Effect of Reformer Gas on HCCI Combustion - Part I:High Octane Fuels 2007-01-0208

Homogeneous Charge Compression Ignition (HCCI) engines offer high fuel efficiency and some emissions benefits. However, it is difficult to control and stabilize combustion over a sufficient operating range because the critical compression ratio and intake temperature at which HCCI combustion can be achieved varies with operating conditions such as speed and load as well as with fuel octane number. Replacing part of the base fuel with reformer gas, (which can be produced from the base hydrocarbon fuel), alters HCCI combustion characteristics in varying ways depending on the replacement fraction and the base fuel auto-ignition characteristics. Injecting a blend of reformer gas and base fuel offers a potential HCCI combustion control mechanism because fuel injection quantities and ratios can be altered on a cycle-by-cycle basis.

This paper describes an experimental study of reformer gas fuel replacement effects on HCCI combustion with base fuels having sufficient octane number to allow spark ignition operating modes. This would be appropriate for engines designed to start and run at high power using spark ignition but using HCCI combustion to improve efficiency and emissions at part load. An experimental study was carried out using a CFR engine which was modified to achieve high compression ratio for HCCI combustion on primary reference fuel with 100 and 80 octane numbers. A variable blend of the base fuel with simulated reformer gas (75% H₂, 25% CO) was used to alter the HCCI combustion characteristics.

Experimental results indicated that combustion was retarded by increasing RG fraction in base fuel. It reduced maximum cylinder pressure and maximum cylinder pressure rise rate, leading to a smoother combustion. NOx remained minimal for all operating points. HC and CO increased slightly with RG addition as an indication of lower combustion temperature. It was found that RG replacement with high octane fuels has two advantages. First, it smoothed combustion by retarding combustion timing while maintaining the same λ. Second, it effectively controlled combustion timing when keeping λ and EGR ratios constant.