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 an innovative concept to control HCCI combustion in diesel-fuelled engines. This new combustion concept is called Homogenous Charge Progressive Combustion (HCPC). HCPC is based on split-cycle principle. The intake and compression phases are performed in a reciprocating external compressor, which drives the air into the combustor cylinder during the combustion process, through a transfer duct. A transfer valve is positioned between the compressor cylinder and the transfer duct. The compressor piston has a fixed delay, in terms of crank-angle degrees, with respect to the combustor piston. The combustion takes place after combustor TDC, thus, during the combustion process, the combustor piston moves downwards whereas the compressor piston moves upwards. As a consequence, the air moves from the compressor cylinder to the combustor cylinder. Contemporary with the air transfer, fuel is injected into the transfer duct, evaporates and mixes with the air, bringing about the conditions needed for homogeneous combustion.
The paper illustrates a CFD study on a new turbocharged HCPC engine with a smaller compressor cylinder. The new engine configuration allows reaching 6000 rpm with high indicated efficiency (45%) and clean combustion.