Simulation Study On The Charge Mixing State Of CAI Based On Internal EGR

It's well known that there are always great challenges we have to face in the 21^st century--the transportation energy sources and environment pollution. However, with the increasing amounts of vehicle in our country, the situation becomes more and more serious. Reducing the fuel consumption and engine emission are two major questions had to be dealt with for further engine development.Conventional engine include spark ignition engine and compression engine. The combustion of spark ignition engine is premixed homogeneous promulgation combustion. The mixture is ignited by the spark. Due to the limits of the characteristic of gasoline and knock of the spark ignition engine, the he Compression Ratio (CR) in the spark ignition engine is small. This leads the decrease of the thermo efficiency and the formation of many Nitrogen Oxides (NOx) and unburned gases. On the other hand, the Spark ignition engine controls the load through the throttle, this leads to the decrease of mechanical efficiency in the middle load. The Spark ignition engine's energy efficiency is 70% of the counterpart in the Diesel engine. This is the insurmountable limitation of the Spark ignition engine Diesel engine's combustion is diffusing combustion. The mixture starts ignition at the end of the compression stroke. Because of the short time to form the mixture, the mixture of the fuels and air isn't homogeneous. This leads to the formation of the high temperature flame region. Because the temperature of diesel engine is very high, NOX emission is vast. The lean of oxygen at flame region with high temperature and consistency of mixture leads to the occurrence of a mass of PM and so there is a limit in the emissions of PM and NOX in diesel engine.CAI (Controlled Auto Ignition) gasoline engines have been received a lot of attention from the world-wide automotive field due to their ultra-low NOx, emissions and high thermal efficiency. However, they are confronted with the problems in ignition timing control and operation range extension. In this paper, the combustion process of a CAI gasoline engine was investigated by numerical simulation. Much of the previous experimental and simulation work related to the CAI engine process has been demonstrating that detailed chemical kinetics was the primary factor which influences on the CAI combustion while other possible effects could be ignored. However, recent research indicated that the charge stratification state played an important role in controlling the ignition timing and the rate of heat release for CAI engine. The flow field and mixture condition were found to be more complicated using internal EGR to achieve CAI combustion. Preliminary simulation work was executed with CFD code STAR-CD with the overall objectives to understand the influence of spatial nonuniformities of tempe- rature and mixture on characteristics of ignition and subsequent combustion in an CAI-like environment and to provide information relevant to the optical research of CAI combustion.Computing grid needs the accuracy of geometry model to ensure. In this paper, the outline of the intake and exhaust port was measured with three-dimensional coordinate measuring instrument or laser measuring instrument. Then, with modeling software the data file of Technology of Reverse Engineering and CAD were employed to rebuild the outline of the port after dealing with the data from measuring. With the common tetrahedron or polyhedron automatic gridding maker PRO-AM, the gridding of the intake port, exhaust port and chamber can be generated automatically. With the engine special moving gridding maker ES-ICE, the moving gridding was made with mapping method. Finally, usable computational gridding was finished by associating the intake port, exhaust port and chamber gridding with moving gridding.In order to understand how speed and valve timing affect the in-cylinder charge condition in intake and compress strokes, characteristics of the in- cylinder velocity field, turbulent kinetic energy field, mixture concentration field, temperature field and NOx concentration field were emphaticly investigated .The items which results in the change of admixture state was also explored. Calculated results indicated that in-cylinder flow was divided into intensive mixing phase and weak flow mixing phase. The gas velocity and turbulent kinetic energy were higher in intensive mixing phase which has the potential to accelerate the mixture between fresh charge and residual gas and the spread of low temperature area, and in weak flow mixing phase, .the gas velocity and turbulent kinetic energy drop signify- cantly, large scale eddy was broke up into small ones which plays the same role in blending the fresh charge and residual gas.The concept of Variance in Statistics was introduced to estimate the level of temperature stratification. The results showed that the uniformity of temperature at the ignition timing became worse as the speed raised and the IVO timing retarded.The spatial location where ignition occurs first could be predicted by the investigation of the maximum temperature point location.