Cam-phasing VVT is the simplest, cheapest and most commonly used mechanism at this moment. However, its performance gain is also the least, a very fair compromise indeed.
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.
Note that cam-phasing VVT cannot vary the duration of valve opening. It just allows earlier or later valve opening. Earlier open results in earlier close, of course. It also cannot vary the valve lift, unlike cam-changing VVT. However, cam-phasing VVT is the simplest and cheapest form of VVT because each camshaft needs only one hydraulic phasing actuator, unlike other systems that employ individual mechanism for every cylinder.
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 its hardly noticeable.
Intake and Exhaust
Some designs, such as BMW's Double Vanos system, have cam-phasing VVT at both intake and exhaust camshafts, this enables more overlapping, hence higher efficiency. This help explains 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 shifts 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
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 enters the chambers via electromagnetic valves which controls the hydraulic pressure acting on the chambers.
For instance, if the engine management system signals the valve at the green chamber to open, then hydraulic pressure acts on the thin piston and pushes the latter, accompany by 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
Toyota's VVT-i (Variable Valve Timing - Intelligent) has been spreading to more and more of its models, from the tiny Yaris (Vitz) to the Supra. Its mechanism is more or less the same as BMWs Vanos, it is also a continuously variable design.
However, the word "Integillent" emphasis the clever control program. Which Not only varies timing according to engine speed, it also considers other conditions such as acceleration, going up hill or down hill etc.