Performance, Efficiency and Emissions Assessment of Natural Gas Direct Injection compared to Gasoline and Natural Gas Port-Fuel Injection in an Automotive Engine

Interest in natural gas as a fuel for light-duty transportation has increased due to its domestic availability and lower cost relative to gasoline. Natural gas, comprised mainly of methane, has a higher knock resistance than gasoline making it advantageous for high load operation. However, the lower flame speeds of natural gas can cause ignitability issues at part-load operation leading to an increase in the initial flame development process. While port-fuel injection of natural gas can lead to a loss in power density due to the displacement of intake air, injecting natural gas directly into the cylinder can reduce such losses. A study was designed and performed to evaluate the potential of natural gas for use as a light-duty fuel. Steady-state baseline tests were performed on a single-cylinder research engine equipped for port-fuel injection of gasoline and natural gas, as well as centrally mounted direct injection of natural gas. Experimental results suggest that similar efficiencies can be achieved in part-load operation for both gasoline and natural gas. While the effects of injection timing are generally minimal for port-fuel injection, varying the injection timing for direct injection, especially after intake valve closure, can speed up the early flame development process by nearly 18 degrees CA. Results at full-load suggest that operation with natural gas regardless of fuel system allows for an efficiency increase. While port-fuel injection of natural gas leads to a power density loss, direct injection of natural gas allows for up to a 10% improvement in full-load power density over liquid and gaseous port-fuel injection for a naturally aspirated engine. In addition to increasing full-load efficiencies, natural gas operation allows for up to a 30% reduction in engine out carbon dioxide emissions at full-load.