Analysis Of Energy Flow, Exergy Flow And Optimization Of Energy Efficiency In An Automotive Turbocharged Diesel Engine

Facing increasingly tense energy supply-demand relationship and stringent energy efficiency regulations,further improving the energy efficiency of traditional automotive internal combustion engine has become the biggest power to promote the development of internal combustion engine science and technology.In order to meet the major demand of energy conservation and emission reduction,solve the existing problems in the study field.In this study,an automotive turbocharged diesel engine was taken as research object,the energy flow,exergy flow and energy efficiency optimization of automotive turbocharged diesel engine were investigated based on engine measurement and control system,one-dimensional,three-dimensional simulation models.The energy flow and exergy flow distribution characteristics,change rules and boundary parameters influence mechanism were determined,moreover,the way to improve energy efficiency of automotive diesel engine by boundary parameter collaborative control was discussed.Main research works and conclusions of this research are as follows:(1)The measurement and control system,one-dimensional and three-dimensional simulation platform with high precision and fast response were established,which can realize the fast and accurate measurement of power,heat,combustion and other parameters of diesel engine,and satisfy the research requirements of energy flow and exergy flow.The energy flow and exergy flow of turbocharged diesel engine evaluation and analysis methods were established,which took residual energy availability coefficient,energy and exergy sensitivity,energy and exergy distortion coefficient,effective thermal efficiency(hereinafter referred to as thermal efficiency),exergy destruction rate and other parameters as the main quantitative indexes.(2)Based on energy,exergy analysis and evaluation methods,the energy and exergy distribution characteristics of turbocharged diesel engine at overall working conditions were studied.Furthermore,the effects of boundary parameters on residual energy availability were studied by means of test.Main conclusions are as follows:1)In overall working conditions,low efficiency region and low load region of diesel engine are basically consistent,high efficiency region is mostly concentrated in high load of medium speed region.The maximum values of thermal efficiency and exergy efficiency are about 43.5% and 41%,respectively.Thermal efficiency is less than 30% at the region with load less than 15%;2)Exhaust energy and exergy have similar distribution characteristics.The region with high speed and high load is the key region of exhaust energy recovery,moreover,exhaust energy availability coefficient is higher at high load condition;3)Heat transfer residual energy availability coefficient at whole working condition is almost independent on speed and positively related to load,the region with high speed and high load is the key region of exhaust energy recovery,moreover,“over cooling” effect should be avoided in the low speed and low load region;4)High exergy destruction region and low load region basically coincide,especially at low load of high speed region,the exergy destruction percentage is even more than 50%.Exergy destruction percentage is the lowest in high load of medium speed region,the lowest value is about 30%;5)Reducing the finite temperature difference between high temperature working medium and coolant can effectively reduce heat transfer residual energy availability,while strengthening the characteristics of low temperature and rapid combustion in cylinder can effectively reduce exhaust residual energy availability coefficient.(3)Intake air temperature,coolant temperature,EGR rate,injection timing and injection pressure were adjusted based on simulation and test means.The effects of boundary parameters on energy and exergy balance relationship,sensitivity,variation,and in-cylinder exergy destruction of turbocharged diesel engine were studied.Main conclusions are as follows:1)Intake air temperature increases from 30℃ to 70℃,energy efficiency and exergy destruction percentage reduce,while the percentage of heat transfer terms and exhaust terms increases.Effective work sensitivity to intake air temperature is higher at low load and low intake air temperature.The root cause of intake air temperature influence on in-cylinder exergy destruction is the influence on local temperature;2)Coolant temperature increases from 50℃ to 90℃,energy efficiency increases but heat transfer terms percentage decreses.At B75 working condition,thermal efficiency increased by 1.99%,while heat transfer energy decreased by 4.05%.The influence of coolant temperature on in-cylinder exergy destruction is weak.Effective work sensitivity is higher at medium and high coolant temperature;3)With EGR rate increasing,efficiency increases first and then decreases,while exergy destruction percentage decreases first and then increases.The root cause of EGR effect on in-cylinder exergy destruction lies in the influence on local temperature and equivalent ratio.4)With injection timing advance,energy efficiency increases slightly.Effective work sensitivity to the front injection timing is higher.Fundamental reason for the influence of injection timing on in-cylinder exergy destruction is the influence on local temperature;5)Energy efficiency is positively related to injection pressure.Injection pressure has a weaker influence on the percentage of energy and exergy compared with injection timing.Effective work sensitivity to injection pressure is smaller.Root reason for the influence of injection pressure on in-cylinder exergy destruction lies in the influence on local equivalent ratio;6)For different working conditions,the energy efficiency optimization direction based on boundary parameter active control has similar characteristics,that is,low intake temperature,high coolant temperature,low EGR rate,front injection timing and high injection pressure.(4)Loading time,intake air temperature,coolant temperature,EGR valve opening were adjusted at typical transient working condition of turbocharged diesel engine.The influence mechanisms of boundary parameters on energy flow and exergy flow of turbocharged diesel engine at transient process were also explored,including energy and exergy balance relationship,distortion characteristics and so on.On this basis,the energy efficiency optimization methods of turbocharged diesel engine at transient working conditions were explored by the piecewise loading strategy.Main conclusions are as follows:1)In transient process,energy efficiency increases first and then decreases,while exhaust terms and heat transfer terms percentage decreases.Energy and exergy distortion degree at short loading time are relatively significant compared with long loading time.Thermal efficiency at the end of 3s loading is about 6% lower than that at the end of 7s loading.Fuel incomplete combustion leads to energy distortion in transient process,energy efficiency deteriorates;2)With intake air temperature increasing,the energy efficiency of diesel engine decreases at the same loading time.Low intake air temperature is helpful to restrain energy efficiency distortion during loading proces;3)With coolant temperature increasing,energy efficiency of diesel engine increases at the same loading time,while the variety of incomplete combustion and exergy destruction proportions are not obvious.Effective work distortion coefficient at high coolant temperature is smaller.Enhancing the adiabatic characteristics of heat transfer system in transient process can effectively improve energy efficiency of diesel engine;4)Compared with other boundary parameters,the influence of EGR valve opening on energy and exergy in transient process is the largest.The superposition effect of EGR and intake delay significantly delays energy and exergy response in transient process,and the delay degree of response,energy and exergy distortion degree are all positively related to EGR valve opening;5)In loading strategies of staged loading,the influence of different loading elements on energy efficiency is from heavy to light: first stage loading time > stagnation time > second stage loading time.Longer first loading and stagnation time are helpful to reduce energy,exergy distortion degree and enhance energy efficiency.(5)The collaborative optimization of boundary parameters for energy efficiency optimization was carried out.According to boundary parameters coordination principle---optimal energy efficiency,the optimal energy efficiency and their solution set at different working conditions were obtained.And then,optimal energy efficiency solution considering the emissions was selected by the way of compromise.Main conclusions are as follows:1)The optimal energy efficiency distribution and their solution are relatively concentrated at different optimal working conditions,they are generally characterized by low EGR rate,high coolant temperature and low intake air temperature;2)In energy efficiency optimal solution which fully considers emissions,with the load increasing,EGR rate moves in the direction of low EGR rate,intake air temperature moves in the direction of high temperature,but coolant temperature has little change;3)Compared with low load working conditions,high load conditions have higher energy efficiency optimization potential,the difference values of energy efficiency before and after optimization are 1.5%,1.8%,2.3% at optimal working condition 1-3,respectively.