Abstract
Keywords: Direct injection, methane/compressed natural gas, injection timing.
A set of engine experiments was carried out on a Ricardo E6 single cylinder engine to measure the combustion and performance of directly injected methane in a spark ignition engine. Direct injection of methane was achieved by mean of Spark Plug Fuel Injector (SPFI). SPFI is a device that combines a fuel injector with a spark plug into a single unit device. Direct injection improves volumetric efficiency by injecting fuel into the cylinder when maximum amount of air has been drawn in after the intake valve closes. The displacement of air by low-density gas associated with port injection or in a mixing chamber is eliminated, therefore improving the absolute heating value of cylinder charge. The higher turbulent intensity induced by a high-pressure methane jet improve mixing and combustion. The engine was run at wide-open throttle and stoichiometric air/fuel ratio. Cylinder pressure data was taken as the main indicator of combustion and engine performance. A set of port-injected data for the same engine running on methane was also taken in order to provide a comparison of performance. Compared to the port injection operation, SPFI direct injection improves volumetric efficiency from 72.4% to 83.4%, which provide a basis for output improvement. Fuel injection timings were varied between 90o and 210o BTDC and engine indicated performance was measured. Results indicate that optimal performance of engine running with SPFI direct injection with 60 bar injection pressure was achieved with fuel injection starts at 190o ATDC when run at 1100 rpm and set at MBT. However, when tested at different injection pressure, the optimal performance proven not to be at the same injection timing. Injection timing is highly dependent to injection pressure and engine speed. In order to gain optimal performance, injection timing must be varied at different engine speeds. This is believed to be the result of temporal and spatial limitations of air-fuel mixing with close-valve direct injection operation. SPFI methane/compressed natural gas direct injection is a practical, simple and low cost conversion from gasoline port injection engine but further work to optimize its performance must be undertaken. These include improving air fuel mixing and developing an adaptive control of injection parameters.