Improving Combustion Of CNG Engine Through Changing Combustion Chamber Shape And Injection Pattern

Given the deteriorating environment and the increasingly tense supply of petroleum, the development of clean and energy-saving "green cars" has become an urgent task for the automobile industry. As a motor fuel, natural gas has outstanding advantages of high economic efficiency and low pollution emissions, so there are broad prospects for the development of compressed natural gas vehicles. There are two main problems on application of natural gas in the car: firstly, the performance of engine drops significantly; secondly, the thermal efficiency is low and the economic performance is far from satisfactory condition. So in the circumstances to maintain its advantages how to improve the performance of natural gas engine by ameliorating combustion is the purpose of this research. However, to make a thorough and meticulous study of the main factors affecting the combustion, only investigating the macro experimental data is far from enough. Computational simulation of the engine combustion process can analyze and predict the changes of turbulence parameters, the concentration of mixture, the temperature distribution, cylinder pressure distribution and the concentration distribution of various products of combustion in intake stroke and combustion stroke, thus it can be used to study the various factors affecting engine combustion and emissions. And there are many advantages of numerical simulation by using computational fluid dynamic software such as low cost, short period and providing some information which can not obtain by test. To study the factors affecting the combustion in cylinder, then improve engine performance by ameliorating combustion, the experiment about the key factors that impact the CNG engine performance was done on CA6SE1-21N CNG engine. It was found that the combustion chamber geometry and dimension of the engine had a major impact on mixture formation, perfectibility of combustion process, dynamic and emissions. And postponing injection timing could improve engine performance, increase lean burn limit and optimize natural gas engine combustion process. It can be seen that the combustion chamber shape and injection law have an important impact on mixture formation process and combustion process. So five different shapes of combustion chamber were designed and the intake process, compression process and combustion process were simulated by using STAR-CD software. Through analysis of the macro and micro parameters, the law how combustion chamber shape affects CNG engine combustion and emissions was found:a) In intake stroke and compression stroke, the flow fields of several combustion chambers are basically same. Near the combustion chamber, because of the dramatic changes in structure, combustion chamber shape has a greater impact on the flow. The reentrant combustion chamber and the trapezoidal combustion chamber have strong squish flow due to their reentrant shape, so they have a longer duration of the swirl. Because vertical jawsω-shaped chamber has convexity and transition arc which have diversion effect at the bottom, a strong turbulence that can promote the combustion of mixture incylinder. Because there are corners in a square combustion chamber, the impact of fluid and a wall engenders a large number of small-scale turbulences. This will be propitious to the formation and rapid combustion of mixture.b) The shape of the combustion chamber greatly affects the turbulent kinetic energy in cylinder. It should be helpful to shape the course of the turbulent kinetic energy changes as follows: turbulent kinetic energy is low when igniting, which is conducive to the formation of a stable flame kernel; subsequently turbulent kinetic energy should be increased gradually, which can help accelerating flame propagation speed and enhance combustion efficiency; turbulent kinetic energy is still high in the late period of combustion, so the mixture in cylinder will burn more completely.c) The shape of combustion chamber has a significant impact on combustion, as is mainly due to differences in distribution of turbulent kinetic energy. Turbulent kinetic energy in vertical jawsω-shaped chamber is relatively weak, so the flame shapes and spreads rapidly. Combustible mixture burns quickly so that the pressure increases rapidly. In main combustion period, because airflow in other chambers is stronger, flame propagates quickly and pressure increases rapidly. In the late period of combustion, pressure and heat release rate in the square combustion chamber are higher than in the trapezoidal combustion chamber in which turbulent kinetic energy is highest. This is caused by their different areas of the flame propagation.d) HC and NO emissions are different due to the variety of combustion chamber shape. Up to ATDC 90°CA, CH4 combustion rate corresponding to the square combustion chamber is about 80%. CH4 combustion rates corresponding to the trapezoid chamber and vertical jawsω-shaped combustion chamber are approximately 70% and the reentrant combustion chamber corresponding rate is 65%. The maximum temperature in reentrant combustion chamber is the highest and high-temperature region is the largest, so the amount of NO is the highest. NO concentration in the bottom of the chamber is up to 0.014. The heat loss of the trapezoidal combustion chamber is biggest, so the maximum temperature in the trapezoidal combustion chamber is lower than in other chambers and the cylinder temperature is more well-proportioned, thus the amount of NO is minimum.Through the simulation we can find that mixture concentration field displays contrary stratification that mixture concentration in upper area of cylinder is leaner and richer in bottom area of cylinder at ignition timing when using the original injection pattern, as is neither conducive to a reliable ignition and stable combustion of the mixture nor helpful to improve lean burn limits. Distribution of local stratification that enhances the concentration in a small area around the spark plugs can guarantee the stability of ignition and improve lean burn limits. Secondary injection can achieve this distribution of the mixture.Because there are corners in square combustion chamber, the impact of fluid and combustion space wall engenders a large number of small-scale turbulence. This will be conducive to increase the combustion rate, so the mixture in cylinder burns more completely and pressure is always higher than other chambers. The wall temperature of square combustion chamber is higher than other combustion chambers, so the quenching layer is thiner .It is benefit to reduce HC emissions. But the amount of NO is appreciably higher. Therefore, considering combustion and emissions, we choose square combustion chamber to simulate mixture formation and combustion process when using secondary injection pattern. The purpose of this study is to find how the injection pattern affects mixture formation and combustion process in cylinder. The simulation software we use is STAR-CD.The result shows that secondary injection achieves the distribution that mixture concentration in region near spark plug is richer and in most other regions of combustion chamber is leaner. This distribution improves the ignition stability of the gas. In combustion delay period flame forms and spreads more quickly than original engine. Combustion delay period and main combustion period are shortened, which is benefit to constant volume combustion. In addition, because the first injection forms a large-scale homogeneous mixture, the flame spreads rapidly in main combustion period. Heat release quantity increases and combustion stability is improved. There is no flame breakdown and partial combustion and late burning happened, so the combustion cycle variability reduces. Thus thermal efficiency is high and the engine performance is improved.