Using Reformer Gas to Enhance HCCI Combustion of CNG in a CFR Engine

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 NO_x 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. Overall, the combustion efficiency increased slightly though there was also an increase in indicated specific CO, possibly because of the CO content in any unburnt fuel.