In recent decades, many countries, engine manufacturers and scientific research institutions have invested a lot of manpower and material resources in the research and development of new technologies based on the requirements of improving the power, economy and reducing pollution of automobile engines. At present, some of these new technologies and methods have been applied to internal combustion engines, and some are in the stage of development and perfection, which may become the development direction of internal combustion engine technology in the future. Variable valve timing (VVT) technology is one of the new technologies gradually applied to modern automobiles in recent years. Using VVT technology can improve the intake charge, increase the inflation coefficient and further improve the torque and power of the engine. 2. Variable valve timing theory It is an extremely important technical problem to choose the valve timing reasonably and ensure the best inflation efficiency hv. By analyzing the working principle of internal combustion engine, it is not difficult to draw the conclusion that the change of late closing angle of intake valve has the greatest influence on charging efficiency hv in the four periods of opening and closing of intake and exhaust valves. The influence of late closing angle of intake valve on charging efficiency hv and engine power can be further illustrated by figure 1 Each curve of charging efficiency hv in figure 1 reflects the relationship between charging efficiency hv and rotational speed at a certain valve timing. For example, when the late closing angle is 40, the inflation efficiency hv reaches the highest value at the rotating speed of about 1800r/min, which shows that the inertial inflation of airflow can be best utilized when working at this rotating speed. When the rotational speed is higher than this rotational speed, the inertia of the airflow increases, so that a part of the gas that could have entered the cylinder by the inertia of the airflow is locked out of the cylinder. In addition, as the rotational speed increases, the flow resistance increases, so the charging efficiency hv decreases. When the rotational speed is lower than this rotational speed, the airflow inertia decreases, and some fresh gas may be pushed back into the intake pipe at the beginning of the compression stroke, and the charging efficiency hv also decreases. The curves of different charging efficiency hv in the figure reflect the relationship between charging efficiency hv and rotational speed under different valve timing. Different intake delay closing angles have different rotational speeds corresponding to the maximum value of charging efficiency hv curve. Generally, the later closing angle increases, and the rotational speed corresponding to the maximum value of the charging efficiency hv curve also increases. Compared with the charging efficiency hv curve with a late closing angle of 40 and a late closing angle of 60, the maximum speed of this curve is 1800r/min and 2200r/min, respectively. Due to the increase of rotating speed, the airflow speed increases, and the large late convergence angle can make full use of the inertia of high-speed airflow to increase inflation. Changing the intake late closing angle can change the change trend of charging efficiency hv curve with speed, thus adjusting the engine torque curve to meet different application requirements. But more precisely, increasing the late closing angle of intake valve and improving the charging efficiency hv at high speed are beneficial to the improvement of maximum power, but unfavorable to the performance at low speed and medium speed. Reducing the intake late closing angle can prevent the gas from being pushed back into the intake pipe, which is beneficial to improve the maximum torque, but reduces the maximum power. Therefore, the ideal valve timing should be adjusted in time according to the working conditions of the engine, and there should be some flexibility. Obviously, for the traditional cam tappet valve train, it is difficult to meet the above requirements because it cannot be adjusted accordingly in the work, thus limiting the further improvement of engine performance. 3. Application in Passat B5 3. 1 Structure and Transmission of Variable Valve Timing Passat B5 recently selected a 2.8-liter V6 engine, which was specially designed for variable valve timing. Seen from the top, Figure 2 shows the transmission mode and the distribution of intake and exhaust camshafts, with the exhaust camshafts installed on the outside and the intake camshafts installed on the inside. The crankshaft first drives the exhaust camshaft through the toothed belt, and the exhaust camshaft drives the intake camshaft through the chain. 