Effect of Reformer Gas on HCCI Combustion - Part II: Low Octane Fuels 2007-01-0206

Homogeneous Charge Compression Ignition (HCCI) combustion offers high fuel efficiency and some emissions benefits. However, it is difficult to control and stabilize combustion over a significant operating range because the critical compression ratio and intake temperature at which HCCI combustion can be achieved vary 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. Because fuel injection quantities and ratios can be altered on a cycle-by-cycle basis during operation, injecting a variable blend of reformer gas and base fuel offers a potential HCCI combustion control mechanism.

The basic diesel exhaust problem is a trade-off between unacceptable levels of NOx and particulate matter, so establishing an HCCI operating mode in a diesel engine is desirable since it simultaneously reduces those two pollutants. Modern diesels using a tightly controlled fuel injection system that allows for multiple, timed injections during each cycle can produce variable amounts of pre-mixture by early injection. Blending the base fuel with variable amounts of light reformer gas can affect HCCI combustion parameters and may provide an acceptable means of combustion control.

This paper examines the reformer gas effect on HCCI combustion of low octane number fuels, including primary reference fuels with 0 and 20 octane numbers. Experimental work was performed in a CFR engine modified for high mechanical compression ratio. A variable blend of base fuel with simulated reformer gas (75% H₂ / 25% CO) was used to alter the HCCI engine's combustion parameters and control combustion onset. PRF0 and PRF20 fuels contain mostly n-heptane so two-stage auto-ignitions were observed for both fuels. Replacing some base fuel with reformer gas generally expanded the engine operating range on the rich side and retarded combustion timing. The retarded ignition, coming despite the tendency of reformer gas to raise compression temperature, indicates that pre-flame chemistry is significantly altered. First stage heat release was reduced by increasing reformer gas content and it delayed the main stage of heat release. The capability to adjust HCCI ignition timing and heat release rate by varying the fraction of reformer gas offers a potential combustion control mechanism.