Study On Optimization Control Of Compression Ratio Of Turbocharged Diesel Engine

The traditional diesel engine geometric compression ratio is determined on the basis of considering the brake efficiency of the engine cycle,and the maximum combustion temperature and the maximum burst pressure in the cylinder cannot be too high,minimize heat loss and mechanical loss while ensuring that the engine has a certain emission performance.Although this method of determining the geometric compression ratio ensures stable operation of the engine over the full operating range,and increases the thermal efficiency of the engine due to the peak pressure approaching the limit of the cylinder under high-speed and high-load conditions,but also reduce the economics of the engine in small speed conditions.The vehicle engine is mostly in the medium and small speed conditions,so the geometric compression ratio of the engine under medium and small speed conditions can be improved to obtain higher overall efficiency.In order to improve the economical efficiency of the diesel engine in small and medium speeds,Based on the original geometrical compression ratio,this paper uses GT-power to improve the geometric compression ratio of diesel at 1000r/min medium load and determine the optimal geometric compression ratio under this condition according to the corresponding constraints.Since GT-power does not provide an effective explanation for gas flow characteristics,combustion and emissions formation mechanisms,it was later analyzed in conjunction with FIRE software.1.Study the change of peak pressure in the cylinder under different working conditions when studying the optimal geometric compression ratio,In view of the operating point of excessive peak pressure at high speed and large load,the idea of Miller cycle is adopted to flexibly change the effective compression ratio of the engine by means of intake valve delay closing,thereby reducing the peak pressure in the cylinder to ensure the working stability of the engine.2.The effect of the variable compression ratio on the charging efficiency and indicated efficiency is analyzed on the basis of engine operating stability.The change of mechanical efficiency at the maximum torque point and the maximum power point after the geometric compression ratio is increased is analyzed to explore the effect of the optimal geometric compression ratio on the mechanical efficiency.3.The reflow state of the airflow in the cylinder at different intake valve closing timings is studied.The optimal geometric compression ratio and geometric compression ratio are fixed to change the effect of the effective compression ratio on the combustion state and emission performance of the engine.The one-dimensional GT simulation study concluded as follows: 1)Increasing the geometric compression ratio of the diesel engine under low-speed and low-load conditions has little effect on the inflation efficiency of the engine,but can significantly improve the in-cylinder pressure and the indicated thermal efficiency under medium and small loads,and improve the engine's power and fuel economy.2)Under the heavy load condition,the method of lowering the effective compression ratio by the method of delaying the closing of the intake valve can reduce the peak pressure in the cylinder and sacrifice part of the economy to ensure the running stability under heavy load conditions.Compared with the original machine,it can improve the power of the engine to some extent.3)The increase of geometric compression ratio leads to an increase in mechanical loss at high speed and a decrease in effective efficiency.When considering the economics at high speeds,the geometric compression ratio of the engine can be appropriately reduced.The 3D FIRE simulation concludes as follows: 1)In the case of heavy load conditions,in order to reduce the peak pressure in the cylinder,the method of closing the intake valve is used.As the intake retardation angle increases,the amount of gas return in the cylinder increases,the speed increases,and creates large eddy currents in the plenum.At the same time,the effective compression ratio of the engine is reduced.2)The optimum geometric compression ratio increases the intake air volume under medium and small load conditions,and the back pressure increases at the end of compression,which reduces the disturbance of the fuel injection to the in-cylinder airflow and the diffusion combustion rate,so that the heat release rate is reduced,and The generation rate of NO is reduced;reducing the effective compression ratio of the engine under large load conditions can improve the combustion state to some extent,but the rate of NO generation increases with the normal closing of the valve.Under the premise of fixed geometric compression ratio,reducing the effective compression ratio of the engine has little effect on the Soot emission under various working conditions.According to the research results of this paper,when the engine's geometric compression ratio is determined at low speed and medium load,This improves the economy at medium and small speeds but results in mechanical losses and peak pressure increases at high speeds.Combined with the effect of geometric compression ratio on effective thermal efficiency under high-speed and high-load conditions,the optimal geometric compression ratio can be determined,which not only improves the dynamic and economical efficiency of the engine in small load,but also ensures the stability under heavy load.Ultimately,the overall performance of the engine is improved.