Variable Valve Timing Control

Fig 1: Identifying Camshaft Position Angle

Courtesy of VOLVO CARS OF NORTH AMERICA.

Continuous variable valve timing (CVVT) control, general

The engine combustion can be improved by changing the shift angle of the camshaft. This reduces emissions, gives lower fuel consumption and better performance. The shift angle of the camshaft is controlled by the control module which changes the pulse ratio of the reset valve. The pulse ratio of the reset valve affects the variable valve timing unit which changes the camshaft position compared to the crankshaft position (CKP). This ensures quick and precise control of the shift angle of the camshaft.

Definition of "Camshaft shift angle" (X°)

When the camshaft (A) is set at the factory it is aligned to top dead center (TDC) on the crankshaft which is 0° (B). The camshaft position when aligned to the crankshaft TDC is the camshaft 0 position (0°). With continuous variable valve timing (CVVT) the camshaft 0 position is offset, the value of the offset of the camshaft 0 position is the camshaft shift angle (x°).

Fig 2: Camshaft Cam Timing

Courtesy of VOLVO CARS OF NORTH AMERICA.

The control module interpretation of camshaft shift angle

(The illustration shows the camshaft shift angle at 10°)

The camshaft shift angle is checked by the control module by comparing the position of the crankshaft with the position of the camshaft.

The camshaft (A) is divided into 4 flanks per camshaft rotation (flanks 1-4). The angles are: between flanks 1-2 = 103°, flanks 2-3 = 90°, flanks 3-4 = 90° and flanks 4-1= 77°. The difference in angle between the flanks allows the control module to determine which flank is currently detected and therefore which cycle the camshaft is in. The control module detects the flanks using the signal from the camshaft position (CMP) sensor.

The crankshaft has four reference positions, one for each camshaft flank. Two reference positions are detected for each crankshaft rotation. Two engine rotations are required to detect all the camshaft flanks: flank 1 at 0°, flank 2 at 206°, flank 3 at 386° and flank 4 at 576° (flanks 3 and 4 are detected during the second rotation of the engine).

The reference positions on the crankshaft correspond exactly to the respective camshaft flank when the camshaft shift angle = 0°. The control module detects the reference positions for the camshaft flanks using the engine speed (RPM) sensor signal.

If, for example, the camshaft shift angle is +10°, the flanks correspond to the crankshaft reference positions for the respective camshaft flank and +10° from the crankshaft reference positions.

Example:

If the camshaft flank 1 is detected at 10° from the crankshaft reference position for flank 1 (0° +10°), the camshaft shift angle is 10°. If the camshaft shift angle is the same (10°), flank 2 is detected at 216° (206° + 10°), flank 3 at 396° (386° + 10°), and flank 4 at 576° (586° + 10°). The shift angle can be positive (+) or negative (-) depending on whether the flank is detected before or after the different crankshaft reference positions.

Fig 3: Identifying Crankshaft Reference Positions

Courtesy of VOLVO CARS OF NORTH AMERICA.

Adaptation of the camshaft shift angle, flanks 1-4

(The illustration shows the adaptation of the camshaft shift angle at 10°)

When the engine has the "basic setting", the ideal value for the camshaft adaptation value is 0°. The different camshaft flanks correspond exactly with the reference positions for the respective flanks. These values may deviate because of tolerances in the materials and installation. By adapting these deviations the camshaft shift angle can be exact.

Adaptation is carried out as follows:

Camshaft control not active, the pulse ratio on the camshaft reset valve is set to approximately 90% by the control module. This means that the camshaft 0 position (mechanical resting position) is assumed and the camshaft angles (x°) for flanks 1-4, which are then detected by the control module, are the adaptation values which are stored. (The adaptation values will be 0° if the camshaft adjustment in relation to the crankshaft is exact). When the control module operates the camshaft to a particular target value, the stored adaptation value is added to the control module's target value.

Example: The control module has adapted flanks 1-4 to an average value of +10° (as the result of an incorrectly set camshaft for example). When the control module needs to operate the camshaft to +10° (target value), the adaptation value is added to the target value which then gives a target value of 20° (10° +10°=20°).

The control module can adapt 20°. A diagnostic trouble code (DTC) is stored in the control module if the adaptation is higher or lower than 10°.

Other information:

If the timing belt is incorrectly positioned by one tooth on the variable valve timing unit, this corresponds to approximately 17° on the crankshaft. This gives a theoretical adaptation value of 17°.
If the timing belt is incorrectly positioned by one tooth on the variable valve timing unit, this corresponds to approximately 34° on the crankshaft. This gives a maximum adaptation value (20°)
The setting of the variable valve timing unit in relation to the camshaft and crankshaft affects the adaptation value (between -20° and +20°)
If the slide in the camshaft reset valve has jammed, the camshaft will be operate at maximum angle. This which will give an adaptation value at the maximum limit (+20° or -20°)
The function of the variable valve timing unit affects the adaptation value (between -20° and +20°).