Introductiontaken from: http://www.helicalcamshaft.com
The Williams Helical Cam (WHC) is a new type of mechanical Variable Valve Actuation system (VVA). More specifically it is a camshaft which allows the valve opening duration to be varied over an extremely wide continuous, stepless, range – all the added duration being at full valve lift.
The WHC was developed in the early years of the 21st century and of all the competing types of mechanical (that is, not “camless” types - electromagnetic or hydraulic) VVA systems it would appear to be the most capable and promising. It is still little-known amongst the mainstream automotive engineering community. One of the surprising and little-known facts about VVA systems in general is that, almost without exception, they are not new ideas. Most were first heard of many years ago – some have their origins in the age of steam engines. That they are not in wide use is a reflection that they are all lacking in some aspect of variable valve actuation.
For example, probably the most widespread in use and the most successful is the Honda VTEC (and the essentially very similar systems from other companies). There is no question that VTEC is very effective – in the Honda S2000 the engine power is virtually doubled compared to a non-VTEC version of the same engine. Otherwise its capabilities are limited. Its operation is strictly a stepped process and this precludes its use in the more exotic forms of engine load control.
Current research into these forms of load control includes Early Inlet Valve Closing (EIVC), Late Inlet Valve Closing (LIVC) and Homogenous Charge Compression Ignition (HCCI). None of the existing available mechanical VVA systems can adequately allow any of these forms of load control. BMW’s Valvetronic probably comes closest to being able to perform EIVC but even here it is not wholly successful. At valve opening durations short enough to allow idle and very low load situations, the unavoidable very low valve lift (that must accompany short duration with this type of system) has a throttling effect – and that is exactly what EIVC is trying to avoid. With the Valvetronic the effect is more “throttling-by-low-valve-lift” than true EIVC. It would seem almost physically impossible to achieve sufficient valve lift at durations short enough to allow true EIVC – maybe not even with electromagnetic/hydraulic VVA arrangements. The BMW Valvetronic is an example of the “oscillating cam” class of mechanical VVA systems – there are many, many other members of this class in the files of the US Patent Office, all with the same limitations.
Details
The WHC is distinctly different from competing VVA systems in that it incorporates a new and unique mechanical principle. Genuinely new mechanical principles are almost unheard of – especially in the automotive world. The WHC 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 in the USPTO files, first appearing as early as the 1920s, number 1527456 from 1925 being a good example). This class of coaxial cam varies the valve opening duration by moving the relative positions of two adjacent cam lobes on the one coaxial camshaft in a circumferential manner – the follower riding on the combined profile of both lobes.
The inescapable problem is that the duration range is not very wide. Not enough for power at really high RPM and certainly not enough to attempt engine load control by LIVC. The WHC importantly differs from other members of the general class by having a unique helical movement – a combined circumferential and axial movement of the two profiles. Surprisingly, because of this movement, the WHC in theory has no practical upper limit to its duration range. That is to say, 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 WHC would have a continuous duration range from about average for a road-going general purpose engine (say, about 250 degrees measured at normal valve clearance) to about 100 or 150 degrees above this. That is, from 250 degrees to 350/400 degrees.
The duration range of the WHC is accomplished in what most car enthusiasts or automotive engineers would agree is the “traditional” or most desirable manner a VVA (or variable duration) system should operate. 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. No matter how short or how long the duration, the opening and closing rates are always unchanged. Another surprising thing here is that this style of duration change over a very wide range has never, in the very long history of the internal combustion engine, been achieved before by a mechanical VVA system no matter how complex or expensive. The only other arrangement that can rival this for width of range is a system where the lobes on two closely-spaced separate camshafts are bridged by a pivoting lifter which operates the valve. The basic idea being that one camshaft opens the valve and the other camshaft looks after the closing of the valve. Changing the relative phase of the two camshafts has the effect of changing the opening duration of the valve. This works well enough but at long durations the lobe on one camshaft may reach full lift before the other even starts to move the follower. This has the effect of halving the full lobe lift (and rate of lift) of each camshaft, at long durations the valve lift (and rate of lift) is somewhat lacking. In most applications the valve lift needs to be at a maximum especially at long durations.
Needless to say this is another of the very old existing ideas which are at regular intervals dug up and proposed as a “new” idea. Recently it has been revived in an arrangement where the two camshafts are combined in the one coaxial shaft with a rocking follower acting on the separate cam lobes. Although more compact than the two separate camshafts layout it still has the same limitations – perhaps even more so.
