| Vehicules are faced with a major challenge of energy shortage and environmental pollution, and hence alternative fuels are becoming very important. Natural gas, whose main composition is methane, is a high efficiency, clean and low cost fuel of engines. As an alternative fuel, natural gas contributes to improve the energy structure and decrease engine emissions. DME, together with Bio-diesel, is a high quality, clean alternative fuel for diesel engines, and it can relieve the current crisis of energy. The research in this dissertation is summarized as follows:1. Experimental study was conducted with a vehicular turbocharged natural gas engine. The results show that the engine power is equivalent to original diesel engine and its torque at low speed is higher than that of diesel engine for natural gas is a smoke free fuel, hence natural gas engine is suitable for city bus.2. The engine work process was simulated using GT-Power software. The inter-cooler model was calibrated for the desired temperature and pressure drop. The model of turbocharger was calibrated in order to provide engine with sufficient air. The models of intake pipe and outlet pipe were established and models of valves were established to determine engine valve timing. The models of combustion, heat transfer and air-flow were established and the model of mechanical efficiency was established as there are frictions between engine parts. The speed characteristics at full load and load characteristics were simulated. The simulated results are consistent with experimental results.3. Based on diesel engine valve mechanism, cam profile was optimized in order to decrease the overlap angle of valves. The buffer segment of cam is constant-acceleration & constant-velocity type and the base segment is polynomial dynamic type. The parameters in the function of tappet lift were adjusted in order to meet the requirements of curvature radius, lubricity and contact stress of cam...
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