Effects of Overall-equivalence Ratios by Varying Intake Pressures and Exhaust Gas Recirculation Rates on the Natural Gas/Diesel Dual-fuel Combustion at a High Load Condition

The effects of intake pressures and EGR rates on the natural gas/diesel dual-fuel combustion under a high load condition (brake power of 100 kW) were evaluated using a 6-L CI engine in this study. Natural gas fraction was fixed as 80% of total input energy for all the dual-fuel combustion operations and engine rotation speed was set to 1,800 rpm. Intake pressure was varied from 0.15 to 0.21 MPa by controlling the waste gate type turbocharger system. EGR rate was changed mainly by controlling EGR valve opening from fully closing to mid-opening to fully opening, but it was also affected when intake pressures were varied. In addition to these intake mixture parameters, diesel injection timing was adjusted in two cases—early and late injections and all the experimental results were compared to those of the neat diesel combustion cases. The results emphasized that the dual-fuel combustion was more sensitive to overall equivalence ratio determined by intake pressures and EGR rates than the neat diesel combustion because it had flame propagation process of premixed natural gas and air mixture. Thus, during the dual-fuel operations, in-cylinder temperature and brake thermal efficiency were decreased as intake pressure was increased.