Mechanism
Although in essence a quite simple idea, to describe the actual mechanism and its operation is almost beyond the power of the written word. Even when held in the hands and worked through its duration range it can be difficult to understand just what is happening and how the WHC generates its continuous range of profiles. The mechanism of the WHC is a coaxial shaft arrangement where the outer shaft carries the main body of the cam lobe. The main body of the cam lobe is in its maximum duration shape (or profile) form. Typically the main lobe body would have a duration of about 400 degrees. The lobe is very long axially, about 45mm, and its profile consists of conventional opening and closing flanks separated by about 170 degrees of constant radius over the nose of the lobe. The lobe has a helical slot machined into it that has a helix angle of about 35 degrees relative to the rotational axis of the camshaft. The width of the slot is equal to the angular extent of the closing flank of the lobe. One edge of the slot extends diagonally the full length of the lobe across the 170 degree constant nose radius. The other edge is ground so that it is all at base circle level. The slot in fact replaces the closing flank on the main body of the cam lobe. Bridging the slot is a segment of lobe (about 10mm in thickness) which is ground to the profile of the closing flank. The segment is attached to the inner shaft. One edge of the slot has a constant cylindrical radius, the same radius as the lobe’s nose radius. The other edge has the radius of the lobe’s base circle. A small region along each edge of the closing flank segment has the same constant radius as the edge of the slot that it is adjacent to. This means the segment can be positioned anywhere along the helical slot and there will always be a smooth transition for the follower to and from the segment. The lobe segment is fixed to the inner shaft so any relative axial movement has the effect of changing the valve opening duration. The follower is arranged so that it always remains aligned with the segment which remains stationary axially. As the slot has a helix angle of about 35 degrees, 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. Make sense? Maybe not. This somewhat tortuous and convoluted explanation disguises the fact that the WHC is actually quite a simple mechanism. This brief description is not the entire story and more details, drawings etc. are available online on the USPTO website – the patent number is 6832586. The slightly indescribable and mind-boggling way it operates probably accounts for the fact that the helical movement principle has never been suggested before let alone built and run in an engine. New mechanisms, even minor detail ones, are almost unheard of – the WHC is a distinct contrast to the literally thousands of VVA designs in the USPTO files alone.
Cam Profile
The base or shortest duration profile of the WHC system is almost identical to a standard production engine profile. The WHC 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. This family of lobe profiles are characterised primarily by not having a very great actual lift directly at the lobe (usually referred to as “lobe lift”). The lobe lift is enlarged by the rocker ratio which often is around 2:1 resulting in quite high lift at the valve. As the duration etc. is as for a normal lobe this results in the nose of the lobe having a very rounded-off (or “snub-nosed”) appearance. The radius of curvature of the nose region (about the axis of rotation of the camshaft) is often very close to being a constant radius over an angular extent of about 20 degrees or so. The WHC principle requires that this 20 degree region is modified to make it a true constant radius. In some cases this requires as little as 0.25mm (or less) to be removed from the nose. The constant radius area is then blended - in to the curvature of the standard opening and closing flanks.
Generally speaking, the modified minimum duration profile is identical in appearance to the standard profile – at least to the naked eye. When measured very accurately, the rates of acceleration, jerk etc. in the nose region are slightly higher than standard but only marginally higher. The WHC expands the duration by adding to the nose constant radius area and removing an equal amount from the base circle constant radius. The 20 degree constant radius area on the lobe nose typically can have about 150 degrees added to it. The 150 degrees is the extra duration. In effect, the opening and closing flanks “open up” exposing progressively more and more constant radius on the lobe nose. As both flanks are unchanged, the rates of lift/acceleration etc. and the total lift are also unchanged – no matter how much constant radius is added to the nose.
A critical question– is this the best way of making a longer duration profile? With the WHC the question is not entirely relevant. The intention is not to use the WHC principle to obtain a “fixed” profile but to develop a minimum duration profile that best suits a cam with totally variable duration control – which is something quite different. If the profile was for a conventional non-varying cam the answer would have to be “probably not”. It would certainly make a quite useable cam but not a really good cam. Certainly in the past, some long duration cams were made exactly in this way – by “opening-up” a standard profile. Even at present many racing cams are made this fashion (or at least with a lot of constant radius on the nose) for some oval track competitions in the USA. In certain classes the valve lift has to conform to the “limited lift rule” – even some very long duration cams. This results in a very distinctive “flat” region on the peak of the lift curve graphs of the cams. These “lift rule” cams rev to high RPM as well (or better) than more conventional racing cams. Usually with a high performance cam the lift is increased, the point being to get more flow into the cylinder. High performance cams generally have greater lift and greater rates of opening and closing of the valve. The aim is to maximise the cylinder filling while keeping the duration as short as is feasible. Keeping the duration as short as possible assists in retaining the lower RPM performance. In a sense the amount of lift/rate of lift etc. can be “traded” against the valve opening duration. If, for a particular application, lower RPM performance is not very important the amount of lift etc. can be kept at moderate levels and the duration increased. That is; the lift etc. and duration are roughly inversely proportional to each other.
