| Under the background of parallel development of conventional fuel vehicles and new energy vehicles,it is an important and urgent research topic to continuously improve the thermal efficiency and optimize the performance of vehicle engines.Low thermal efficiency and unsatisfactory energy distribution are found in conventional Otto cycle gasoline engines,whose heat rejection energy and exhaust energy account for about 2/3 of the total energy.In this paper,in order to optimize the energy distribution characteristics of the gasoline engine,the original Otto cycle was modified to the Atkinson cycle with relatively longer expansion stroke,combined with low heat rejection condition in cylinders,to improve the indicated thermal efficiency by reducing heat loss and exhaust energy loss.Combined with simulation,test and theoretical analysis,the intake characteristics and combustion characteristics of low heat rejection gasoline engine with Atkinson cycle on different loads were revealed under the condition of 2000r/min and theoretical equivalent ratio.The distribution characteristics of indicating work,heat rejection energy and exhaust energy on variable working cycles,heat rejection conditions and loads are explored.Besides,The effect of early exhaust valve opening angle and exhaust gas recirculation rate(EGR rate)on the performance of low heat rejection gasoline engine with Atkinson cycle were studied.The research contents and conclusions are shown below:(1)Firstly,the one-dimensional and three-dimensional simulation models of a 1.6L gasoline engine were established,coupled and calibrated according to the structural parameters and test data of the prototype engine,and the prototype models were modified after the verification.To achieve the Atkinson cycle,the throttle valve was canceled,and three sets of intake valve lift curves with 90°CA,120°CA,and 140°CA of late intake valve closing angle were designed to match the load of 80%,49%,and 27% respectively.The geometrical compression ratio of the prototype was increased from 10 to 13 by reducing the top dead center clearance height.Based on the thermophysical properties of the 1mm YSZ insulation layer,the heat transfer model was improved to achieve low heat rejection condition in cylinders.The modified simulation model of low heat rejection gasoline engine with Atkinson cycle was obtained by the above methods.(2)On the three load conditions,the optimal ignition advance angle of the low heat rejection engine with Atkinson cycle is greater than that of the normal heat rejection engine with Atkinson cycle,but smaller than that of the prototype with Otto cycle.Compared with the Otto cycle,higher mixture pressure and temperature in the intake manifold and less intake air mass per cycle are found in the low heat rejection engine with Atkinson cycle,which is caused by the larger late intake valve closing angle and the higher in-cylinder temperature at the start of intake valve opening.The combustion rate of the high compression ratio Atkinson cycle is faster than that of the Otto cycle,and its in-cylinder heat convection is stronger during the combustion process and weaker after the end of combustion.The low heat rejection engine with Atkinson cycle further shortens the combustion duration,and its in-cylinder heat convection remains weak level throughout the expansion stroke.(3)Compared with the Otto cycle,less exhaust energy and larger indicated thermal efficiency are found in Atkinson cycle because of longer expansion stroke and less pump loss,and more obvious improvement effect on thermal efficiency of Atkinson cycle is found on low load.The in-cylinder heat loss of the low heat rejection engine with Atkinson cycle is greatly reduced that 10% lower than the prototype under variable loads,and more than 70% of the energy saved by low heat rejection could push the piston to work,resulting that the indicated thermal efficiency is 7%,8.1%,and 8.6% higher than that of the prototype under high,medium and low load.Although the low heat rejection engine with Otto cycle can greatly reduce the heat loss,its effective expansion stroke length is close to the effective compression stroke length,so only about 30% of the saved energy can be converted into the indicated work.(4)No matter the exhaust valve opens too early or too late,it would cause more exhaust loss.Therefore,by optimizing the exhaust valve lift curve,the effect of the early exhaust valve opening angle on the energy distribution characteristics of the low heat rejection engine with Atkinson cycle was explored in the range of 20-60 °CA before bottom dead center.With the increase of the early exhaust valve opening angle,the in-cylinder temperature decreases rapidly and the in-cylinder heat convection is weakened because the high temperature gas in the cylinder flows to the exhaust manifold earlier,resulting that the proportion of heat loss energy is gradually reduced.The larger early exhaust valve opening angle leads to more expansion loss,and the smaller early exhaust valve opening angle causes more pumping loss,so the proportion of exhaust energy decreases first and then increases with the increase of the early exhaust valve opening angle.The heat loss and exhaust energy could be reduced simultaneously by properly increasing the early exhaust valve opening angle,and the maximum indicated thermal efficiency is obtained when the early exhaust valve opening angle is 40°CA.However,the early exhaust valve opening angle barely influence the energy distribution characteristics and less saved energy from low heat rejection converts into the indicated power at low load,caused by the less combustible mixture and limited working capacity.(5)The in-cylinder thermal load of the low heat rejection engine with Atkinson cycle can be reduced effectively by proper application of EGR,but the engine dynamic would be degraded when the EGR rate is too large.Therefore,under the premise of ensuring that the effective torque under different conditions is not lower than the prototype,the performance of low heat rejection engine with Atkinson cycle and EGR rate of 0-10% was explored.With the increase of EGR rate,the maximum in-cylinder temperature gradually decreases,the heat release rate slows down,and the heat convection weakens,so the ratio of heat loss energy gradually reduces.The percentage of exhaust energy does not change much with variable EGR rate at medium and high load,however,the application of EGR at low load deteriorates the combustion performance,so the proportion of exhaust energy increases as the EGR rate increases,and the thermal efficiency is lower when the EGR rate is large.(6)When the EGR rate is 10%,the brake torque of the low heat rejection engine with Atkinson cycle under different loads is lower than that of the prototype with Otto cycle.However,when the optimal condition of 7.5% EGR and 40°CA early exhaust valve opening angle is selected,not only the in-cylinder thermal load can be effectively reduced,but also the low heat rejection engine with Atkinson cycle combines with good dynamic and fuel economy.The brake specific fuel consumption could be reduced by 16%,19.4%,and 21.5% at high,medium and low load respectively,indicating that the indicated thermal efficiency can reach 44%,43%,and 39.7%,respectively. |
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