. As well, a moderate level of external cooled EGR allows reducing NOx emissions up to levels that are typical of low-temperature combustion.