Experimental Study of Operation Stability of a Spark Ignition Engine Fueled With Coal Bed Gas

Experimental research has been carried out on a single cylinder naturally aspirated spark ignition engine which was modified to operate with coal-bed gas fuel to investigate the method of improving operation stability and lean burn limit. Varied fuel composition with methane concentration 30–100% and CO₂ volumetric fraction 0–0.7 was employed to simulate coal-bed methane (CBM) and coal mined methane (CMM), respectively. Hydrogen was then employed to improve operational stability and lean burn limit. The results show that a stable operation range of the engine was obtained under most of the fuel compositions even if up to CO₂ volumetric fraction = 0.6 was employed. Besides lean burn limit, the unstable operation with COV_IMEP > 10% only appears at lean burn limit as well as CO₂ volumetric fraction = 0.7 at each equivalence ratio. The lean burn limit of coal-bed gas has been significantly enlarged from the equivalence ratio equals to 0.6–0.4 by hydrogen addition. Stable operation with COV_IMEP < 5% at the equivalence ratio equals to 0.4 has also been obtained at some high hydrogen concentration conditions. Hydrogen addition induced the reduction of both carbon monoxide (CO) and total hydrocarbon (THC) emissions at all equivalence ratio conditions, especially at the equivalence ratio equals to 0.4 and 0.6. CO₂ addition improves NO_x emission significantly; however, high CO₂ volumetric fraction will lead to unstable operation, which results in deteriorated CO and THC emissions. Hydrogen addition has benefits of improving operation stability and enlarging lean burn limit of coal-bed gas engine, which has practical significance to improve the application of coal-bed gas engine technology.