| The " carbon peak,carbon neutrality " action is a major strategic decision-making deployment for the establishment of a clean,low-carbon,safe and efficient energy system in China,which poses an important challenge to the internal combustion engine industry to improve the energy efficiency of fossil fuels and reduce pollution emissions.Considering that the direction and distribution of energy flow in diesel engine can be used to characterize the distribution,change process and influencing factors of energy output from fuel combustion to indicated work,heat transfer,exhaust and other energy losses.In this study,CA6DL2-35E2 heavy-duty turbocharged diesel engine was used as the test object.By building a test bench,the steady-state condition test and transient condition loading test were carried out to obtain the test data required for threedimensional simulation.The three-dimensional simulation software CONVERGE was used to check and verify the cylinder of CA6DL2-35E2 heavy-duty turbocharged diesel engine.The key combustion parameters such as cylinder pressure and heat release rate were compared to prove the accuracy of the model.The energy balance thermodynamic system model of diesel engine cylinder is established,and the energy balance equation and exergy balance equation are established.The medium speed of 1650 r·min-1 and the load of 25%,50% and 75% were selected as the steady-state conditions of the study.The transient conditions of 25%,50% and 75% of the same fuel injection volume during the transient process of constant speed and torque increase of 1650 r·min-1 and 5 s uniform loading are extracted as the transient conditions of the study.The performance and energy flow characteristics of the two are compared.The results show that the typical 75% load performance and energy flow characteristics of the transient conditions are the most serious.The specific research conclusions are as follows :(1)Due to the intake hysteresis effect,the intake air volume under transient conditions is less than that under steady-state conditions,resulting in different degrees of deterioration of combustion parameters,dynamic performance,emission performance,energy flow and other parameters under transient conditions compared with steady-state conditions.With the increase of load,the deterioration is more serious.Especially under the condition of 75% load under heavy load,the cylinder pressure,heat release rate,average temperature in cylinder,combustion phase and combustion duration deteriorate seriously,and the harmful pollution emissions such as Soot,HC and CO increase.(2)Comparing the energy flow characteristics of typical load steady-state conditions and transient conditions,the results show that the main energy changes in transient conditions and steady-state conditions are intake and exhaust energy.The proportion of intake and exhaust energy in 25%,50% and 75% load transient conditions and steady-state conditions increases by 1.80%,3.76% and 3.73% respectively,and the indicated thermal efficiency decreases by 1.51%,1.92% and 4.33% respectively.The energy loss of 75% load transient conditions increases by 1.63% compared with that of steady-state conditions.The decrease of intake energy is mainly due to the intake hysteresis of the turbocharger during the transient loading process.The increase of exhaust energy is due to the serious combustion after the combustion speed is too slow in the transient condition,resulting in the increase of exhaust temperature and the increase of exhaust loss.On the other hand,the increase of unburned ratio leads to the increase of other energy losses,which distorts the energy distribution under transient conditions.From the perspective of in-cylinder energy distribution,energy efficiency can be improved by alleviating intake hysteresis,reasonably optimizing heat release law,reducing afterburning loss and reducing the proportion of unburned fuel.The available coefficients of heat transfer energy and exhaust energy increase gradually with the increase of load.The available coefficients of heat transfer energy and exhaust energy under transient conditions are higher than those under steady conditions,mainly because the exhaust temperature and heat transfer wall temperature under transient conditions are higher than those under steady conditions.The heat transfer energy is divided into three parts: piston,cylinder wall and cylinder head.It is found that there is an opposite trend between the heat transfer ratio of piston and the heat transfer ratio of cylinder wall.The heat transfer ratio of piston decreases with the increase of load,and the heat transfer ratio of cylinder wall increases with the increase of load.The heat transfer ratio of cylinder head does not change much,about 20%.The heat transfer loss of transient condition is less than that of steady state condition,which is mainly due to the small heat transfer coefficient of near wall side of transient condition.Although the wall temperature of transient condition is higher,the main factor is the heat transfer coefficient of near wall side.In view of the above research,it is found that the performance and energy flow distribution characteristics of diesel engine under transient conditions are significantly worse than those under steady-state conditions.In order to explore the influence factors of different combustion boundary conditions(fuel injection parameters and intake parameters)on the energy flow characteristics of diesel engine under transient conditions,and to seek the potential of different combustion boundary conditions(fuel injection parameters and intake parameters)to optimize the energy flow distribution characteristics of diesel engine under transient conditions.Taking 75% typical load under transient loading conditions as an example,single factor is used to adjust different combustion boundary conditions(fuel injection parameters and intake parameters).The specific research conclusions are as follows :(1)When the injection timing is delayed from 2 °CA to 2 °CA earlier than the original engine,the proportion of irreversible exergy loss decreases from 23.09% to17.27%,and the indicated thermal efficiency and indicated exergy efficiency increase by 2.84% and 2.66% respectively.Increasing the injection pressure can also reduce the unburned loss,improve the indicated thermal efficiency and reduce other energy losses.When the injection pressure is increased by 30 MPa,the indicated thermal efficiency is increased by 1.18%,and the proportion of other energy losses is reduced by 1.67%.The advance of injection timing and the increase of injection pressure not only improve the indicated thermal efficiency and reduce other energy losses,but also increase the heat transfer energy loss and the intake and exhaust energy loss.However,with the advance of injection timing and the increase of injection pressure,the maximum utilization coefficient of heat transfer energy and exhaust energy is also increasing.(2)When the transient loading condition is 75% load,reducing the intake air temperature is beneficial to alleviate the shortage of intake air in the transient condition,and can optimize the heat release law of fuel combustion and shorten the combustion duration.When the intake air temperature is reduced from 335 K to 295 K,the indicated thermal efficiency increases by 1.95%,the proportion of heat transfer energy decreases by 0.13%,the proportion of other energy loss decreases by 2.12%,and the proportion of unburned energy decreases by 2.04%.The decrease of intake air temperature reduces the proportion of fuel afterburning and exhaust temperature,so that the exhaust energy loss decreases with the decrease of intake air temperature.At the same time,the shortening of combustion duration shortens the heat transfer time,which leads to the decrease of heat transfer energy loss.Increasing the EGR rate of the intake air can effectively reduce the average temperature in the cylinder,so that the heat transfer energy loss and exhaust energy loss decrease together.When the EGR rate increases from 0% to 20%,the proportion of heat transfer energy loss and intake and exhaust energy loss decreases by 0.49% and 2.5% respectively.At the same time,due to the increase of EGR rate,the oxygen content of the intake air is reduced,and the unburned ratio is increased.When the EGR rate is 20 %,the unburned ratio is as high as 12.09%,resulting in other energy loss and irreversible exergy loss. |
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