This is the situation to some extent with engines driving water or air propellers, the engine more-or-less running at constant speed. With wheeled vehicles, no matter what the application, generally there is some need to restrain the amount of duration to retain at least some lower RPM performance. Even in racing, most types of race are standing start or if not still require accelerating from rest after pit stops, slow corners etc. The situation with the WHC is totally different. Even with an engine that must retain a lot of low RPM performance the maximum amount of duration that can be used is not restricted – it can be literally whatever is needed to produce maximum power at the maximum RPM the engine is capable. The implication would seem to be that both the rates of lift and the total lift need not be so high with the WHC.
This is a situation that cam designers have never really had to consider – there was no point in thinking about what having total freedom of duration choice would mean to the required lift, etc, if there was no mechanism available to allow wide range variable duration. If the WHC was forced to operate at the one fixed duration setting it probably would not be quite as good in performance as the best conventional cam of the same duration – but it would be far from being bad.
However, the fact that the WHC has an almost unrestricted duration range really makes it far superior to any conventional “fixed lobe shape” cam. It is almost a case of “in the land of the blind, the one-eyed man is king”. Having unlimited variable duration is an enormous advantage to a camshaft. An interesting aside here is that the style of profile generation and variable valve opening of the WHC is virtually identical to that of most hydraulic/electromagnetic “camless” systems as is its range of full-lift durations. This would seem to imply that there would be little point in using a “camless” system. The WHC can do the same sort of things but is much simpler, cheaper, more reliable and is not rev-limited (and does actually exist in a more-or-less production practical form – unlike the “camless”).
Applications
The “traditional” application of VVA (especially variable duration) is to match the engine RPM to the valve opening duration (this is very roughly what the VTEC does). The general idea being to improve the high RPM performance without the associated problems of a long duration “racing” cam which are lack of lower RPM power, rough idle, etc. Engines typically need a roughly linear increase in duration as the RPM rises. The aim is to maximise the torque at every point in the allowable RPM range. This means that with the WHC the old concept of a maximum power point in an RPM range no longer applies. The WHC employs a totally standard pivoting (or “finger”) follower and the whole WHC system is not rev-limited – or at least no more than any other finger follower layout. As most Formula 1 engines use finger followers it would seem that this type of follower does not hinder high RPM too much. This is in distinct contrast to most other VVA arrangements – “camless” included. With the WHC the power continues to build until the “breathing” limit of the induction system is reached – or more likely, the mechanical strength limit of the engine’s connecting rods, etc is reached. The WHC’s typical 250 degree to 350 + degree duration range basically means that a suitably robust engine could “pull” strongly from about 1500 RPM to maybe 20,000 + RPM and still idle smoothly at, say, 500 or 600 RPM.
This is not just “in theory” speculation. If you have a VVA system with an unlimited duration range and the VVA system is not rev-limited, then the type of performance just described would be quite possible. It is probably not well-known that competition engines that are intended for ultimate power output at extreme RPM (like Formula 1 or MotoGP) are still slightly limited in how long a valve opening duration they can use – they could actually make more power with even longer duration cams. As with lesser engines, consideration has to be given to lower RPM performance and power delivery characteristics. In a word, the car or bike (bikes especially) must remain at least reasonably “driveable”. Most of these engines are right on the limit of manageability (- if you can call F1’s 3000 RPM idle and no power under 10,000 RPM even vaguely manageable by most standards). For an engine intended for competition only, the idea could be expanded to possibly even a rev limit of maybe 30,000 + RPM as there would be fewer worries about “driveability” with the WHC. (As a point of interest, some small capacity competition two-stroke engines can run to well over 40,000 RPM so astronomical RPM levels are not unknown).
To repeat what has been said above; 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 WHC 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 WHC 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 (which, of course, is one of the main points of VVA and the WHC).
Efficiency
In these days of extreme oil prices, the application of the WHC in its fuel saving guise is possibly an even more important application than just to maximise the power output of an engine. Testing of a WHC prototype in a Suzuki GSX 250 cc engine has a shown a remarkable improvement in fuel economy at idle speeds. This particular WHC is arranged so that all the duration increase is on the closing side of the intake cam lobe, the opening point of the intake valve remaining as standard on a Suzuki GSX 250 engine. The object of this was to test the effectiveness of LIVC on the idle fuel consumption.
