The Cycle Simulation And Optimum For The Single Fuel Of The Natural Gas Internal Combustion Engine

Due to the large consumption and pollution to environment of the oil as the fuel ofinternal combustion engine, natural gas as an alternative fuel obtains more and moreattention, and the natural gas vehicle is developing rapidly worldwide. Because of theadvantageous and clean physicochemical property of natural gas, study on theperformance of single fuel natural gas internal combustion engine becomes one of themost important research areas. However, there is also a series of problems relative to thenatural gas engine, such as inadequate power output, slow combustion flame speedcomparing to the gasoline engine, unclear thermodynamic process in the cylinder and soon. Hence, carry out researches on the single fuel natural gas internal combustionengine to improve the working performance is very significant.In this paper, the performance of a four-stroke internal combustion engine iscarried out by zero-dimension thermodynamic model and three-dimensional CFDsimulation, respectively, and the results of these two models is verified based onexperimental data, and then, more study was carried out on the whole cycle of a naturalgas internal combustion engine. After that, the thermodynamic cycle characteristics andcombustion process of the natural gas engine are analyzed in detail, and the optimumspecification for the natural gas engine is proposed to improve the engine performance.According to the CFD simulation on the performance of a pre-mixed spark ignitionnatural gas internal combustion engine, it is found that the turbulence fluctuation in thecylinder curls the flame boundary during the combustion process, which increases theinterface area between the burning flame and the unburned fuel and accelerates the fuelcombustion speed. Also, the core of the flame is affected by the turbulence flow in thecylinder significantly and moves to the left side of the cylinder, which caused uniformtemperature distribution appearing and the temperature in the left side is higher than thatin the right side. In the pre-mixed spark ignition gas engine，the combustion speed isdepended on the flame propagation, and that is the reason why the natural gas in thecylinder burns slowly comparing with that of gasoline. So, in the design of natural gasengine, the spark advancing angle of the gasoline engine is not suitable and will causethe primary combustion of natural gas occurs in the expansion stroke, and reduce thepower output and thermal efficiency. In addition, the simulation result shows that theformation and distribution of NO are affected by the flow turbulence and temperature profile obviously. The formation rate increases significantly when the temperature ishigh and, and keep almost constant during the later stage of the combustion process.In the paper, the homogeneous charge compression ignition (HCCI), as a newtechnology of combustion which can realize high thermal efficiency and cleancombustion, is also studied in detail. In this study, a chemical reaction mechanism ofnatural gas combustion based on Chemkin is coupled with the three-dimension CFDsimulation. Comparing with the spark ignition combustion process of natural gas, theburning rate with HCCI is dominated by the chemical reaction kinetics and affected bythe flow turbulence mildly. What is more, once the mixed air is compressed to beignited, the combustion of the natural gas will process with a very high speed and theresulted pressure in the cylinder will increase quickly, which will lead to the cylinderknocking and other problem if the air-fuel ratio is too small. Besides, the workingperformances of natural gas engine with HCCI under different excess air ratio andinhaled air temperature are studied. The results show that, with increasing excess airratio, the timing of ignition should be delayed to achieve a good engine performance.Meanwhile, the peak temperature in the cylinder will decrease, which decreases theformation of the NO. Due to the high ignition temperature of natural gas, thecombustion process depends on the temperature of fresh air inlet obviously.The Weibe-function, as an expression of the heat release rate during thecombustion process, is very important to the thermodynamic cycle simulation in thecylinder. In this paper, based on the results of CFD simulation of pre-mixed sparkignition natural gas combustion, a Weibe-function for natural gas combustion isproposed. Then the whole cycle of the natural gas internal combustion engine issimulated based on the zero-dimension model. According to the zero-dimensionsimulation, the key specifications, including spark timing, compression ratio, excess airratio and so on, are compared and optimized to obtain the best performance of thenatural gas engine.