Combustion initiation in an HCCI engine is dependent of several parameters that are not easily controlled like the temperature and pressure history in the cylinder. So achieving the same ignition condition in all the cylinders in a multi-cylinder engine is difficult. Factors as gas exchange, compression ratio, cylinder cooling, fuel supply, and inlet air temperature can differ from cylinder-to-cylinder. These differences cause both combustion phasing and load variations between the cylinders, which in the end affect the engine performance. Operating range in terms of speed and load is also affected by the cylinder imbalance, since misfiring or too fast combustion in the worst cylinders limits the load.
The cylinder-to-cylinder variations are investigated in a multi-cylinder Variable Compression Ratio (VCR) engine, and the effect it has on the engine performance. Different strategies to balance the cylinders are tested, i.e. static balancing of cylinder individual compression ratio and inlet air temperature, and closed loop control of cylinder individual combustion phasing using fuel offsets or inlet air temperature. The best engine performance regarding fuel consumption, combustion stability and emissions was found with cylinder individual combustion phasing control using inlet air temperature.
The test engine is a, 1.6L, four-stroke five-cylinder engine with Port Fuel Injection (PFI), Thermal Management of inlet air temperature, and Variable Compression Ratio (VCR). HCCI is achieved and combustion phasing is controlled with compression ratios up to 21:1 and adjustable inlet air temperature. The test fuel fulfills the specifications of US Regular and has a RON/MON of 92/82.