Cam-Phasing VVT


    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.

    Macro illustration of the phasing actuator