Decreasing hydrocarbon and carbon monoxide emissions of a natural-gas engine operating in the quasi-homogeneous charge compression ignition mode at low loads

Abstract

Experimental studies have been carried out on decreasing the hydrocarbon (HC) and carbon monoxide (CO) emissions of a compressed natural-gas (CNG) engine operating in quasi-homogeneous charge compression ignition (QHCCI) mode at low loads. The effects of three technical approaches including partial gas cut-off (PGC), intake air throttling, and increasing the pilot fuel quantity on emissions and the brake thermal efficiency of the CNG engine are studied. The results show that HC and CO emissions can be reduced with only a small penalty on the brake thermal efficiency. An increase in the brake thermal efficiency and reductions in HC and CO emissions can be simultaneously realized by increasing the pilot fuel quantity. It is also indicated from experiments that the HC and CO emissions of the engine can be effectively reduced when using intake air throttling and increasing the pilot fuel quantity are both adopted. However, nitrogen oxide (NO_x) emissions increase with increase in the throttling and the pilot fuel quantity. Under PGC conditions, NO_x emissions are lower than those in the standard mode; however, they increase and exceed the values in the standard mode in increases in the load and natural-gas supply.