Effect of gas composition on the performance and emissions of a dual-fuel diesel-natural gas engine at low load conditions

Abstract

Areas located at or near natural gas (NG) well or storage sites can use NG to partially or totally replace the diesel fuel used to power equipment or for local transportation. This study investigated a dual-fuel diesel-NG engine operating at low load conditions, as literature shows that such conditions are the most sensitive to NG composition. Part of the diesel fuel was replaced with four different gas blends containing methane, ethane, and propane. No EGR was employed to avoid it interfering with the individual fuel component evaluation. Results show up to 80% higher rates of pressure rise compared to the diesel baseline, which limited the diesel substitution rate to 40% without EGR. While pure methane affected the most the diesel fuel vaporization and ignition delay, important differences were seen during the late-stage fuel oxidation. The location and magnitude of the 2nd heat release peak changed with gas composition, with propane gas mixtures showing the largest and most advanced peak. In-cylinder pressure correlated with the gas mixture’ autoignition temperature. As a result, the mixtures with propane had the best performance and gaseous fuel oxidation, with opposite results for the methane-only gas. As propane gas mixtures performed better than mixtures with ethane, it suggests maintaining a higher propane fraction in the NG for dual-fuel operation. CO2 and NOx emissions reduced up to 6.8% and 20%, respectively. However, the reduction was offset by the large increase in CO and HC emissions, which could require after-treatment modifications. Finally, the results suggest that the switch from diesel only to low-load dual-fuel operation can be performed without major modifications in terms of engine control and aftertreatment changes, at least for the low load conditions and NG compositions used here.