Helical camshaft

[citation needed] 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.

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.

The lobe has a helical slot machined into it that has a helix angle of about 35° relative to the rotational axis of the camshaft.

[clarification needed] One edge of the slot extends diagonally the full length of the lobe across the 170 degree constant nose radius.

Bridging the slot is a segment of lobe (about 10 mm in thickness) which is ground to the profile of the closing flank.

The lobe segment is fixed to the inner shaft so any relative axial movement has the effect of changing the valve opening duration.

As the slot has a helix angle of about 35°, any axial movement of the outer shaft causes the segment to rotate, exposing more or less of the nose constant radius and thus changing the duration.

The helical camshaft base profile belongs to the general group of lobe shapes which are used with pivoting cam followers, especially those with a fairly high rocker ratio, around 2:1.

[citation needed] When measured, the rates of acceleration and jerk in the nose region are slightly higher than standard.

This means that with the helical camshaft the old concept of a maximum power point in an RPM range no longer applies.

[citation needed] There has never been a mechanical VVA system that had either the duration range at full lift or the high RPM capability to do anything like this.

“Camless” electromagnetic/hydraulic systems do have similar duration/lift ranges to the helical camshaft but at present their high RPM ability is strictly limited.

On a possibly somewhat more practical level, dynamometer testing of road engines has shown that even with the helical camshaft limited to only about 30 degrees increase in duration, a typical road engine can increase its power by 25% to 30% at the same RPM power peak as the standard cam – and the idle and low RPM behaviour are totally normal.

Testing of a helical camshaft prototype in a Suzuki GSX 250 cc engine has a shown a remarkable improvement in fuel economy at idle speeds.

These pumping losses are greatest at idle, progressively reducing as the manifold pressure (and the power output) increases.

Just how a modern multi-cylinder fuel injected car engine would behave with the helical camshaft is as yet untested.

Having discussed the use of the helical camshaft to aid high RPM power and also for load control by LIVC it should be made clear that there is no reason why both functions could not be used in the same engine.

With a twin helical camshaft arrangement and suitable controls, an engine could have both extreme power output and also be very fuel efficient.

The helical camshaft and the general principle of LIVC also allow the possible use of a very high compression ratio (CR).

The more the expanding hot gases move the piston the more the heat energy is converted into useful work and the higher the thermal efficiency is.

(Strictly speaking the Atkinson cycle refers to an engine with mechanically different length compression and expansion strokes.

In modern practice, the compression pressure is limited by a fixed amount of intake valve late closing – this has exactly the same effect as the different stroke lengths).

The high CR would allow even greater amount of LIVC to be used at idle thus further reducing pumping losses and improving efficiency.

The duration of the helical camshaft is changed by moving the outer shaft of the coaxial arrangement in a lengthwise (or axial) direction.

For HCCI operation the picture is less clear but the very short (and thus very fast) axial movement that would be needed to change the compression pressure would seem to make the helical camshaft very suitable for this process.

[citation needed] The various prototypes have never[peacock prose] shown any wear or ultimate strength (breakage) problems in the many hours of testing (some at very high RPM) they have undergone.

But as a production car camshaft must ideally last for the life of the vehicle, there must remain some doubt until really long-term testing is carried out.

have made it appear that it is something of a virtue that their particular VVA system produces very short durations and the linked low valve lift as they really have had no choice.

Figure 1 : A prototype helical camshaft. The camshaft is in the minimum duration position.
Figure 2 : A helical camshaft for the intake on a Suzuki GSX 250 motorcycle engine, with a standard GSX camshaft for comparison
Maximum duration position
Minimum duration position. There is a stepless range of duration settings between these two extremes.
The area of constant nose radius is marked by white masking tape on a black background.
The constant radius base circle is in masking tape on a red background.
The camshaft in its minimum duration position fitted in a head. Note that the lobe segment is aligned with the follower. The black felt tip pen marking is roughly the constant radius area.
The camshaft is in its maximum duration position. Note that the segment remains aligned with the follower.
A comparison of the standard Suzuki camshaft lobe profile with the base duration profile of the helical camshaft shows that they are virtually identical.