Implementation and control of stoichiometric natural gas combustion to enable low-emission diesel engines
Keywords: Mechanical Engineering.

The expected growth in the heavy-duty transportation sector necessitates the development of engine technologies able to increase efficiency and reduce emissions without sacrificing power output. Previous research has demonstrated that reducing heat transfer losses from the cylinder can enable significant efficiency gains in Diesel engines. The high in-cylinder temperatures generated in this engine architecture enable the use of low-cetane fuels with the potential for low-soot operation. Low soot emissions allow the equivalence ratio to be increased to stoichiometric which increases power, and could allow the existing Diesel aftertreatment system to be replaced with a less-expensive three-way catalyst. Natural gas is a promising candidate for stoichiometric, high-temperature, Diesel-style combustion. Its high hydrogen-to-carbon ratio should be able to reduce both soot and carbon dioxide emissions, and its wide distribution as a commercial and residential fuel provides existing infrastructure to speed deployment in transportation applications. This thesis demonstrates stoichiometric, Diesel-style combustion of neat methane as a single-component surrogate for natural gas. It explores the challenges of injecting a gaseous fuel at high pressures, and demonstrates the fuel's capacity for low emissions. It then provides a preliminary investigation into multiple-injection strategies for controlling combustion behavior and emissions in a stoichiometric, high-temperature engine architecture. First, fuel system hardware is developed to enable gaseous operation and preliminary experimentation is accomplished with methane. A fuel compression system is designed to supply methane at pressures suitably high to achieve good mixing and short injection durations, and a solenoid-actuated Diesel fuel injector is modeled and modified to inject methane at these pressures. This fuel injection system is then implemented on a single-cylinder engine. An insulated piston face, air cooled head, and intake preheating achieve suitable start of injection temperatures to ignite methane. Intake preheating is varied at low equivalence ratios to determine the sensitivity of engine performance to temperature at the lowest-load, lowest-temperature conditions of interest. A sweep of equivalence ratio demonstrates soot emissions roughly four times the current EPA limit for heavy-duty vehicles and combustion efficiencies of approximately 92% at stoichiometric fuel loading. High soot levels and low combustion efficiencies are also seen at the lowest equivalence ratios investigated. This suggests poorly mixed combustion, and poor injector performance. Second, injector dynamics are examined in greater detailed, and emissions performance is characterized with improved injector performance. High-speed Schlieren imaging is able to determine the injection dynamics contributing to high low-load emissions. A parametric modeling investigation suggests that reducing the injector plunger length is able to remove flow rate oscillations seen at long injection durations, and that the addition of dry friction is able to reduce the magnitude of low-momentum post injections occurring after injector closing. Dry friction is implemented using PTFE O-rings installed between the injector body and plunger. Imaging is used to confirm that a shortened plunger is able to remove long-duration oscillations, and to determine the number of O-rings necessary to suitably reduce post injection magnitude. The improved injector is used to repeat the sweep of equivalence ratios and demonstrates improved soot emissions at all operating conditions. Most notably, low-load soot emissions are reduced by more than a factor of ten, demonstrating the effectiveness of improved injector performance for improving emissions. Techniques for further improving injector performance and potential changes to injector design are discussed. Finally, the prospects for controlling combustion in a stoichiometric, low heat rejection Diesel engine using multiple injections are discussed and experimentally investigated. The applications and effects of multiple injection strategies in traditional Diesel engines are explored, and their potential extension to stoichiometric engines is discussed. Methanol engine operation enables the use of a fast-actuating piezoinjector and the realization of short injection pulses. A range of two-injection strategies are implemented in order to determine the sensitivity of engine operation to pilot, split-main, and post-injection timing and duration. Small pilot injections are found to have control authority over rate of pressure rise and peak pressure and show some promise for improving combustion efficiency. Post injections demonstrate authority over peak pressure and combustion efficiency. All of these effects are accomplished with minimal impact on engine work output. The experiments of this thesis demonstrate that, even with course control of injection, high-temperature, stoichiometric combustion of methane is able to greatly reduce soot emissions over traditional Diesel engines. Improved injector dynamics and the implementation of multiple injection strategies further improve emissions and combustion performance, suggesting substantial room for refinement of the technology and motivating the continued development of injector hardware and injection strategies. The ability to operate a Diesel engine at stoichiometric fueled only by natural gas and to employ a three-way catalyst for emissions abatement makes this strategy a clean, efficient, high-torque, and low-cost solution for heavy-duty transportation.