The structure and working principle of electronic valve

BMW's electronic valve (VALVETRONIC) is composed of a fully variable valve travel control device and a variable camshaft control device (double VANOS), which can arbitrarily select the closing time of the intake valve. The electronic valve largely assumes the function of the throttle valve. For this purpose, a fully variable valve travel control device is required.

1. The structure of the electronic valve

The structure of the BMW electronic valve is shown in Figure 1. When the intake valve is open, the intake volume is achieved by adjusting the valve stroke, so that the optimal cylinder gas volume can be determined.

1. Servo motor

The servo motor, also known as the actuator motor, is arranged above the camshaft. The worm of the servo motor is embedded in the worm wheel installed on the eccentric shaft. The servo motor is a typical closed-loop feedback system. The reduction gear set is driven by the motor, and its terminal (output end) drives a linear proportional potentiometer for position detection. The potentiometer converts the angular coordinates into a proportional voltage and feeds it back to the control circuit board. The control circuit board compares it with the input control pulse signal, generates a correction pulse, and drives the motor to rotate forward or reverse, so that the output position of the gear set matches the expected value, thereby achieving the purpose of precise positioning of the servo motor.

2. Roller valve stem

The contact surface between the roller valve pressure rod and the intermediate push rod is a ramp. Rolling friction occurs at the ramp to reduce mechanical losses. The pressure rod and push rod are divided into different grades, and components of the same grade are always installed on the same cylinder.

3. Eccentric shaft sensor

The eccentric shaft sensor, as shown in Figure 2, measures the eccentric shaft angle to provide actual parameters for the adjustment function. The eccentric shaft sensor sends the eccentric shaft position to the valve stroke control unit or DME. Its measurement angle range is 180°.

The eccentric shaft sensor works on the principle of magnetoresistance: When the nearby magnetic field changes position, the ferromagnetic conductor changes its resistance. For this purpose, a magnetic wheel with permanent magnets is mounted on the eccentric shaft. When the eccentric shaft rotates, the magnetic field lines of these magnets pass through the magnetically conductive material in the sensor. The resulting change in resistance is converted into valve travel by the engine control unit. Therefore, the magnetic wheel must be fixed to the eccentric shaft with a non-magnetic fixing screw, otherwise the sensor will not work properly.

2. Working Principle of Electronic Valve

1. Working process of electronic valve

The engine has very different requirements for valve stroke at different speeds. At low speeds, due to the small intake volume, if the valve stroke is large, it will not be able to generate enough intake negative pressure. After the fuel injector injects fuel, it cannot fully mix with the inhaled air, resulting in low combustion efficiency, a significant reduction in low-speed torque, and increased emissions. In this case, a smaller valve stroke should be used. Due to the small valve stroke, the intake negative pressure is increased, and the large amount of vortex generated can fully mix the mixture to meet the normal operation of the engine at low speeds. At high speeds, the situation is just the opposite. At this time, the intake volume is very large. If the valve stroke is too small, it will cause excessive intake air resistance and fail to inhale enough air, thus affecting the power. Therefore, at high speeds, a larger valve stroke is required to obtain the best valve timing requirements.

To reduce fuel consumption, the amount of air introduced into the engine by the adjustable valve mechanism of BMW cars is adjusted not through the throttle but through the adjustable lift of the intake valve. Through the electrically adjustable eccentric shaft, the middle lever changes the effect of the camshaft on the roller valve pressure rod, thereby producing the adjustable lift of the intake valve. The throttle is only used during starting and emergency operation. In all other operating conditions, the throttle is fully open, with almost no throttling effect. Electronic valve technology achieves the best balance of power and torque output under different engine speed conditions by realizing stepless adjustment of the valve stroke.

