Variable Duration Camshafts

Thursday, May 13, 2010

Tags :  camshafts

camshaftsGiven enough time in engine research and development, only someone who is very blinkered could say they've never come across something a bit, shall we say, unusual. Devices to increase fuel economy substantially tend to be the norm here. Now and again I get a full-size drawing of an engine concept, and invariably the sender is looking for introductions or funding. This month, however, I have a camshaft, and for once its owner/designer has sent me the actual component, which looks interesting.

The idea comes from Danny Williams, of New South Wales, Australia, who has applied some ingenuity and a lot of hard work to come up with a variable duration camshaft that maintains the opening and closing characteristics - velocity, acceleration and jerk - of a traditional engine camshaft, but which changes the phasing at which these occur.

In essence, he has taken an opening flank and a closing flank and joined them together with a 'bridge' that keeps the valve open for a variable period of time. Calling it a 'helical' cam, he not only designed it but built one to test in a Suzuki 250 GSX, in which it has run on the bench only, for about 15 hours.

camshafts figure-1

Now I have to say that it is a lot easier to play with the device than describe its action, but described by Williams as a 'coaxial shaft, combined profile cam', the unit consists of two shafts, one inside another.

The outer shaft carries with it the normal cam opening profile at one end which, as we travel along its length in the axial direction, blends into a lobe of constant radius. The inner shaft moves in and out co-axially and has the closing flank and ramps attached to it (see Figs. 1 and 2).

camshafts figure-2

As the two shafts move relative to each other at one end of the cam, the profile is that of profile A in the drawing here (Fig. 3), while at the other end profile C is followed. At one end the duration is about 240 crank degrees; at the other about 320.

The interesting point about this cam is that, as the inner shaft moves and rotates, the period of opening - the duration - is continuous and directly proportional to the position along the axial width of the main cam. Suitable only for a finger-follower system, in this particular example the drive comes in through the centre from a roller chain and sprocket assembly.

In theory, the valve dynamics are little changed from the standard GSX profile - apart from the extra accelerations and jerks caused by the constant lift portion of the curve - but the main concern would appear to be the apparent complexity of the assembly process and the accuracy of the fit of the sliding parts required during manufacture.

camshafts figure-3

The angle of helix in this example is about 30º, and obviously the wider the main cam or the shorter the change in duration then the less this angle would be. But in trying to make this as compact as possible, the risk must surely be in the mechanism jamming. Williams, however, assures me that this is not the case in practice.

It's a refreshingly simple approach and one that would be good to evaluate on a dyno, despite the difficulties and costs associated with manufacturing it. But what do you think?

Fig. 1 - Closing flank at a mid-position slides along between the end of the nose and the beginning of the base circle
Fig. 2 - Another view
Fig. 3 - Cam profiles(s)

Written by John Coxon

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Comments

Amazing!
This is a very slick solution to an age-old problem!  I'm sure that CNC-controlled equipment could manage the fabrication and tolerances without too much fuss... Simple, elegant and mostly self-contained.  Any idea what kind of control mechanism(s) will work with it?
Very good, if it works, and I hope it does. I would like to see more so I can understand the motions
Frank - it actually does work as neatly described by John Coxon.  You can see the cam running in videos on the "helical camshaft" website.
  I also find the "motions" hard to picture and especially difficult to describe in words.
Bearing in mind that this development is fairly “low tech” in its approach (i.e. does not rely on state of the art computerised, camshaft-less valve actuating systems that the major automotive manufacturers are apparently developing for their next generation models), what would be the likely market(s) for this camshaft ?
Would it be possibly a way for developing nation manufacturers to obtain more performance (both output, and fuel economy) from their necessarily simpler designs of motors ?
Or maybe, seen as an after-market sales opportunity for camshaft developers in, say, the United States ?
Robert Bates - Control for LIVC purposes only can be by direct unpowered mechanical connection to the accelerator pedal (or twist grip).  Control for duration increase with rpm increase only can be by a simple centrifugal mechanism.  However control to utilise both LIVC and duration-with-rpm simultaneously could be a bit trickier.  Although a mechanical solution is possible, control through a ECU/hydraulic/electric etc. system would probably be more effective.
Robert Jamieson - The likely market would appear to be the OEMs.  Even though the helical cam essentially replaces a normal camshaft it is not really a "drop-in" replacement usually.  I would think that it would be suitable for both developing and developed nations - but I see your point.
There is an earlier, simpler, non-LIVC version of the cam that may be more suitable for aftermarket use and simpler designs.  The earlier version has actually had more testing (road and dyno) than the later type of camshaft.
I need this for my drag bike like yesterday. I sent him an email, hopefully we can work something out. I have already got MMC pistons, a air spring valve setup is in the works and a i "obtained" motogp stlye throttle bodies. this would COMPLETE my goal of "worlds most advanced and powerful GSXR1000 Engine." :D