The basic aim of LIVC is to reduce the intake pumping losses. These pumping losses are greatest at idle, progressively reducing as the manifold pressure (and the power output) increases. The test Suzuki engine consistently recorded a slightly startling 40% improvement in economy at idle when compared to the same engine with the standard camshaft fitted. This may seem a little unlikely, but it should be remembered that it has been estimated that at idle about 80% of the fuel used is just to overcome the intake pumping losses. Any reduction in pumping losses thus has a major and direct effect on the idle fuel use. As the power output rises, the 40% would quickly drop away but for an engine in typical road/traffic use an overall figure would be probably between 10% to 20% improvement.
The Suzuki idled at about 55 or 60 extra degrees of late closing. That is; about 120 degrees after bottom dead centre. This means that the total duration required was around 320 degrees. Engine load control by LIVC needs very long durations. Usually a much longer duration is needed for load control by LIVC than would be needed for high RPM power, especially for a general-purpose road-going application. Importantly all this very long valve opening duration, when used for LIVC, must be at full valve lift. The valve lift must be at a maximum so as not to impede the flow into and out of the cylinder. Any restriction to the flow causes pumping losses which defeats the whole purpose of LIVC. Needless to say, the WHC is the only mechanical VVA system that exists (or has ever existed) that has a wide enough continuous duration range to properly allow LIVC.
Having discussed the use of the WHC 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. Realistically the WHC principle can only be applied to twin cam engines. For maximising power output both the intake and exhaust cam would need to be of the WHC type. The increase in duration needed for high RPM performance needs to be roughly equal on both the intake and exhaust cams, and roughly a symmetrical increase about the base duration lobe profile centre line. For LIVC operation alone, only the intake camshaft needs to be a WHC. With a twin WHC arrangement and suitable controls, an engine could have both extreme power output and also be very fuel efficient.
There is also the possibility of even greater fuel efficiency at the expense of outright power. The WHC and the general principle of LIVC also allow the possible use of a very high compression ratio (CR). The idea here being to use a very high geometrical CR but limit the compression pressure by LIVC so as to avoid detonation. The expansion ratio after combustion still remains high. It is the expansion ratio that fundamentally converts the heat energy of the burning fuel/air mixture into useable mechanical energy. The more the hot gases are expanded by the moving piston the more the heat energy is converted into useful work and the higher the thermal efficiency is. This general principle is usually called the “Atkinson Cycle”. (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). With the Atkinson Cycle the added efficiency is at the expense of reduced overall power. For example, if an engine had a geometrical CR of 18:1 it would have to be restricted to about half its full charge of air/fuel mixture to avoid detonation. The resulting effect would be that at full load the engine would use half the fuel but the power would be not half but roughly two-thirds or three quarters that of the equivalent “normal” engine – the net result being an increase in thermal efficiency. Such an engine would be economical but it would still suffer from intake pumping losses.
The WHC would allow both the Atkinson Cycle and LIVC to be applied simultaneously. The high CR would allow even greater amount of LIVC to be used at idle thus further reducing pumping losses and improving efficiency. The resulting engine would have a fuel economy very similar to (or better than) a diesel – and it could run on the cheaper LPG fuel. It would also be lighter in weight and cheaper to make than a diesel. A car fitted with such an engine would appear to be a much simpler and cheaper alternative to a “hybrid” car. (But a hybrid fitted with a WHC/Atkinson/LIVC engine would be more economical still).
One of the more recent “fashionable” areas of engine research at present is the Homogenous Charge Compression Ignition (or HCCI) engine. It amounts to running a spark ignition engine at light or part load in a similar fashion to a diesel engine. HCCI requires the compression pressure to be very quickly and accurately altered so that the more-or-less controlled compression ignition doesn’t suddenly blossom into full-blown detonation. One of the main strengths of the WHC is that it can do exactly that. However, it would seem that the easily-controlled LIVC (with or without Atkinson high CR effects) is a much simpler way to control an engine than the decidedly risky HCCI process – and it is doubtful that HCCI is more fuel-efficient than LIVC, etc.
