Variable valve gear / Valvetronic
The variable valve gear was developed to reduce fuel consumption.
The air volume supplied to the engine is not set by the throttle valve but by the variable stroke of the intake valve. An electrically adjustable eccentric shaft uses intermediate levers to change the effect of the camshaft on the rocker arms. This produces an adjustable stroke of the valves.
The throttle valve is now only used for starting up and for emergency operation functions. In all other operating states, the throttle valve is opened so far that it only has a very slight throttling effect. For tank ventilation, a certain vacuum is still necessary.
The corresponding position of the variable valve gear is calculated in the engine control unit from the position of the accelerator pedal and other variables. The variable valve gear is moved by its own control unit and servomotor. The servomotor is fitted to the cylinder head and it drives the eccentric shaft via a worm drive in the oil chamber of the cylinder head.
The communication between the engine control unit and Valvetronic control unit is on a separate LoCAN bus. All functions are calculated in the engine control unit. The Valvetronic control unit evaluates the signals of the position sensor and controls the servomotor of the eccentric shaft.
Position sensing
The current position of the eccentric shaft is sensed by a special position sensor. The sensor is equipped with two independent angle probes. The engine management controls the position by means of the electric drive until the current position corresponds to the set position. For safety reasons, two angle probes with opposing characteristic curves are used. The two signals are sent to the Valvetronic control unit in digital form.
The Valvetronic control unit powers both potentiometers with a voltage of 5 V.
Monitoring the signal voltages
The Valvetronic control unit permanently monitors the two voltage signals of the potentiometers. The monitoring function checks whether the voltages are within the permissible range, i.e. whether there is a short-circuit or sensor defect.
The two signals must not deviate from each other.
In the event of a fault occurring, the eccentric shaft is set to maximum possible valve opening. The air volume is now restricted by the throttle valve. If it is not possible to detect the current position, the valves are opened to the maximum without control.
Monitoring the eccentric shaft position
The Valvetronic control unit continuously checks whether the actual position of the eccentric shaft corresponds to its set position. This makes it possible to determine when a valve is sticking. In the case of an error here, the valves are opened as wide as possible and the air supply is controlled by the throttle valve.
If the engine control unit is unable to communicate with the Valvetronic control unit due to a LoCAN bus fault, emergency communication is set up across a separate line (P_VVTEN).
Learning function / adaptation
In order to achieve the correct valve opening height, all the tolerances in the valve gear must be balanced by an adaptation. In this learning procedure, there is a slow approach to the mechanical adjustment limits of the eccentric shaft. The positions thus reached are stored and used in each operating point as the basis for calculating the current valve stroke.
The learning procedure runs automatically if, for example after a repair, a difference between the last stop position and the new starting position (with terminal 15 on) is detected. The adaptation can also be requested via the DIS-plus Tester.
Vanos.
Variable camshaft timing control VANOS
The VANOS control unit serves the purpose of increasing torque in the lower and medium engine speed ranges. Reduced valve overlap results in lower residual-gas quantities while idling. Nitrous oxide components are reduced by internal exhaust-gas recirculation in the partial load range. This achieves faster heating of the catalytic converters, lower untreated emissions after cold starting and reduced fuel consumption.
VANOS intake/exhaust
A regulated VANOS control unit for intake and exhaust is fitted in this engine. It is activated by means of an electromagnetically operated 4/3-way valve.
The required position of the intake and exhaust camshafts is calculated from the engine speed and load signal depending on the intake air and engine temperature and the VANOS VANOS adjustment unit is controlled accordingly by the engine control unit. The inlet and exhaust camshafts can be variably controlled within their maximum adjustment range, i.e. any arbitrary positions are possible corresponding to the specified values of the engine control unit. When the relevant optimum camshaft position is reached, the solenoid valves maintain a constant oil volume in the adjustment cylinder on both sides of the chamber so that the camshafts remain in the corresponding positions.
When the engine starts up, the intake camshaft is in its end position, i.e. it is initially in the retard position. During engine start, the exhaust camshaft is held in the advance position by means of a spring.
