The abundance of natural gas in the United States and low price relative to diesel fuel has generated interest in dual fuel engines where natural gas is substituted for diesel fuel. The factors limiting the natural gas (NG) substitution rates are: minimum diesel injector pulse width, cycle-to-cycle variation in net indicated mean effective pressure (NIMEP), engine knock, peak cylinder pressure, compression ratio, boost pressure and lean air/fuel limits leading to misfire among others.
The objective of this study was to explore the highest natural gas substitution for a commercially available heavy duty diesel engine for several of the 13 Mode European Stationary Cycle (ESC) and US EPA Supplementary Emissions Tests (SET) speeds and loads while maintaining acceptable engine performance levels. A heavy duty 2012 Navistar MaxxForce 13® engine was retrofitted to accommodate dual-fuel operation. The engine was operated over several different speeds and loads to determine the possible NG substitution rates at different diesel injection timings, diesel injection pressures and equivalence ratios, while maintaining combustion phasing.
The data showed that dual fuel operation at high NG percentages was stable over several speeds and loads with brake thermal efficiencies comparable to 100% diesel operation. The introduction of NG generally demonstrated reductions in peak cylinder pressure and cylinder pressure rise rate at a given speed and load point. Increases in hydrocarbon and greenhouse gas emissions and a decrease in nitrogen oxides were observed during dual-fuel operation.
