Study On Combustion System Of The High Performance And Large Power Natural Gas Engine

Facing the serious challenges of energy and environmental protection, all the countries in the world began to think about adjustment and sustainable development of the energy utilization structure seriously. Natural gas has the advantages of rich resources, low emission, and low price and so on, which should be a suitable alternative fuel for engine, is playing more and more important role in meeting energy demands. Researchers began to research natural gas engine in our country and have made tremendous progress since the eighty’s of last century, and have made tremendous progress. But there is still a big gap with foreign advanced technology in the research and development of big power natural gas engine, which restricted the use of natural gas engine in the fixed purpose, such as power generation applications, so R&D technology of high power natural gas engines are the big problems that we desiderate to solve.Based on a large bore natural gas engine, the pre-combustion chamber type combustion system which can fulfil IMO Tier Ⅱ was studied by the methods of theoretical analysis, the combination of numerical simulation and experimental verification; and the combustion system meets Tier III emission standard was researched. The main jobs of this thesis include:(1) The new R&D method of performance for multi-cylinder large bore natural gas engine was put forward in this thesis combining the thermodynamic simulation and single cylinder testing engine, it.improve the development process and the accuracy greatly. Firstly, based on the parameters of similar engine and R&D experience, the thermodynamic model of multi-cylinder engine was established; secondly, the thermodynamic model of single cylinder engine was established according to the simulation results of multi cylinder engine and the single cylinder testing engine was designed and created, the operation boundary conditions are all come form the thermodynamic simulation of single cylinder engine, and then the performance test and components test are carried in this engine; thirdly, the testing results such as the heat release rate was used to modify the thermodynamic model of single cylinder engine, and then the calibrated model was explanted to multi-cylinder engine model, and the performance parametes were forcasted accurately using this thermodynamic model.(2) Combined with reduced chemical kinetic model of CH4, the effects of connecting channels between the pre-chamber and the main chamber of the pre-chamber combustion system to overall combustion were studied in detail using the three-dimensional numerical method. The more channels with smaller diameter are adverse to the burning velosity as the throttle is serious when the total flow area fixed. On the other hand, the fewer channels with bigger diameter will affected the combustion speed because the vortex motion in cylinder is weak. Compared to the10and6connecting channels,8channels with2mm diameter can obtain the most rapid combustion.(3) The smallest channel section would not conductive to the flame propagation from the pre-chamber to main combustion chamber; the biiger channel diameter should beneficial to the rapid combustion of lean mixture in main combustion chamber.(4) The flame will be spread faster along with the direction of the movement of the piston with the smaller channel angle; while he flame will be spread faster along with the direction of the diameter of the cylinder with the larger channel angle. Slightly larger channel angle is more conducive to the rapid combustion of the gas mixture in main combustion chamber because of the larger diameter, weak swirl and shallow basin shape combustion chamber.(5) Numerical studies of different precombustion chamber volume to combustion found that the smaller pre-chamber volume can obtain richer mixture than bigger one in the same chamber gas supply, so the higher combustion pressure in the pre-chamber could be achieved, which can accelerate the flame propagation velocity of lean mixture in main combustion chamber.(6) The concept of knock index was put forward to detect and evaluate the detonation of natural gas engine, and the detonation limit of this engine was achived by the test. The last verification testing found the developed combustion system can meet the IMO Tier II standard, and achieved all the requirements.(7) Based on the numerical results of combustion system, the exploitative tests of combustion system were carried on the test bench of single cylinder testing engine. The effects of two open pressures of check valve to combustion were investigated, the results show that lower opern pressure will bring more time for gas mixture in pre-chamber, so the combustion speed is faster than that of bigger open pressure, so it will have more energy to ignite the lean mixture in the main combustion chamber.(8) The gas movement in the cylinder was investigated in order to study the combustion system which can meet more strict emission standard. The different steady evaluation methodologies were comparative analyzed, and the instant evaluation parameters were used to study the gas movement of intake stroke and compression stroke, and the effects to combustion were discussed.(9) The concept of flame penetration was put forward to evaluate the combustion speed of lean mixture in main combustion chamber. And the effects of different swirl rate and concentration of mixture to combustion were studied. The results show that the swirl ratio nearly no effect on combustion speed when the λ less than1.8, the driving force of the lean mixture burning in main combustion chamber is the higher differential pressure between pre-chamber and main combustion chamber which is achieved by combustion in pre-chamber. The swirl ratio is benefit to the faster propagation of flame in lean mixture.(10)The matching studies of gas movement and combustion system were carried on. And a new project was put forward which could meet IMO Tier III standard, it include the combustion system with pre-chamber, higher intake swirl ratio, strong Miller cycle in which the intake valve close at520deg, and higher pressure ratio with3.8.Bsed on CAE and testing, the developing method of multi-cylinder bigger bore natural gas engine and the combustion system was studied, at the same time the evaluation of detonation and the relationship between the gas movement and the excess air factor were investigated in this thesis. These would give some theoretic and experimental references for the researches and the development of high performance, large power pre-chamber type natural gas engines.