Abstract
A set of engine experiments to measure the combustion and performance of directly injected methane in a spark ignition engine was carried out on a Ricardo E6 single cylinder 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 after 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 facilitate 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. Results indicate that optimal performance of engine running with SPFI direct injection was achieved with fuel injection starts at 190o ATDC when run at 1100 rpm and set at MBT. 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. The combustion of SPFI direct injection is faster than the one of port injection which also shows faster rate at the later part of combustion due to the presence of fuel stratification. The combustion also shows comparable steadiness with port injection operation. However, the indicated performance of SPFI direct injection was less than those of the port injection with reductions in mean effective pressure and indicated fuel conversion efficiency by 6.5% and 18.9% respectively while the indicated specific fuel efficiency was increased by 23.3%. At higher engine speed, the performance of SPFI direct injection deteriorates due to increased effect of delayed fuel arrival in the cylinder. The low-than-potential performance is because of poor air-fuel mixing due to temporal and spatial limitation. Moreover the gas jet from the SPFI nozzle has a narrow spray cone angle of 9o which was shown by PLIF imaging and has a negative effect on the mixing process. SPFI methane direct injection is a practical short cut and low cost conversion from gasoline port injection engine but further work to optimise its performance must be undertaken. These include improving air fuel mixing and developing a better control of injection parameters.
Keywords: Direct injection spark ignition engine, methane, combustion, performance, fuel conversion.