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Although HCCI engines promise low NOx emissions with high efficiency, they suffer from a narrow operating range between knock and misfire because they lack a direct means of controlling combustion timing. A series of previous studies showed that reformer gas, (RG, defined as a mixture of light gases dominated by hydrogen and carbon monoxide), can be used to control combustion timing without changing mixture dilution, (λ or EGR) which control engine load. The effect of RG blending on combustion timing was found to be mainly related to the difference in auto-ignition characteristics between the RG and base fuel. The practical effectiveness of RG depends on local production using a fuel processor that consumes the same base fuel as the engine and efficiently produces high-hydrogen RG as a blending additive.

This paper describes use of reformer gas (RG) to alter and control combustion in a CNG-fueled HCCI engine. Experimental work used a mixture of simulated RG (75% H2 and 25% CO) to supplement base CNG fueling in a CFR engine upgraded to achieve high compression ratios. RG was used to improve the engine's operating performance and to control combustion onset in experiments conducted at three different compression ratios. A combination of high compression ratio (18.5) and high intake temperature (140°C) was observed to be appropriate to run the CNG-fueled CFR engine in HCCI mode. RG replacement of CNG altered combustion characteristics and expanded the operating range on the lean side. Use of RG decreased knock severity and reduced NOx emission. At constant relative air/fuel ratio (λ) it advanced combustion timing, moving the maximum cylinder pressure earlier in the cycle and increasing maximum pressure.