The main benefit of HCCI engines compared to SI engines is improved fuel economy. The drawback is the diluted combustion with a substantially smaller operating range if not some kind of supercharging is used. The reasons for the higher brake efficiency in HCCI engines can be summarized in lower pumping losses and higher thermodynamic efficiency, due to higher compression ratio and higher ratio of specific heats if air is used as dilution. In the low load operating range, where HCCI today is mainly used, other parameters as friction losses, and cooling losses have a large impact on the achieved brake efficiency.
To initiate the auto ignition of the in-cylinder charge a certain temperature and pressure have to be reached for a specific fuel. In an engine with high in-cylinder cooling losses the initial charge temperature before compression has to be higher than on an engine with less heat transfer. The heat transfer to the combustion chamber walls is affected by parameters such as area-to-volume ratio and in-cylinder gas motion, i.e. turbulence.
In this paper the performance of three multi-cylinder HCCI engines with different displacements are compared. The engines are a five-cylinder 1.6dm3 VCR engine, a four-cylinder 2.0dm3 engine, and a six-cylinder 11.7dm3 truck engine. All engines are port fuel injected and run with a RON91/MON82 gasoline. Combustion phasing is mainly controlled with inlet air temperature. The engines have about the same indicated efficiency but different brake efficiency. The truck engine has 32.3% brake efficiency at 2bar BMEP, followed by the 2.0dm3 engine with 29.8%, and the 1.6dm3 VCR engine with only 24.4%.