Overall Efficiency Optimization Strategy Of Diesel Engine With Electric Turbocompounding Based Whole Energy Route

Electric turbocompounding technology has been regarded as a promising technique to recover waste heat of engine exhaust.However,it is still a problem to improve the fuel economy of vehicles under driving cycle condition due to the strong coupling phenomenon between internal combustion engine and electric power turbine and dynamic characteristic of turbocompounding system.A whole energy route is established including electricity generation by serial power turbine,electricity storage by battery and electricity consumption by engine thermal management based on heavy duty diesel engine.The coupling effect,dynamic effect and operational conditions adaptability of the system are uncovered.The energy management strategy is investigated from the perspective of overall efficiency optimization and miles of vehicle tanks enhancement.The investigation work aims at providing reference for the match and energy management ideas for the electric turbocompounding system.Firstly,the whole energy route test rig and the applicative whole route simulation platform are built to investigate the basic characteristic and energy management strategy of the system.The power turbine and super high speed three phase AC permanent magnet generator are developed,the issue of power turbine and high speed generator integration is solved.The experimental prototype of diesel engine serial electric turbocompounding system is established and the rated electric power generation is 20 kW.The electric energy storage unit is chosen and the diesel engine electric thermal management system is integrated and tested as energy consumption subsystem.The whole energy route experimental platform is accomplished.What is more,the applicative whole route dynamic simulation platform includes the model of diesel engine characteristic,cooling system,vehicle,driver and road condition is established.The operational conditions adaptability and energy management strategy of the whole energy route is investigated on the simulation platform.Secondly,the coupling effect and influence law of key state parameters on coupling effect is investigated based on the experimental test rig developed.Results show that exhaust energy recovery by power turbine is at the cost of pumping loss and gross work reduction among which pumping loss is the main contradiction.Net power of the system depends on the break even state of cost and recovered power: the overall efficiency of original engine cannot be enhanced at low load condition,however,improvement of overall efficiency exists at medium and high load condition.The improvement percentage increases as engine load becomes higher.Turbine efficiency and expansion ratio are vital state parameters influencing the coupling effect.Research shows that there exists a basic condition as critical expansion ratio of power turbine for the effectiveness of serial turbocompounding technology.Moreover,an efficiency dividing line exists for the expansion ratio within the ciritical value,above which the overall efficiency of the system will be enhanced.Optimal expansion ratio exists corresponding to the maximal overall efficiency if power turbine efficiency is considered as a constant.Proper expansion ratio split between charging turbine and power turbine is important and the ratio of expansion ration between charging turbine and power turbine should be higher if the efficiency of power turbine decreases.Regulation is necessary due to the coupling effect between internal combustion engine and power turbine.The regulation principle of bypass valve opening degree,rotating speed of power turbine and injection timing of engine is investigated.Results show that exhaust bypassing is necessary in low load region in order to avoid the deterioration of overall efficiency.The overall efficiency reaches a maximum only when the bypass valve is fully closed in the engine operating region where the overall efficiency can be enhanced.The efficiency of generator and power turbine varies significantly with revolution speed.There exists an optimal rotating speed corresponding to the maximum overall efficiency under a certain exhaust mass flow rate.Investigation of injection timing shows that retardation of injection timing cannot be enhanced the overall efficiency under the current turbine efficiency level.Then,the dynamic effect,operational conditions adaptability of the whole energy route and basic energy management principle are investigated based on the simulation platform.The accumulated dynamic factor is put forward to describe the dynamic characteristic of the whole energy route.It is found that the time-average efficiency of power turbine goes lower when the accumulated dynamic factor becomes higher.The time-averaged efficiency of power turbine is between 46%-66% under driving cycle condition,which is much lower than optimized value.A new assessment factor named general fuel reduction percentage is put forward to describe the operational conditions adaptability of the whole energy route.Results show that the whole energy route is applicable to the driving cycle featured by high vehicle speed.The percentage of high load condition increases when vehicle speed becomes higher.However,the general fuel reduction percentage increases with slower amplification.The match between energy generation and consumption under steady and driving cycle conditions indicates that balance between electric power generation and consumption is necessary for the fuel tank mileage maximization.Finally,simplified efficiency model of diesel engine serial turbocompounding system is established and a two layer energy management strategy based on efficiency model is put forward.The efficiency model is used in the planning layer and the decision variables are determined through traversal solution process.An equivalent factor which is adjusted according to the SOC of battery on line and a “least bypass valve opening time” strategy are used in the execution layer to elevate the time-averaged efficiency of power turbine and to keep balance between electric energy generation and consumption.Simulation results indicate that the fuel consumption per 100 km reduces by 5.7% in HWFET driving cycle compared with basic engine powered vehicle if rule based control strategy is adopted.The accumulated dynamic factor reduces from 0.25 to 0.19,the time-averaged efficiency of power turbine increases from 57.9% to 67.3% and the fuel saving percentage can be up to 6.6% if the strategies provided above are used.