Study On Matching Method And Switching Boundary Of Diesel Engines With Turbocharing Systems At Different Altitudes

The steady performance of turbocharged diesel engines is maintained based on that turbine power ability can guarantee required compressor power.The changing operating conditions and different altitudes situation would complicate contradictory problem between required compressor power and turbine power ability.This disequilibrium problem can decrease the output power and increase the brake specific fuel consumption at high altitudes.The design of highland turbocharging system becomes an effective solution to achieve the target of power recovery and improve engine adaptability at different altitudes.However,this solution would bring the following new challenge: The problems of more matching hardness and unwarrantable matching accuracy can be brought out with traditional matching method to determine the turbocharging system scheme;Traditional determination methods of switching boundary have the limitations of expiration within special zones,huge calculated amount for simulation and experiment processes.In this paper,the steady matching problem can be resolved on the purpose of power recovery with the matching method based on boost pressure recovery and considering overall efficiency of turbocharging system.The determination problem of switching boundary can be simplified with the buildup of calculation model for turbocharging system switching boundaries at different altitudes.Turbine equivalent area model and isentropic efficiency equation are deduced on the basis of turbine flow characteristic model.The equivalent area relational expression at different altitudes is achieved wth the target of boost pressure recovery.With the combination with prediction calculation of overall efficiency at different altitudes,the new matching method is proposed based on boost pressure recovery and considering overall efficiency of turbocharging system.The matching schemes of different diesel engines are determined with this matching method.The rationality and effectiveness of matching schemes are verificated with the simulated and experimental results at different altitudes.The determination method of optimal design parameters is built up through the process of theoretical analysis and equation solution.This method can obtain intake requirement characteristic for power recovery at different altitudes,optimal pressure ratio distribution equation corresponding to the highest overall efficiency of turbocharging system,and selection principle of matching point according to variable altitude adaptability.The results show that the total pressure ratio is related to performance parameters,different altitudes,power recovery target,and mechanical efficiency at plain region.For ideal situation,the optimal pressure ratio distribution is only related to the overall efficiency of two-stage turbochargers.With the consideration of the changing intake and exhaust inlet conditions,the selection of optimal pressure ratio distribution should make the intake wasted work of high pressure turbocharger close to low pressure turbocharger to minimize the total intake compression work.The altitude range of turbocharging system can be affected by the selection of matching point and bypass flowrate ratio of turbine bypass valve,and the matching point of turbocharging system usually should be fixed at maximum torque speed not rated speed.The switching boundary calculation model of turbocharging system is proposed to simplify the determination problem of switching boundary at different altitudes.This calculation model is constructed with two relational expressions: the corresponding relationship between turbine equivalent area and operating consitions,and the corresponding relationship between turbine inlet temperature and engine torque at same speed.With the matching schemes of single turbocharger system and regulated two-stage turbocharging system,the switching boundaries of waste-gate valve and turbine bypass valve are calculated at different altitudes.The verification analysis is carried out with simulation method and plain region experiment.The results show that the relative error of engine torque is less than 5% between simulated results,experimental results at plain region and calculation model results,and the switching boundary calculation model has sufficient accuracy.A regulated two-stage turbocharging system is designed for a highland diesel engine,and a simulated test bench is built up to verificate the matching scheme,engine performance and switching boundary at different altitudes.The operating modes of turbocharging system are determined on the basis of switching boundary for two different target boost pressure at different altitudes.The results show the matched scheme has achieved the power recovery target at the altitudes of 3000 m and 4500 m.The fuel economy can be improved by 11.9%,and the thermal load can be reduced by 146.6K with the matched scheme.The switching boundary of turbine bypass valve will move down,and the operating zones of closing mode will reduce gradually with the increase of altitudes.