Effect Of EGR And Excess Air Coefficient On Performance Of Natural Gas Engine

With the deficient of non-renewable petroleum resources and gradually growing concern about the environmental protection of human beings, and for maintaining the sustainable development of human beings, the pressure and challenge of vehicle engines energy saving and emission reduction, lead the research-development and application of the alternative fuel to irreversible trend. Natural gas has been paid a great of attentions because of its "clean" emission and abundant resources. However, the burning of natural gas has some defects such as high ignition temperature and slow burning rate. As the scope of natural gas vehicle applications continue to expand and these laws of engine fuel economy and emission limitations become more rigorous, how to ensure that the economy and low emissions of engines has become one of the problems which must be solved, therefore experimental researches of the combustion and emissions in natural gas engine is meaningful.Exhaust gas recirculation (EGR) technology has been widely used by engine researchers as its high efficiency in NOx emissions reduction. It also becomes one of the key technologies which could meet the stringent vehicle emission regulations. It is well known that lean-burn is effective in increasing thermal efficiency and reducing exhaust emissions, nevertheless, if only use EGR or air dilution, the lean-limit of combustion is normally limited. The combustion and emission will dramatically worsen while it is over the limitation.In order to reduce the emissions, improve the fuel economy and expand the lean-burn limitation, EGR and extra air are all applied to dilute the mixture gas in this research. The experiment is based on CA6SE1-21N heavy duty CNG(compressed natural gas)engine with cylindrical combustion chamber. The research analyse influences of combustion and emission on the CNG engines with different EGR rate and excess air coefficient. It also optimizes the injection timing and ignition timing to further improvements of the fuel economy and exhaust emission for the CNG engines. The engine speeds from ESC 13-mod were selected as test operations. Under the condition of part loads, constant speeds, EGR valve opening has been adjusted when the excess air coefficient is on the specified condition, the impacts generated by these two parameters has been studied using combustion analyzer and emission analyzer. The results show that the exhaust gas recirculation and lean burn operation have significantly impacts on the combustion process of CNG engine, meanwhile NOx emissions effectively reduced by these methods, and then, through optimizing the boundary conditions, fuel economy and NOx emissions also can be further improved. The research results are as follows:1.Under the equal working condition, with the increase of EGR rate and excess air coefficient(λ),the in-cylinder mixture concentration becomes leaner, the maximum combustion temperature decreases, the center of the heat release rate and the timing of combustion all move backward, the main combustion duration becomes longer.2. With the increase of EGR rate and excess air coefficient(λ),the engine cycle-by-cycle variation increases, and it shows that lambda influences the cycle-by-cycle variation more obviously.3.With the increase of the EGR rate, the mass emission of NOx presents downtrend, the maximum mass emission of NOx will reaches when lambda=1.1,when lambda is more than 1.1, the mass emission of NOx decreases gradually with the increase of lambda. When natural gas engine is running under a Stoichiometric condition, the effect of EGR on reducing the mass emission of NOx is not obviously; on the contrary, EGR is very effective when engines operate on the lean-burn condition. With the increase of EGR rate,the mass emission of HC increases. It will decrease when the engine loads increase.4. When the excess air coefficient is lower, the gas consumption presents downward trend with the excess air coefficient increases; with further increase of excess air coefficient, the trend of gas consumption is volatile. When the air-fuel mixture is rich, with increase of EGR rate, the gas consumption drops gradually. When the excess air coefficient is higher, the gas consumption increases with the rises of EGR rate.5.Through the ignition timing and injection timing optimizing, the economy and NOx emissions of the engine is able to be improved further.