Investigation of Performance and Emission Characteristics on a Large-Bore Spark-Ignition Natural Gas Engine with Scavenged Prechamber and Miller Cycle Attribute

Adoption of methane scavenged prechamber has been considered as a promising solution to extend the lean limit of large-bore natural gas engines. In this paper, the effects of prechamber energy fraction, Miller cycle, excess air ratio, intake temperature, fuel methane number, and engine load are examined by a full set of experiments and detailed analysis on a single-cylinder large-bore natural gas engine. The results indicate that brake thermal efficiency (BTE) could be improved by increasing prechamber energy fraction, reducing excess air ratio, and using fuel with a higher methane number. It is also observed that brake thermal efficiency is higher at higher engine loads. Moreover, knock intensity is shown to be higher at higher prechamber energy fractions, higher intake temperatures, and lower methane numbers. For a methane scavenged prechamber engine, the cyclic variation of indicated mean effective pressure is maintained at a low level, attributed to the strong ignition energy. However, reducing air excess ratio and utilizing valve timing with a higher Miller degree are capable of reducing cyclic variation to a lower level. Additionally, carbon monoxide (CO) emissions are lower at higher prechamber energy fractions, higher excess air ratios, and lower Miller degree, which first increases and then decreases with the increase of methane number. Hydrocarbon emissions could be controlled by increasing intake temperature, reducing prechamber energy fraction and excess air ratio, and using a higher Miller degree. Meanwhile, lower NOx emissions can be obtained by raising prechamber energy fraction and excess air ratio. It is also deduced that implementation of higher Miller degree has positive effect on NOx reduction. (c) 2017 American Society of Civil Engineers.