what is Variable Valve timing?

Variable valve timing and lift: the new must-have tech

what is Variable Valve timing?

VARIABLE VALVE TIMING – VVT for short – has had a long and somewhat bumpy history when it comes to bikes. The earliest iterations of the idea date right back to the 1980s but in 2019 we’re seeing the most significant adoption of the idea yet with BMW introducing its ShiftCam set-up on the R1200 GS, RT, RS and R models as well as the all-new S1000RR superbike.

Why are we excited about BMW’s system? After all, Kawasaki’s now-departed GTR1400 had VVT right back in 2006, multiple Ducatis feature a similar arrangement and Suzuki’s GSX-R1000 also has VVT. What’s more, Honda has had its own take on variable valve operation, first appearing as the 1983 CBR400’s ‘REV’ system and later evolving into ‘Hyper VTEC’.

The reason is that BMW’s ShiftCam system is more than just variable valve timing – in fact it’s a cam-switching system that alters not only when the valves are opened and closed, but the entire cam profile. That means valve lift, duration and overlap are all changed.

Let’s get that terminology straight first of all. Valve operation is defined by the shape of the cam lobes, which force the valves to open as the camshaft rotates. There are four key elements to consider here: timing, duration, overlap and lift.

Timing is exactly what it sounds like, defining when the valve is opened. Duration is how long the valve remains open for. Overlap describes the period when both the exhaust and intake valves are open at the same time. Finally, lift describes how far the valve opens; more lift means more airflow.

Early days

So far, motorcycles have seen three distinct types of variable valve control. The first was Honda’s REV (above) setup back in 1983. Unlike a true variable valve timing system, REV involved taking a four-valve-per-cylinder engine and turning it into a two-valve-per-cylinder motor at low revs. In the REV system, each valve was operated via finger-follower that sat on top of the valve stem and below the camshaft but the cam lobes only acted directly on half of them – one exhaust and one intake for each cylinder. At high revs, oil pressure forced a metal pin to connect the active finger-follower to its inactive neighbour, bringing all the valves into operation.

Later, Honda’s Hyper VTEC system – as used in the VFR800 – eliminated the finger followers from the arrangement, a bucket and shim system to let the cam lobes operate directly on the valves. However, it still used oil pressure to push metal pins into place at high revs to enable half the valves in the engine.

The Honda systems are basically unlike any other variable valve arrangement. They don’t alter the lift, duration, overlap or timing, but simply halve the valve area of the engine at low revs. That alters the engine’s behaviour, boosting low-end torque, but doesn’t have the same finesse as a truly variable valve tech.

Cam-phasing systems

The most common type of variable valve timing in cars, where the tech has been around for decades, is also the one that’s used by Ducati and by Kawasaki’s GTR1400. It’s also the type used – in a rather different way – by Suzuki’s latest GSX-R1000.

Called ‘cam phasing’ these setups add an additional component between the camshaft and its sprocket. This component, the cam phaser, can alter the rotational position of the camshaft in relation to that sprocket, perhaps by as much as 30°. These phasers usually work via oil pressure, with electronic controls diverting oil into chambers inside them to rotate the camshaft in relation to the sprocket.

These phasers help because there’s no way to have perfect cam timing across an engine’s entire rev range.

At high revs, you need lots of valve overlap. The intake valves need to open early, while the engine is still in its exhaust stroke, to give enough time to fill the cylinders effectively. And the exhaust valves need to stay open late, after the intake stroke has started, to give the waste gases enough time to exit the cylinder.

But at low revs – when all the valves remain open for longer simply because the engine is running more slowly – so much overlap gives time for unburnt fuel/air mixture to escape through the still-open exhaust valves, with horrible effects on economy and emissions.

Cam phasing means you can alter the overlap depending on revs. In the simplest systems only the intake cam has a phaser, and it just switches between two timing options at a fixed point in the rev range. More sophisticated versions can phase both the intake and exhaust camshafts, and operate gradually over a range of revs to slowly increase overlap as revs rise.

At the moment, motorcycles tend to use the simple systems.

Suzuki’s set-up is slightly different. Although the GSX-R has a cam phaser, it’s not operated by oil pressure or electronic control. Instead it uses centrifugal force to mechanically shift the camshaft in relation to the sprocket – a system the firm devised for MotoGP to get around rules that ban electronic or oil-operated variable valve timing.

Why is ShiftCam better?

Although cam phasing helps, it only alters the valve timing and overlap. The two other key components – valve duration and lift – remain unchanged throughout the rev range.

BMW’s new ShiftCam system changes both of them as well as the cam timing, giving a massive advantage over the simpler setups.

It’s a clever system, and one that’s surprisingly simple in concept. Fitted only to the intake camshaft (at the moment, at least – it wouldn’t be hard to also use it on the exhaust, although the benefits wouldn’t be so significant), it’s basically two camshafts in one.

There are two sets of lobes for each intake valve. One set is ‘mild’ – with low lift, short duration and little overlap, timed to open relatively late, ensuring plenty of low-end torque and good economy and emissions. The other set of lobes is ‘wild’ – high lift, long duration and lots of overlap, with advanced timing to work well at high revs.

The lobes are right next to each other, so by sliding the whole camshaft along by a few millimetres – during the two engine strokes when the valves are closed and the lobes aren’t engaged – you can switch from one set to the other.

On the inline-four S1000RR, the system is slightly more complex. Because at least one set of intake valves is always being opened or closed, BMW has split the intake camshaft into two halves, each dealing with two cylinders, which can be moved independently a fraction of a second apart from each other when there’s a moment that they’re not engaged in opening a set of valves.

Although lacking the continuously-variable element of some cam-phasing VVT systems, the benefit of also being able to change the valve lift and duration outweighs that. In theory, a cam phaser could always be added to ShiftCam as well, giving an extra dimension of adjustment for future engines. Some car engines, like Toyota’s VTL-i and Porsche’s Variocam Plus, already combine cam-phasing with variable lift and duration, albeit with completely different systems to BMW’s ShiftCam.

The future – camless engines

For years – even decades – engineers have striven for the ultimate four-stroke engine, which would dispense of camshafts altogether and rely on pneumatic, hydraulic or electrically-operated valves that could be opened and closed at the command of a computer rather than a mechanical cam lobe.

Although theoretically ideal, such systems have so far generally failed to get far beyond the prototype stage.

At the moment, perhaps the most promising is the design from Freevalve, a sister company of supercar firm Koenigsegg, which has developed several camless engines including one intended for mass production by Chinese car firm Qoros. Others, including Lotus, have made running prototypes that demonstrate the idea, but getting the systems to operate at the sort of high revs associated with bike engines would be a tough challenge.

Although still a technology with potential, it’s a race against time to see if camless engines can be perfected before electric cars (and motorcycles) start to overtake their internal combustion-powered rivals in performance and popularity.

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