IMPACT OF NATURAL GAS DIRECT INJECTION ON THERMAL EFFICINECY IN A SPARK IGNITION ENGINE PDF Download

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Abstract : Interest in natural gas as an internal combustion engine fuel has been renewed due to its increasing domestic availability and stable price relative to other petroleum fuel sources. Natural gas, comprised mainly of methane, allows for up to a 25% reduction in engine out CO2 emissions due to a more favorable hydrogen-to-carbon ratio, relative to traditional petroleum sources. Traditional methods of injecting natural gas can lead to poor part-load performance, as well as a power density loss at full load due to air displacement in the intake manifold. Natural gas direct injection, which allows the fuel to be injected directly into the cylinder, leads to an improvement in the in-cylinder charge motion due to the momentum of the gaseous injection event. While research performed with natural gas typically occurs at full load, the current research project focused on a part-load condition as this was most representative of real world driving conditions, becoming increasingly relevant for a downsized boosted application. The goal of this research was to further the understanding of natural gas direct injection and its resulting effect on the thermal efficiency of a GDI engine at a part-load condition. Key objectives were to measure and quantify the effects of injection location, injection timing, and exhaust gas recirculation on the thermal efficiency of the engine. A single-cylinder research engine was equipped for natural gas direct injection at Argonne National Laboratory, with detailed tests and analysis being performed. Experimental results show that the injection location played a crucial role in the mixture formation process; injecting along the tumble motion led to a greater thermal efficiency than injecting directly towards the piston due to improved mixing. The start of injection had a strong impact on the thermal efficiency, which agreed well with literature. While injecting after intake valve closure led to increased mixture flame speeds, there was a decrease in thermal efficiency due to decreased mixing time leading to increased stratification. An advanced start of injection timing led to the highest thermal efficiency, as this provided the best tradeoff between mixing time and resulting heat losses. In addition, the injection location and timing directly influenced the dilution tolerance. Injecting along the tumble motion produced the highest dilution tolerance due to the gaseous injection event amplifying the tumble motion, improving in-cylinder mixing.