Improvement of Performance and Reduction of Exhaust Emissions by Pilot-Fuel-Injection Control in a Lean-Burning Natural-Gas Dual-Fuel Engine

The purpose of this study is to determine a pilot injection control strategy for the improvement of dual-fuel combustion with a lean natural gas/ air mixture. Experiments were performed using a single cylinder test engine equipped with a common-rail injection system. The injection pressure, timing and quantity were varied at a fixed overall equivalence ratio of 0.5. The results of single-stage-injection experiments show that middle injection timings (-20 to -10 degATDC) produce low emissions of unburned species, because the pilot-fuel vapor spreads into the natural-gas lean mixture and raises the effective equivalence ratio, which leads to fast flame propagation. Early injection (-35degATDC) is advantageous for low NOx emission; however, increased emissions of unburned species are barriers. Two-stage pilot injection offers improved thermal efficiency and reduced exhaust emissions when early first-stage injection (-45 to -40degATDC) and moderately early second-stage injection are selected. Analysis of the heat release rate data indicates that the second injection controls the start of heat release, while the first-stage injection affects the overall flame propagation speed. Increased ratio of the first-stage injection quantity is useful for low hydrocarbon (HC) and NOx emissions with high thermal efficiency.