Basically, it varies
the
valve timing by shifting the phase angle of camshafts. For example, at
high speed, the inlet camshaft will be rotated in advance by 30° so
to enable earlier intake. This movement is controlled by engine
management
system according to need, and actuated by hydraulic valve gears.
Continuous or Discrete
Simpler cam-phasing VVT has just 2 or 3 fixed shift angle settings to choose from, such as either 0° or 30°. Better system has continuous variable shifting, say, any arbitary value between 0° and 30°, depends on rpm. Obviously this provide the most suitable valve timing at any speed, thus greatly enhance engine flexiblility. Moreover, the transition is so smooth that hardly noticeable.
Intake and Exhaust
Some design, such as BMW's Double Vanos system, has cam-phasing VVT at both intake and exhaust camshafts, this enable more overlapping, hence higher efficiency. This explain why BMW M3 3.2 (100hp/litre) is more efficient than its predecessor, M3 3.0 (95hp/litre) whose VVT is bounded at the inlet valves.
In the E46 3-series, the Double Vanos shift the intake camshaft within a maximum range of 40° .The exhaust camshaft is 25°.
| Advantage: | Cheap and simple, continuous VVT improves torque delivery across the whole rev range. |
| Disadvantage: | Lack of variable lift and variable valve opening duration, thus less top end power than cam-changing VVT. |
| Who use it ? | Most car
makers, such as:
|
From
the picture, it is easy to understand its operation. The end of
camshaft
incorporates a gear thread. The thread is coupled by a cap which can
move
towards and away from the camshaft. Because the gear thread is not in
parallel
to the axis of camshaft, phase angle will shift forward if the cap is
pushed
towards the camshaft. Similarly, pulling the cap away from the camshaft
results in shifting the phase angle backward.
Whether push or pull is determined by the hydraulic pressure. There are 2 chambers right beside the cap and they are filled with liquid (these chambers are colored green and yellow respectively in the picture) A thin piston separates these 2 chambers, the former attaches rigidly to the cap. Liquid enter the chambers via electromagnetic valves which controls the hydraulic pressure acting on which chambers. For instance, if the engine management system signals the valve at the green chamber open, then hydraulic pressure acts on the thin piston and push the latter, accompany with the cap, towards the camshaft, thus shift the phase angle forward.
Continuous variation
in timing
is easily implemented by positioning the cap at a suitable distance
according
to engine speed.
 |
Macro illustration of the phasing actuator |
However, the word "Integillent" emphasis the clever control program. Not only varies timing according to engine speed, it also consider other conditions such as acceleration, going up hill or down hill.