Numerical Investigation of Methane Number Effect on Combustion, Performance, and Emissions in a Heavy-Duty Spark Ignition Natural Gas Engine

Abstract In this study, the effects of methane number (MN) on the numerical conversion of a heavy-duty diesel engine to a spark ignition (SI) natural gas engine were investigated. A high-compression-ratio configuration was considered, and six natural gas compositions with MN values ranging from 50 to 100 were analyzed. The results showed that increasing MN improved engine performance, with gross indicated power, indicated mean effective pressure, and indicated thermal efficiency increasing by 2.2%, 2%, and 4.1%, respectively, while indicated specific fuel consumption decreased by 3.9%. Higher MN values resulted in increases in CA0-10 and overall combustion duration by approximately 26% and 34%, respectively. The slower combustion reduced peak in-cylinder pressure and temperature, decreasing temperature gradients and limiting heat transfer losses to the cylinder walls. Consequently, thermal efficiency improved and fuel consumption decreased. In contrast, low MN fuels exhibited higher reactivity, which advanced combustion phasing under constant spark timing. This caused rapid early energy release, increasing heat losses and promoting thermal quenching during the expansion stroke. As a result, incomplete oxidation in cooler boundary layers led to higher CO and unburned hydrocarbon (UHC) emissions. Regarding emissions, NOx increased with MN due to prolonged high-temperature combustion, rising by about 26% from MN 50 to MN 80, followed by a slight decrease of 3.8% at MN 100. Overall, optimizing MN is essential to balance engine performance, efficiency, and emissions.