Realizing Stoichiometric, Natural Gas-Fueled Combustion in Diesel Engines 2018-01-1148

For high-load applications, natural gas represents a clean burning, readily available, and relatively inexpensive alternative to number 2 Diesel fuel. However, the fuel’s poor ignitability has previously limited implementation to spark ignited and dual-fueled engines. These approaches suffer from reduced peak load and high engine-out particulate emissions, respectively, requiring lean operation and expensive aftertreatment to meet regulatory standards. A high-temperature combustion strategy can overcome the difficult ignitibility, allowing for true Diesel-style combustion of pure methane-the least ignitable and least sooting component of natural gas. In order to achieve this result, a compression system was designed to supply fuel at pressures suitably high to achieve good mixing and short injection durations, and a solenoid-actuated Diesel fuel injector was modified to function at these pressures with a gaseous fuel. This fuel supply system was paired with a single-cylinder research engine equipped with an insulated piston face. An intake preheat temperature of 250 °C was shown to provide the best combination of ignition delay and engine performance. A sweep of equivalence ratio then demonstrated soot emissions close to or below the current regulatory limit and combustion efficiencies greater than 96% up to stoichiometric fuel loadings. However, both soot emissions and combustion efficiency were worse than expected at low fuel loadings, and cycle-to-cycle variability was high throughout. Schlieren imaging and numerical investigation of the injection process demonstrate oscillatory dynamics and poor control over the end of injection. This indicates that further improvements to performance and emissions could be made by developing purpose-built gaseous fuel injectors. However, even with coarse control over injection, the improvements in particulate emissions over number 2 Diesel fuel and potential for stoichiometric operation-enabling NOx control using a three-way catalyst-make a strong case for further investigation and refinement.