What can be seen in the above picture, is without a doubt, a cylinder head from a inline four-cylinder engine with two overhead camshafts. Indeed, for both the inlet- and the exhaust valves, three cams are available. To solve the puzzle quickly, we'll compare the middle, apparently superflous cam, with the other two. It is wider, has more lift and would open a valve earlier and close it later.
What we have here, is one of the older VTEC-systems, although this one was originally used as a motorcycle engine. The basic idea was, to control a valve using at least two cams, one for good torque in the lower RPM-range and the other for higher performance in the higher RPM-range. The question was, how to switch over when the engine is running.
The next picture shows, that the two cams on the left and the right, do not operate their valves directly. The following picture shows that this is done by rocker-arms. Indeed, the middle cam also has a rocker-arm. With this, we've almost arrived at the solution to the problem, thus, the switch-over follows by means of a mechanical connection from the middle- to one or both of the other rocker-arms.
Here, we find crosspins, which are thus shifted by the engine-oil, that a connection is established. Of course the oil pressure is activated electronically. If the oil pressure decreases, then the connection is broken and each valve again has it's own cam. They can quite possibly, as can be seen in the picture, be various sizes.
You can see here, that such a system is also possible in the case of four-valve engines with only one camshaft. Only the spark plug can no longer be installed straight down from the top. One picture further and you can see the roller-angle levers used here, as a cut-away. It should now be easier to imagine, how the middle cam is connected to the others.
Of coure the engine RPM does not determine the switch-over point.
The system is also open for an adjustment of the camshaft, thus also for the additional varying of the valve-timing. Then again however, to do this, two camshafts would be needed. The name then changes to 'i-VTEC'. Should it now be given direct-injection as well, the name changes to 'i-VTECi'. The same way that one can connect cams to valves, it is also possible to de-activate them completely. The valves then remain closed, e.g., when an electric motor is used (hybrid - video no. 4). 01/14