Investigation Of Combustion Process And Development Of Combustion System Of Lean-burn Nature Gas Engine

In order to satisfy the stringent emission regulations, it is important for the engine researchers to develop low emissions medium- and heavy-duty single fuel Compressed Natural Gas (CNG) engine used in the city bus. Through the theoretical analysis, numerical simulation and experimental study, the combustion process and combustion system are studied on a spark ignition CNG engine which was refitted from 6102 type diesel engine.With the combination of experimental test and Computational Fluid dynamics (CFD) simulation, the combustion chamber of prototype engine was optimized. Some conclusions are gained by the analysis of gas flow and flame propagation. The turbulence intensity has small influence on the formation of flame kernel during the early stages of flame development, but it has direct influence on flame propagation speed in the fast combustion duration. The flame propagation velocity is faster where turbulence kinetic energy is strong. The turbulence kinetic energy and crushing strength is increased with the increasing of squish area. In this paper, the effect of the spark plug location on combustion is analyzed. The results indicate that the flame propagation velocity is increased when the spark plug is placed in the center of chamber, but at the same time, the rate of pressure rise and NOx emission also increase greatly.The valve train system was optimized in order to reduce the HC and NOx emissions. The effect of cam phase on combustion and exhaust is studied by comparison the original and optimized cam profiles on the test bench. The conclusions are showed as follows. With small valve overlap, the exhaust of HC and NOx are reduced, but too small valve overlap at high speed deteriorates the HC emission. The ignition timing should be advanced properly at high speed, in order to keep the peak pressure timing before 20 oCA atdc, to avoid the post combustion and to reduce the exhaust temperature.In order to maintain the ignition stability and to increase the flame propagation velocity, the helical intake port was optimized on the base of the original intake port by altering the valve room height H and the relative angleθof scroll case. The numerical simulation under steady and transient state is carried out. The results show that, the turbulence kinetic energy is increased with the reduction of swirl ratio, and with higher turbulence kinetic energy the flame speed is increased, but flame extinction appears with very high turbulence kinetic energy.Finally, after the control strategy of the CNG engine is investigated, the overall test of CNG engine is carried out. The effects of ignition timing and air excess ratio on combustion are studied. The lean burn limit is investigated under various operation conditions. At last, the ETC emissions experiment is carried on. The conclusions are showed as follows. When the intake pressure and engine speed are constant, the combustion becomes unstable and NOx emission decreases with the reduction of fuel concentration. When the fuel concentration is constant, the combustion is retarded with the reduction of ignition timing. Moreover, the lean burn limit is decreased and the combustion becomes unstable. The lean burn limit and the stability of combustion are increased at maximal torque condition. The ETC emissions experiment with oxidation catalyst is carried out under the condition that the dynamics and economy are not worse than the prototype engine. Two tests results indicate that the CNG engine meets the National emissions standardⅣregulation in the ETC experiments.