The engine's air intake is controlled by the electronic throttle and the electronic valve stroke control mechanism. During the start-up process, the air intake is mainly controlled by the electronic throttle. After the engine is running, the throttle is fully opened, and the air intake from idle to high speed is mainly controlled by the electronic valve stroke control mechanism. Since the intake duct no longer has the throttling effect of the throttle, the intake resistance is greatly reduced, thereby effectively improving the engine's output torque.

The servomotor is arranged above the camshaft. The servomotor is used to adjust the eccentric shaft, while the servomotor's worm is embedded in the worm wheel mounted on the eccentric shaft. After adjustment, there is no need to lock the eccentric shaft specifically, because the worm drive has sufficient self-locking capacity. The rotation of the eccentric shaft causes the intermediate push rod on the mounting to move in the direction of the intake camshaft. However, since the intermediate push rod also rests on the intake camshaft, the position of the roller valve push rod relative to the intermediate push rod changes. The rotation of the camshaft and the movement of the cam towards the intermediate push rod cause the ramp on the intermediate push rod to come into play. The ramp pushes the roller valve push rod, which causes the intake valve to move further downwards, so that the intake valve continues to open.

The intermediate push rod changes the transmission ratio between the camshaft and the roller push rod. In the full load position, the valve stroke and the duration of opening are at their maximum. In the idle position, the valve stroke and the duration of opening are at their minimum. Since the minimum valve stroke at idle speed is very small, it is necessary to ensure that the cylinder charge is evenly distributed and all valves are opened to the same degree. For this reason, the roller push rods and the associated intermediate push rods are divided into different grades. The components of different grades can be distinguished by the marked parameters. Components of the same grade are always installed on the same cylinder. The factory-assigned roller push rods and intermediate push rods ensure that the valves are evenly charged even at the minimum valve stroke.

2. The principle of joint regulation of electronic valve and VANOS

The electronic valve uses VANOS and a fully variable valve mechanism to adjust the stroke and closing time of the intake valve, so that the ideal mixture quality is achieved in the combustion chamber when the "intake valve is closed". As shown in Figure 3, after the electronic valve is used, the ventilation loss is greatly reduced, and the intake valve is always closed during the intake stroke, which is different from the ordinary electronic fuel injection engine. The intake valve of the ordinary electronic fuel injection engine is closed at the beginning of the compression stroke, that is, the intake valve is closed late, in order to make full use of the inertia of the intake flow to increase the intake. However, since the electronic valve has no throttling in the intake duct and is directly connected to the atmosphere, it does not need to be closed late. As the intake valve lift increases, its closing time is closer to the bottom dead center, the closing time is relatively later, and the intake volume is also increasing, which is exactly matched with the engine load. After the intake valve is closed, the further expansion in the closed cylinder and the subsequent compression process will hardly produce energy loss, so the intake loss is reduced, but this ventilation advantage continues to weaken with the increase of load. The ventilation advantage is zero at full load, because the throttle of the ordinary electronic fuel injection engine is also fully open at this time.

When the load is small, the intake valve opening time must be very short, which can only be achieved by greatly reducing the valve stroke, which will reduce the valve opening cross-section and produce a significant throttling effect, but the intake speed at the valve clearance increases from 50 m/s to more than 300 m/s, and the airflow flows evenly around the entire valve, so the oil droplet size is reduced, the best mixture formation process is achieved, combustion is sufficient and power output fluctuations and HC and NOx emissions are reduced. According to experimental measurements, fuel consumption can be reduced by 20% at idle speed. As the load increases, the fuel saving potential decreases, but even if the engine runs at an ideal air-fuel ratio, it can still save 10% of fuel.

To sum up, BMW's electronic valve technology can greatly reduce ventilation losses by adjusting the intake volume through the intake valve when the throttle is fully open when the engine is idling or partially loaded. As someone described it: when an ordinary electronic fuel injection engine is idling or partially loaded, due to the throttling effect of the throttle, it is like breathing with a "mask"; while the electronic valve engine breathes without the "mask" under the same working conditions, greatly reducing ventilation losses and improving engine efficiency.