Computational And Experimental Study On Variable Geometry Exhaust Manifold Turbocharging System

Constant-pressure turbocharging system and pulse turbocharging system are the two basic styles of turbocharging system. The turbine efficiency of the constant-pressure turbocharging system is higher and the negative pumping work of it is less than that of the pulse turbocharging system during the high speed operation. The constant-pressure turbocharging system works better during the high speed operation, while it works worse during the low speed operation because the pulse energy can't be used sufficiently. The pulse turbocharging system has a relatively small exhaust manifold volume, which can avoid scavenging interference and get the pulse energy better utilized. It works better during the low speed and the transient response operation. But the negative pumping work of the pulse turbocharging system is bigger than that of the constant-pressure turbocharging system during the high speed operation. Variable geometry exhaust manifold (VGEM) turbocharging system can realize the switch between two charging modes by the switch valve. The VGEM turbocharging system works as a pulse turbocharging system when the switch valve is closed at the low speed or the transient response operation, and it works as a semi-constant pressure turbocharging system when the switch valve is opened at the high speed operation. The VGEM turbocharging system has the advantages of the constant pressure and the pulse turbocharging system, it can improve the coordination of the high speed operation and the low speed operation, and it has the potential application value. So, computational and experimental study on the VGEM turbocharging system has been performed in this paper.In this paper, firstly, the program simulation of the VGEM turbocharging systems for five-cylinder, six-cylinder, seven-cylinder, eight-cylinder, nine-cylinder marine diesel engines has been done respectively. The fundamental simulation model is built on the Z8170 marine diesel engine. The excellent design programs of the five engines have been found by comparing the program simulation results. The general design method of the VGEM turbocharging system has been put forward. The exhaust manifold junction model is very important for the design of the exhaust manifold in the VGEM turbocharging system. The cold wind tunnel experiments with higher Mach number for two oblique triangle"T"junctions have been carried out in order to improve the simulation accuracy of the total pressure loss coefficient. The corrected formula of the total pressure loss coefficient has been put forward by using of the experimental results and the existing theoretical formula. The corrected formula related with the Mach number. The simulation accuracy of the total pressure loss coefficient can be improved by the corrected formula.In order to study on the overall performance of the VGEM turbocharging system with one switch valve installed on the exhaust manifold, detailed simulation of the VGEM turbocharging system for the six-cylinder vehicle diesel engine and the eight-cylinder marine diesel engine has been done respectively after the junction model is corrected. For the VGEM turbocharging system of the six-cylinder vehicle diesel engine, the switch point at full load characteristic has been defined, and the simulation results indicate that the BSFC can be reduced by 3% on the rated operation condition. For the VGEM turbocharging systems of the 1200 r/min and the 1000 r/min eight-cylinder marine diesel engine, the switch points of the propeller law have been defined. The performance of the new designed VGEM turbocharging system is excellent compared with four pulse, PC,and MPC turbocharging systems. The transient simulation results of the propeller law indicate that the transient performance can be improved obviously when the switch valve of the VGEM turbocharging system is closed; the time for the intake pressure increased to the 90% stable pressure can be reduced by 27% compared with the state that the switch valve is opened, and it can be reduced by 32% compared with the MIXPC turbocharging system.Simulated experiments for the VGEM turbocharging systems of the six-cylinder vehicle diesel engine and the eight-cylinder marine diesel engine have been performed respectively in order to certify the simulation results. Based on the principle of optimum fuel economy, the switch rules of the switch valve are found by comparing the steady experimental results. For the VGEM turbocharging system of the six-cylinder vehicle diesel engine, the BSFC can be reduced by 4.7g/kW.h on the rated operation condition when the switch valve under the opened-state. The transient experimental results indicate that the time for the turbocharger speed became stable can be reduced by 30%, the time for the intake pressure increased to the 90% stable pressure can be reduced by 16%, and the peak value of the smoke opacity can be reduced by 15.3% when the switch valve under the closed-state. For the VGEM turbocharging system of the eight-cylinder marine diesel engine, the lower load performance can be improved. The BSFC can be reduced by 7.5% and the exhaust temperature before turbine can be reduced by 32% at 25% load when the switch valve under the closed-state. The experimental results have proved that the simulation results are reasonable, and the VGEM turbocharging system is excellent compared with other conventional turbocharging systems.