Study On Lean Burn Characteristics Of Direct Injection CNG Engine

With the growth of world population and the development of automotive industry and economy, the problems of the global petroleum resource shortage and environmental pollution have become increasingly serious. People began to realize the importance of adjusting the energy structure of auto industry, and actively search for the new fuel that is bigger and cleaner to replace the petroleum. Natural gas, as the new fuel, from the reserve, is the world’s third largest natural energy after coal and oil; from the environmental protection, is a clean fuel. In the complete combustion, compared with gasoline, the emission of HC is reduced by 72%, NOx is reduced by 39%, CO is reduced by 97%, and compared with diesel, the emission of SO2 is reduced by 90%, soot is reduced by 93%. Therefore, compressed natural gas(CNG) has become the ideal alternative fuels for internal combustion engines. With the wide application of CNG in auto engine, CNG engine technology has been developing rapidly. At present, the domestic and foreign researches focused on the in-cylinder direct injection technology and the lean-burn technology. As the direct injection has equivalence ratio control precision, the high volumetric efficiency and a wide limiting range of lean-burn, so it easy to realize the lean combustion. The lean combustion technology will help to improve the thermal efficiency of the engine, and as the combustion temperature could be reduced by the lower mixture concentration, the decomposition of combustion products would be decreased, that’s more conducive to the complete combustion, so that to reducing the engine’s emissions of CO, HC and NOx.This paper is based on the China Postdoctoral Science Foundation Project(2013M541295), “Study on the Flame Propagation Theory of Lean Burn in CNG DI Engine”. Combined with the results of visual test of the flame propagation process in CNG DI engine that were studied by the research group member early in foreign using optical test sample of CNG DI engine, using computer to modeling and simulation for computing, it made a depth study on the 3D micro physical fields variation law of concentration field, temperature field and the NO formation rate of the mixture in-cylinder by different injection methods and different ignition methods. And combined with the test sample’s heat release rate, indicator diagram and formation of NO, it also studied the influence of the parameters such as equivalent ratio, injecting position, injecting time and ignition methods, etc. on the flame propagation theory and NO formation. According to these, an effective ways are given to improve the speed of flame propagation, to get combustion stability, to achieve higher thermal efficiency and to reduce NO emission. On this basis, combine with the project from the enterprise, “Development of 2.0L High Pressure Common Rail Diesel Engine Combustion System”(2011220101001190), using CNG fuel injection model and combustion model determined by simulation in CNG test sample, and take the actual operating conditions of 2.0TCI high-speed DI diesel engine as the boundary conditions(such as inlet air flow conditions and temperature conditions), and after adjusted and verified the simulation model and algorithm, studied the influence of the air flow mode in numerical simulation by the parameters of combustion chamber shapes, engine speed, and compression ratio, and then, with the certain injection methods and ignition methods, analyzed and studied, under the three conditions above, the dynamic characteristics of mixture formation and the influence of heat release rate, micro physical fields variation law in cylinder, and NO formation. Above these all, proposed an improvement scheme to modify the structure of combustion system from 2.0TCI diesel engine to CNG engine, and a way to improve the engine power, economy and emission performance.Research results show that:For the test prototype of direct injection CNG engine:(1) Increasing the equivalence ratio in lean combustion region appropriately will lead to increasement of the mixed gas concentration gradient and enhancement of the flame propagation speed, the combustion heat release rate and the combustion pressure, thus reducing the cyclic variation and stabilizing the lean burn process. But at the same time, the higher combustion temperature results in a larger amount of NO generation. Furthermore, if the equivalence ratio is too high, the overrich mixture which can make flame propagation characteristics worse will be formed. Therefore, in the practical application, the equivalence ratio should be selected appropriately.(2) When making the two sparkplugs ignite at different time, the later generated flame pushs the former generated flame propagation, thereby further enhancing the speed of the whole flame. It is noteworthy that under the same engine power performance condition, comparing to singe sparkplug ignition, two sparkplugs ignition will generate less NO.(3) The relative position between the ejector and the sparkplugs has a certain effect on the flame propagation. The more the jet air of the ejector closes to the sparkplugs, the faster the flame kernel forms.(4) When the injection mass and the ignition method are fixed, injection time mainly determines the mixure formation duration and further affects the distribution characteristics of mixure concentration in cylinder. Advancing the injection time properly can enlong the mixure formation duration, make the mixure tend to be homogeneous, slow down the flame speed, decrease the heat release rate, temperature, temperature gradient and area of high temperature in cylinder, as a consequence, reducing the NO formation.(5) In rapid combustion period, the temperature in flame zone is higher than 2000 K, thus there is a little NO formation in high concentration anoxic area of mixture. This shows that both prompt NO and Zeldovich NO will be generated in stratified lean combustion process based on the gradient distribution of mixture concentration field. Temperature dependent Zeldovich thermal NO is not generated on the flame zone, but generated in high temperature oxygen-enriched zone after the flame propagation. In short, through optimized matching of methods of injection and ignition, the flame propagation process can be controlled effectively, thus the NO formation can be effectively suppressed meanwhile improving the lean burn speed and stability.For the CNG engine modified from 2.0TCI high speed direct injection diesel engine:(1) The design of reentrant and bulge of combustion chamber has a great influence on the air flow pattern in the formation process of mixture. The higher central bulge with concave slope can guide the air flow better, thus getting bigger turbulent kinetic energy(TKE) near the bottom wall of the combustion chamber. When reducing diameter ratio is less than 1, the guidance of air flow is good, but the mixture concentration field and distribution of TKE around the spark plugs is poor at the ignitiontime. If using expanding opening chamber profile, squeeze flow area will be reduce, which is not conducive to flame propagation. Using straight opening chamber profile in which reducing diameter ratio equel to 1 can ensure keeping a certain turbulence intensity in the combustion chamber, at the same time, the high TKE zone distributing near the wall of the combustion chamber, which is beneficial to the formation of the mixture in the near-wall region, and can effectively improve the flame propagation speed.(2) At different engine speeds, the difference of the combustion chamber shape can cause different combustion characteristics. For the combustion chamber with bulge and contracting opening profile, engine speed has a greater impact on the flame propagation speed. With the increase of engine speed, the total combustion period becomes longer, the flame propagation speed is slowed down, as a consequence, the combustion heat release rate decreases. For the canister combustion chamber, the effect of engine speed on the flame propagation speed is much smaller.(3) Increasing the compression ratio of the DI CNG engine can make the strong squeeze flow formed earlier, thus increasing the average TKE, accelerating the flame propagtion, making the heat release gravity closer to top dead center, and enhancing the heat release rate. But the thermal efficiency not linearly increases with the compression ratio. The increment of heat release rate decreases with the augment of compression ratio. And the change of NO emission is opposite, that is, its increment increases with the increase of the compression ratio, resulting in a sharp increase in the amount of NO generated by compression ratio increased. Therefore, under the small increase NO emission, to enhance appropriately compression ratio can improve the power and economy. However, if the compression ratio increases too much, not only the thermal efficiency increase any more but also amount of NO generate. For above, the compression ratio of CNG engine must be controlled in right range. According to the research results, the compression ratio of the test sample is 12.