Helical camshaft

A helical camshaft is a type of mechanical variable valve actuation (VVA) system. More specifically it is a camshaft that allows the valve opening duration to be varied over a wide, continuous, step-less range, with all of the added duration being at full valve lift.

In this article a “variable duration camshaft” refers to a camshaft with a design that is intended to replace a conventional camshaft in a cylinder head and operate the valves through conventional followers. Further qualifications are:

These qualifications must be made as there have been many claims over the years of purely mechanical variable duration camshafts but none have been able to meet all these requirements.

Despite enormous effort and expense being expended by both large organisations and private individuals, camshaft arrangements like U.S. Patent 1,527,456 have never been significantly improved upon and have remained unused by the mainstream automotive community. The opinion of many engineers (and others) on the possibility of a workable variable duration camshaft being developed after so much effort and so many years of unsuccessful attempts was that it was highly unlikely that it ever could be done and would remain an unobtainable “holy grail.”

The helical camshaft very distantly belongs to the very numerous general “coaxial-shaft-combined-profile” class of cams as most recently typified by the work from Clemson University (whose cams are essentially identical in principle to many other cams, such as U.S. Patent 1,527,456).

The helical camshaft importantly differs from other members of this general class by having a unique helical movement – a combined circumferential and axial movement of the two profiles.

The duration can be increased until the closing flank of the cam lobe reaches the opening flank – a duration of 720 degrees. In a typical application the helical camshaft would have a continuous duration range from about average for a road-going general purpose engine (about 250 degrees measured at normal valve clearance) to about 100 or 150 degrees above this.

The valve is opened at normal rates of acceleration, jerk, etc., and then held open at its maximum lift for whatever duration is required before being closed at a normal rate.

A helical camshaft effects its duration change basically by rotating the opening and closing flanks away from each other starting at a split line on the nose of the lobe. As the flanks move apart the nose region is “filled-in” with an area of constant radius about the centre of rotation of the camshaft. An equal angular amount is removed from the constant radius base circle as the nose constant radius increases.

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Wikipedia