Strategies to Improve Combustion and Emission Characteristics of Dual-Fuel Pilot Ignited Natural Gas Engines

Dual-fuel pilot ignited natural gas engines have several intrinsic advantages relative to spark ignited; mainly higher thermal efficiency and lower conversion costs. The major drawback is associated with light loads. This paper discusses objectives, approaches, methods and results of the development of strategies which overcome the drawbacks and enhance the advantages. Development of a pilot fuel injection system, having a delivery of only 1 mm³ at a duration of 0.6 ms, was described in a previous paper. This paper concentrates on the results of strategies to reduce unburned methane in the exhaust and to increase the substitution of gas at light loads through skip-fire, by-passing boost air and exhaust gas recirculation techniques. Engine tests proved that with these strategies, diesel fuel replacement of more than 95% over the entire engine operating map, including idle, can be achieved and current and anticipated future emission standards satisfied. The effect of each strategy on combustion and emission characteristics is experimentally studied, using a turbocharged, intercooled Navistar DT 466, 7.6L diesel engine. This concept, called micropilot compression ignited natural gas is proven to be a rational approach to the dedicated alternative fuel engine, and has potential for further improvements.