Research On Combustion And Emission Of Pre-chamber Stratified Lean Combustion System In Natural Gas Engine

Natural gas has become to be the promising alternative fuel for internal combustion engines due to the advantages of rich reserves, low pollution emission and good economy. Developing natural gas engine can replace part of the diesel and gasoline engine, and reduce environmental pollution. Typically, there is no need of after treatment apparatus, because gas engine with lean burn technology can meet for the current emission regulations. However, there are instability and upheaval and other problems under the lean burn gas engine combustion cycle. The way to enhance ignition energy is needed for the lean mixture. In this research, the pre-chamber combustion system is applied. By using independent enriching injection, near stoichiometric mixture is formed in the pre-chamber, which has stable ignition and fast combustion, providing extremely high ignition energy as well as high intensity of turbulence for the lean mixture in the main chamber. As a result, stable ignition and rapid combustion occur at the main chamber, and the thermal efficiency of the engine is improved.Numerical simulations have been proceeded by using the CONVERGE code. The effect of different pre-chamber geometry parameters(channel number、β value and channel angle), main chamber geometry and operating parameters such as spark advanced angle, enrichment injection strategies were analyzed. And the formation of NOx emission, stratification process in pre-chamber also has been uncovered. The influence of different parameters can be quantitatively analyzed. The main work are as follows:(1) By researching on per- chamber geometers, it is found that selecting appropriate pre-chamber parameters(4-channel number, β value of 0.35, the channel angle 140°) can achieve uniform distribution of the fuel/air mixture in the pre-chamber, realizing stability and rapid combustion.(2) By researching the effect of different ingniton advance angle on the combustion system, ignition advance angle has an important influence on the performance parameters of the pre-chamber system. Under the condition of the ignition advance angle of 18°, it can obtain the best balance on indicated power, fuel consumption and NO x emissions. Finally, this ignition advance angle and the aforementioned pre-chamber parameters are confirmed as the optimized parameters, which can remarkably enhance the combustion rate, leading to a reduction of 41.5% for NO x emission rate and 6.8% for fuel consumption and increase of 14.1% for indicated power, respectively.(3) Enrich injection strategy, including injection timing and the injection quantity are studied. It is found that the injection quantity should not be too high, and the injection timing should be earlier. In conclusion, strategy program 16 is more suitable for the study.(4) Main combustion chamber geometry has a great impact on the pre-chamber combustion system, which changes the combustion flame development process. Matching open cylinder-shaped combustion chamber can effectively improve the utilization of the air, and accelerate the combustion, improving the thermal efficiency.