Valve stroke control, Valvetronic
An electrically-adjustable eccentric shaft changes the action of the camshaft on the roller cam follower via an intermediate lever. The result of this is variable valve lift.
One special feature is that the eccentric shaft sensor is no longer located on the eccentric shaft; it has been integrated in the servomotor.
Valvetronic III is used. The differences between Valvetronic III and Valvetronic II lie in the arrangement of the Valvetronic servomotor and of the sensor. With Valvetronic III, the level of turbulence at the end of compression is increased to optimize mixture preparation with advance and masking, as was already the case with Valvetronic II. The charge movement improves combustion in the partial load range and heating mode of the catalyst.
Advance
The advance leads to a difference in the stroke of the two intake valves of up to 1.8 mm in the lower partial load range. This swirls the fresh gas that is drawn in, making it rotate.
Masking
Masking is the shape at the valve seat. The effect of this shape is that the incoming fresh air is aligned in such a way that the desired charge movement results. The of these measures is that, for example, the delay on combustion is reduced by approx. 10° crank angle. Combustion is completed more quickly and a greater valve overlap can be used. This enables a significant reduction in NOx emissions.
The response characteristics can be improved by combining Valvetronic III, direct fuel injection and turbocharging. As in the case of the naturally aspirated engine with Valvetronic, the response characteristics up to naturally aspirated full load are shortened, as the filling procedure of the air intake system for intake air is not required. The subsequent build-up of torque on start-up of the exhaust turbocharger at low engine speeds can be accelerated by setting a partial stroke. This promotes the purging of residual gas, which leads to faster build-up of torque.
A new brushless direct current motor is used. The Valvetronic servomotor has the following special features:
- Open concept (supplied with oil)
- Angle of the eccentric shaft is calculated from revolutions of the engine
- Power consumption reduced by approx. 50%
- Greater dynamics of the adjuster (for example, cylinder-specific adjustment or idle speed control)
- Reduction in the weight (approx. 600 grams).
The third generation Valvetronic servomotor also contains the sensor for identifying the position of the eccentric shaft. Another special feature is that the Valvetronic servomotor is supplied with and surrounded by engine oil. An oil spray nozzle ensures that the screw drive for the eccentric shaft is lubricated.
Valvetronic was developed to reduce fuel consumption. Activation of the Valvetronic is now integrated in the Digital Engine Electronics (DME). The air supplied to the engine when Valvetronic is active is adjusted by the variable valve lift on the intake valve and not the electrical throttle-valve actuator.
The following graphic shows the Valvetronic components in engine N55
| Item | Explanation |
|---|---|
| 1 | Intake VANOS unit |
| 2 | Exhaust VANOS adjustment unit |
| 3 | Camshaft housing |
| 4 | Valvetronic servomotor |
| 5 | Torsion spring |
| 6 | Gate |
| 7 | Intermediate lever |
| 8 | Valve head |
| 9 | Roller cam followers |
| 10 | Intake camshaft |
| 11 | Eccentric shaft |
| 12 | Oil spray nozzle |
| 13 | Line for blow-by gas |
The electrical throttle-valve actuator is activated for the following functions:
- Engine start (warm-up)
- Idle speed control
- Full load operation
- Emergency operation.
In all other operating conditions, the throttle valve only remains open far enough to induce a slight vacuum. This vacuum is required to ventilate the tank, for example. The Digital Engine Electronics (DME) calculate the associated position of Valvetronic using the position of the accelerator pedal and other variables. The Digital Engine Electronics (DME) activate the Valvetronic servomotor at the cylinder head. The Valvetronic servomotor uses a worm gear to drive the eccentric shaft in the oil chamber of the cylinder head. The two signals of the eccentric shaft sensor are permanently monitored by the Digital Engine Electronics (DME). Checks are made as to whether the signals are plausible in their own right and also in relation to one another. The signals may not differ. Where a short circuit or fault develops, the signals lie outside the measuring range. The Digital Engine Electronics (DME) control unit continuously checks whether the actual position of the eccentric shaft corresponds with its desired position. This makes it possible to determine when a valve is sticking. In the event of malfunctions, the valves are opened as wide as possible. The air supply is then controlled by the throttle valve. If the actual position of the eccentric shaft cannot be detected, the valves are opened to the maximum extent without regulation (controlled emergency operation). In order to achieve the correct valve opening, an adaptation must be made to balance all tolerances in the valve gear. During this adaptation process, the mechanical limit positions on the eccentric shaft are adjusted.
The positions registered are subsequently saved. These positions are used as the basis for calculating the actual valve lift in every situation. The adaptation process is automatic.
Each time the engine is restarted, the position of the eccentric shaft is compared with the values registered. If following a repair, for example, a different position of the eccentric shaft is detected, the adaptation process is carried out. In addition, the adaptation can be initiated via the diagnostic system.
The following graphic shows the components in engine N55
| Item | Explanation |
|---|---|
| 1 | Cam in shell shape |
| 2 | Corrugated tubing |