Impacts of Gaseous Fuel Quality on Combustion and Regulated Emissions for a Small, Low-Pressure Direct Injected Natural Gas Engine 2019-01-0560
Recent research has focused on the development of small combined heat and power systems for decentralized power generation. Such technologies have the ability to improve grid resilience and improve overall efficiency while reducing emissions. Our research focused on the design of a two-stroke natural gas combustion cylinder as the prime mover in either slide-crank or novel linear generator applications. The engine includes electronic ignition, tuned intake and exhaust systems based on resonance theory, and a modified porting system. Nearly two-thirds of US homes have either natural gas or liquefied petroleum gas available at low pressures. In order to reduce compression power we modified the engine to use low-pressure direct injection (LPDI) of gaseous fuels. We tested a 34cc, air-cooled, two-stroke engine with three different natural gas blends (each with a methane number (MN) of around 75), pure methane (MN = 100), and pure propane (MN = 34.2). We examined regulated gaseous emissions, greenhouse gas emissions, and combustion trends over a range delivered air fuel ratios. Start of Injection (SOI) occurred at either 180 or 190 CA BTDC and improved efficiency by reducing fuel slip However, for natural gas blends the predominant emissions were methane - potent greenhouse gas. We showed that while propane had the highest CO2 emissions, it also produced the lowest CO2 equivalent emissions. However, propane also tended to have the highest NOx, NMHC+NOx, and CO emissions. As expected, propane and the natural gas blend with high ethane content tended to produce the highest peak cylinder pressure while methane produced the lowest. CNG blends two and three yielded the lowest COV of IMEP over the broadest range of delivered air fuel ratios, while pure methane yielded the highest. For all fuel blends, LPDI operation showed the capability to meet all current regulated gaseous emissions standards while providing improved efficiency.