Study On The Air Injection Of Turbocharged Natural Gas Engine For Improving Power Performance

Natural gas, as a clean alternative fuel, can effectively improve the engine’s economic and emission performance. Due to the low volume efficiency of natural gas engine, and the lag-effect in turbocharger, the turbo gas engine exists the problems of insufficient torque at low speed and transient response lag. To solve the above problems, a study of air injection was conducted on turbocharged natural gas engine with premixed gas supply system. Four projects were proposed in current study, namely, air injection before air mixer, after intercooler, at intake manifold inlet, and before turbine. And the engine bench test was conducted based on different air injection schemes.Take the air injection before air mixer as an instance, the influence of air injection on intake and exhaust performance and power performance was analyzed in detail, and the effects of air injection pressure and engine speed on the results of air injection and turbocharger surge were also discussed. According to the results of engine bench test, a comparative study of effects of four air injection projects on engine intake performance, dynamic performance and cylinder combustion process was investigated. Aming at the change of mixture gas caused by air injection before air mixer and at intake manifold inlet, the influence on combustion process and engine performance was analyzed. The results show that when air injectes before gas mixer, air inflow and torque are rapidly improved; when the speed is constant, the growth in torque increases with air injection pressure, but excessive pressure will cause the air compressor surge. The other three schemes can make the engine’s low-speed torque obviously improved, except for air injection at the intake manifold inlet. The main reasons for the decrease of torque caused by air injection at intake manifold inlet are that mixture gas is diluted by air injection, which makes excessive air ratio increased, combustion duration prolonged, and 50% burning point position away from TDC, leading to the decrease of indicated thermal efficiency. During the air injection before gas mixer, mixture gas is enriched at part of transient operating conditions, thus preferable indicated thermal efficiency and the mean effective pressure is obtained.Finally, the scheme of air injection before gas mixer is selected and applied to the vehicle, and the results of engine bench test are verified with vehicle test. The test results show that with application of air injection, the power performance of vehicle and the turbocharger lag-effect is significantly improved; the number of sharp changes in the throttle is reduced during the climbing process, and the climbing speed increases; the vehicle climbing performance is improved, and the vehicle can climb in high gears, which can contribute to its economic performance.Through the research, the potential of various air injection for improving the supercharged natural gas engine power performance and transient response is revealed; the effects of different air injection parameters on the natural gas engine performance are concluded; an effective way to improve the turbocharged natural gas engine low-speed torque and transient response is provided.