A basic study on the ignition position of natural gas direct-injection super-lean combustion

A basic study on the ignition position of natural gas direct injection super-lean combustion was studied by using a rapid-compression machine. The results show that an optimum ignition position exists for a specified overall equivalence ratio where it has short combustion duration, high value of pressure rise, high combustion efficiency, rapid heat-release rate, low concentration of CO, and unburned hydrocarbon. The optimum ignition position will be close to the injector nozzle tip with decreasing overall equivalence ratio. Lean burn limit can be extended to even smaller overall equivalence ratios as the spark electrode gap approaches the injector tip, whereas the range of overall equivalence ratios for the flammability becomes narrow when closing the spark electrode gap to the injector tip. High combustion efficiency can be maintained at the overall equivalence ratio φ ≥ 0.1 and decreases remarkably at φ < 0.1 due to the increasing ratio of unburned fuel to injected fuel.