Operation
The duration of the WHC is changed by moving the outer shaft of the coaxial arrangement in a lengthwise (or axial) direction. The helix angle of the WHC is probably always going to be around the 30 to 35 degree mark. This translates to a figure of around 3.5 (crankshaft) degrees per millimetre of axial movement. 30mm of movement would give 105 degrees of duration change. Although the WHC is capable of far more than this, it has been found in testing that this amount is sufficient for most purposes. Little force is needed to move the shaft axially so there is a possibility that when using the WHC for LIVC load control alone the axial movement could be connected directly and mechanically to the accelerator pedal. Similarly, if the WHC is used to improve high RPM power only a simple self-contained (that is; it does not need an additional power source) centrifugal controller/actuator could be used. Some prototypes have run very well using centrifugal controller/actuators. If it was desired to operate the WHC to use both the LIVC and the high RPM aspects of the cam it would probably require hydraulic actuators on each cam (of a twin-WHC layout) to enable the LIVC to be used. Each WHC would also need a phase-changing mechanism for the high RPM use. At low RPM and part-load the WHC would be all LIVC. At high RPM and full load it would still require long duration from the WHC but the phase changing mechanism would need to alter the all-on-the-closing-flank duration increase to something of a more symmetrical duration increase. All this possibly could be done mechanically but the sensible arrangement probably would be an externally-powered arrangement with a computer/microprocessor to sort out the required amounts LIVC and phasing. 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 WHC very suitable for this process.
Considerations
One slight drawback (and maybe the only thing that could be called a possible problem) would seem to be the cost of the WHC. Even though it is a fairly simple device it requires very accurate helical machining and very careful assembly. The WHC prototypes typically cost about $1500 in machining and materials. This figure would reduce greatly in production. The cost of the WHC is really only high when considered in comparison to a conventional camshaft which reportedly cost the manufacturers only a few dollars per unit to make. This fact does tend to make the WHC look more expensive than it really is. Having said this, the cost of the WHC (and associated controls etc.) is probably very similar to (or even cheaper) than other production VVA systems.
The various prototypes have never 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. However, indications are that there would probably be no insoluble long-term problems.
It probably doesn’t come into the category of a possible problem but realistically the WHC must operate through a lift-multiplying pivoting follower. The WHC could not really be used with an inverted bucket type of follower. Even though the inverted bucket is still used, it is being increasingly replaced both in road and racing engines by the pivoting “finger” follower. As well as needing a pivoting follower, if the engine has four valves per cylinder (and there are very few newly-designed engines that don’t) then the follower must be forked so that the one WHC lobe operates two valves. This is more a characteristic than a problem. If a really wide duration range is required, because axial space is somewhat limited along the camshaft, usually only room for one WHC lobe (and its operating space) can be found.
Also in the category of “not really a problem,” is the fact that the WHC does not “do” very short durations or variable lift. Many companies and manufacturers 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. Up until the appearance of the WHC there was literally no method of mechanically producing continuous, wide-range full-lift variable duration valve timing. So it was either try to promote the reasonably-easily achieved short duration/low lift (usually by oscillating cam systems like the Valvetronic) or nothing. The long duration/full lift characteristics of the WHC are the commonsense way a VVA system should operate. It should be noted that there is no physical reason why a WHC could not be the “driving” cam in a Valvetronic-type oscillating cam setup. (But it would be quite complex and the Valvetronic part of the arrangement would limit the WHC’s high RPM capabilities).The result would be an almost unbelievable array of possible duration/lift combinations. This could be very useful in research.
However, in the real world probably 95% of the combinations have no really useful relevance to the four-stroke cycle. This, of course also applies to the WHC to some extent. What possible use could a duration of, say, 600 degrees be? It is hard to imagine a use for more than about 400 degrees – and the WHC potentially has another 300 or so degrees in hand. This is also true of the “camless” types. They have even greater duration ranges than the WHC but again, most of the range is just for novelty’s sake – it is not actually very useful. Besides this, “camless” VVA doesn’t really exist in any useable production form at present and may never do so.
The Williams Helical Camshaft would seem to have a bright future (and deservedly so). Most of its capabilities could be used to produce very fuel-efficient, powerful and cheaper engines. Oil prices will almost certainly never be low again, and the WHC has appeared just when it is most needed.
The WHC has one main obstacle to overcome. This is to convince engineers and the general public that for the first time in the entire history of the internal combustion engine a really effective system of mechanical wide-range, full-lift continuously variable valve opening duration has been discovered.
The WHC principle is one of the cleverest examples of mechanical lateral thinking that has been seen in many, many years.
If this camshaft makes it into production, the aftermarket camshaft manufacturers will have a stiff competition to match already. Imagine being able to change the duration as well as the lift to match your engine setup.
Though this technology is up and coming for stock production bike, eg, Kawasaki's GTR14oo. It'll be still exciting for the aftermarket to get a taste of it on older engines.