3.2 The variable valve timing regulator is shown in Figure 3. (a) The figure shows the position of the engine at high speed after increasing the late closing angle of the intake valve in order to make full use of the flow inertia of the gas entering the cylinder and improve the maximum power (automobile engines usually work at high speed, so this position is a general working position). (b) The picture shows the position where the late closing angle of the intake valve is reduced in order to improve the maximum torque when the engine is at low speed. The intake camshaft is driven by the exhaust camshaft through a chain, and the variable valve timing regulator is located between the two shafts. Under the action of internal hydraulic cylinder, the regulator can rise and fall. When the engine speed drops, the variable valve timing regulator drops, the upper chain is loosened, and the lower chain acts on the rotational tension of the exhaust cam and the downward thrust of the regulator. Because the exhaust camshaft cannot rotate counterclockwise under the action of the crankshaft timing belt, the intake camshaft is subjected to two forces: one is the tension of the chain driven by the normal rotation of the exhaust camshaft; The second is the tension that the regulator pushes the chain to transmit to the exhaust cam. The intake camshaft rotates clockwise by an extra angle θ, which accelerates the closing of the intake valve, that is, the delayed closing angle of the intake valve decreases θ. When the speed increases, the regulator rises and the lower chain is loosened. When the exhaust camshaft rotates clockwise, the lower chain must be tightened to a tight edge, so that the intake camshaft can be driven to rotate by the exhaust camshaft. In the process of changing the lower chain from loose to tight, the exhaust camshaft has turned θ, the intake cam starts to move, and the intake valve closes slowly, that is, the late closing angle of the intake valve increases θ. 3.3 Two working states It is not difficult to see from Figure 2 and Figure 3 that the working directions of the variable valve timing adjusters on the left and right sides of the engine are always required to be opposite. When the left variable valve timing regulator of the engine moves down, the right variable valve timing regulator moves up, with the tension side of the left chain at the bottom and the tension side of the right chain at the top. When the regulator moves down, the tight chain changes from short to long. When the engine speed of Passat B5 is higher than 1000r/min, the intake valve is required to be closed in advance, as shown in Figure 4(a). The variable valve timing regulator corresponding to the left bank cylinder moves downward, and the upper chain changes from long to short, and the lower chain changes from short to long. The variable valve timing regulator corresponding to the right bank cylinder moves upward, and the upper chain changes from short to long, and the lower chain changes from long to short. Under the combined action of two forces, the intake camshafts corresponding to the left and right rows rotate clockwise by an additional angle θ, which accelerates the closing speed of the intake valve and meets the requirements that the low-speed intake valve closes earlier and the maximum torque can be improved. When the engine speed of Passat B5 car is 3700r/min, the intake valve is required to close late, as shown in Figure 4(b). The variable valve timing regulator corresponding to the left bank cylinder moves upward, and the upper chain changes from short to long, and the lower chain changes from long to short. The variable valve timing regulator corresponding to the right bank cylinder moves down, the upper chain changes from long to short, and the lower chain changes from short to long. In the process that the lower chain of the left bank cylinder and the upper chain of the right bank cylinder are changed from long to short at the same time, the exhaust camshaft has turned θ, the intake cam starts to move, and the intake valve closes slowly, which meets the requirements of high speed and delayed closing of the intake valve and can improve the maximum power. 4. Microcomputer control of variable valve timing The variable valve timing system of Passat B5 2.8 liter V6 engine is controlled by Motronic M3.8.2 engine control unit. The microcomputer control relationship is shown in Figure 5. The variable valve timing mechanisms corresponding to the left and right columns are all provided with variable valve timing solenoid valves, as shown in Figure 6. After obtaining the information of the speed sensor, the engine correctly selects the control mode of the variable valve timing solenoid valves corresponding to the left and right banks, and controls the valve body action. When different valve body positions are obtained, the oil path leading to the hydraulic cylinder in the variable valve timing regulator is changed, so that the variable valve timing regulator rises or falls, thereby enabling intake valves corresponding to the left and right rows of cylinders to obtain different delayed closing angles. 5. Conclusion The variable valve timing system of the engine mentioned above is to control the lifting of the variable valve regulator by a microcomputer to obtain the relative position change between the toothed pulley and the intake cam (intake valve). This structure belongs to the camshaft valve timing variable structure, which can generally be adjusted by 20%. ~30。 Crankshaft angle. Because the camshaft, cam profile and intake duration angle of this mechanism are unchanged, although the intake delay closing angle can be increased at high speed, the valve overlap angle is reduced, which is its disadvantage. Generally speaking, the engine variable valve timing technology is relatively mature, and more and more high-performance gasoline engines will adopt this technology in the future.