Optimization Research On Compression Ratio﹑ Spark Time And Valve Timing Of CNG Engine

The emergence of internal combustion engine and vehicle has made a great contribution to the human society. But with the increment of vehicles, the problem of energy crisis and environmental pollution is getting worse and worse. So it is urgent to look for a kind of alternative energy. As a kind of high-quality energy, nature gas has been widely used in vehicle, and nature gas also has the characters of rich,low pollution emissions,cheap and so on. So it has become the preferred alternative energy of vehicles. But when nature gas was used in vehicles, it will make the engine power reduced. Now many experts and scholars both foreign and domestic have made a lot of research on nature gas engine. Much of these research was made when nature gas engine at full load. The research on the performance of nature gas at Medium and low load is still less.Optimize the parameters of natural gas engines is a good way to improve its power. In the paper a simulation model of gasoline engine was built in the GT-power software. And the fuel model of ethane and butane was built in the fuel library of GT-power, and then generated the fuel model of nature gas in Sichuan. In order to study the problem of nature gas engine power decline, the performance of the nature gas engine was simulated with the gasoline engine model and nature gas fuel model. Then analyzed the variation of nature gas engine performance with compression ratio,spark time and valve timing under different load, and optimized compression ratio,spark time and valve timing of the nature gas engine.Nature gas engine performance after optimization was compared with the performance of the nature gas engine before optimization and the gasoline engine. The results showed that the power and economy of the nature gas after optimization was significantly improved, but compared with the gasoline engine was still less. After optimization the maximum torque was increased from 53.9N·m to 56.8N·m, and the maximum power was increased from 11.1kW to 12.6kW at the 25% load, with the enhance range of 3.1%,6.4%; At the 50% load the maximum torque was increased from 108N·m to 113.5N·m, and the maximum power was increased from 22.4kW to 24.1kW, with the enhance range of 5.1%,7.4%; At the 75% load the maximum torque was increased from 120.7N·m to 128.1N·m, and the maximum power was increased from 39kW to 41.7kW, with the enhance range of 6.1%,6.9%; At the 100% load the maximum torque was increased from 143.2N·m to 159.6N·m, and the maximum power was increased from 67.2kW to 70.6kW, with the enhance range of 11.5%,5.1%. Some conclusions were obtained in the process of simulation and optimization analysis: 1.Compared with gasoline engine, the compression ratio of nature gas engine should be increased. Between 10.5 and 12 was good; 2.Compared with gasoline engine, the spark time of nature gas engine at medium speed should be earlier, with the range in advance of about 3°CA; 3.The fixed valve timing was optimized at medium speed, could improved the nature gas engine power at low and medium speed, but the nature gas engine power was reduced at high speed. The result of the paper has some reference value on the development of natural gas engine.