The Simulation Research On Combustion Process Of The CA4102 CNG Engine

Nowadays the crisis of energy and the pollution of environment become serious. As a clean alternative fuel, CNG (Compressed Natural Gas) has been widely used in the engine. Currently natural gas engines used in China are modified from gasoline engines or diesel engines, however, because of the slow flame diffusion speed of natural gas, such problems as low average efficient pressure, low thermal efficiency and high local heat load are universe on the CNG engine. So the root way to improve the engine performance is improving the combustion in the cylinder. The combustion is a complicated physical and chemistry process. Test is the main way for research on the combustion process in the past time. Since the 1960s, along with the appearance of computer and its extensive application, adding with the further research on the computational hydrodynamics, computational heat transfer, and chemistry dynamics, the numerical simulation and computation has become a efficient way to study the engine. The numerical simulation to the processes in the engine cylinder with CFD technology provides the thorough and complete information about all relevant variables, and allows the researchers to clearly know the changes in the combustion process so as to identify combustion inadequacies and to take the necessary measures to improve combustion.In this thesis, with AVL FIRE software, the modeling and simulation to compression, ignition, combustion and the expansion process in a CA4102 CNG engine which is modified from a CA6102 engine are carried out. The model is validated by the experiment, and then the influence of combustion chambers and ignition energy to the performance and emission of the engine is studied. The distribution of velocity field,concentration field of CH4 and temperature field in the cylinder is analyzed emphatically.The main work includes:1. The test-bench is set up. And the experiment scheme is designed. The parameters to validate the model are measured from the experiment. 2. The geometry model of the combustion chamber of the CA4102 natural gas engine is built with CATIA software. And then the model is meshed with automatic mesh generating order. The combustion process is studied and analyzed through the simulation on the condition that the model is validated by the experiment.3. In order to study the influence of different shapes of the combustion chamber to the combustion process, three different shapes of the combustion chamber are designed and the simulation computation of the three models is done after they have been meshed.4. The influence of different ignition energy to the combustion process in the natural gas engine is studied through the simulation.The conclusions are drawn via the simulation:1. Flow motion has a greater influence on the combustion process. During the initial stage of ignition, the flame core leaves away the spark plug and moved towards the airflow. Along with combustion, the flame diffusion speed in the positive side of airflow is larger than it in the opposite side. When the piston is closed to the TDC, the boss on the piston crown nearing to the squash side strengthens the squash, increases the turbulent strength of the combustion nearing the TDC, which is benefit to improve the flame diffusion speed. But, on the other side, the boss decreases the area of the former flame, which will affect the combustion velocity. The highest temperature of in-cylinder appears at the earliest ignition zone which is near spark plug ,and spark plug is always in the high temperature zone, and the highest temperature was above 3100K. So much attention should be paid to the spark plug cooling of the CNG engine.2. The shape of combustion chamber has a major affection on the process of flow and combustion in cylinder. 1) On initial stage of ignition, reducing the mixture's flowing intensity near the spark plug is conducive to the formation of flame center after igniting, and to shorten the delay period. 2) Strengthening intensity of squeezing flow near the side of spark plug, which makes the direction of squeezing flowing consistent with the flowing direction of mixture on the right side of chamber that caused by combustion expansion, is conducive to the transmission of flame to the chamber on the opposite side of spark plug. The mutual cooperation of squeezing flowing direction caused on each side of the combustion chamber, which increases the turbulence intensity in chamber near TDC, is conducive to the rapid combustion of mixture in burning emergency period. 3)As for combustion rate, the 4# combustion chamber has the best performance in the four combustion chambers which are designed.3. Ignition energy has major influence on the process of combustion. Increasing ignition energy is conducive to the formation of flame center on initial stage of ignition, to shorten delay period and burning emergency period and to increase the maximum pressure and heat release rate of combustion in cylinder and combustion speed, which makes the peak of heat release rate near TDC in order to decrease the loss of high temperature radiation in late burning period. However, when the ignition energy increased to a certain value, it has a little influence on the process of combustion. The appropriate ignition energy of the CA4102 CNG engine is about 120mJ according to the simulation results while the life and the reliability of the ignition system are considered.