Page "Variable valve timing" Paragraph 59

This system is distinctly different to the Mechadyne system in that, although similarly coaxial with pairs of lobes acting on the one follower etc.

the actual lobe profile has a distinctly different profile and acts on a conventional ( although maybe somewhat wider than normal ) follower.

The profile used has a characteristic “ snub-nosed ” appearance with usually about a twenty degree angular region of true radius about the axis of rotation of the camshaft on the lobe nose.

This type of profile is actually very little different from some conventional lobe profiles that act on valve lift multiplying followers of a fairly high ratio of lobe lift to valve lift.

The essential operating principle is that the one follower spans the pair of closely spaced lobes.

Up to the angular limit of the nose radius the follower “ sees ” the combined surface of the two lobes as a continuous, smooth surface.

When the lobes are exactly aligned the duration is at a minimum ( and equal to that of each lobe alone ) and when at the extreme extent of their misalignment the duration is at a maximum.

The basic limitation of the scheme is that only a duration variation equal to that of the lobe nose true radius ( in camshaft degrees or double this value in crankshaft degrees ) is possible.

In practice this type of variable cam has a maximum range of duration variation of about forty crankshaft degrees.

Any attempt to drastically increase this range ( by increasing the angular extent of the true radius lobe nose region ) soon gives rise to problems with excessive rates of valve acceleration etc.

This range limits the usefulness of the cam to some extent.