Variable Valve Timing: the next big thing?

GSX-R1000 Variable Valve Timing

Sometimes a new technology will burst onto the motorcycling scene, sweeping all before it and becoming a factor that clearly marks out bikes that came before or after that watershed. Traction control, multiple riding modes, TFT displays, for instance. All are things that were nowhere to be seen a handful of years ago but are fast becoming standard fare.

Other tech takes a little longer to reach that tipping point. ABS brakes aren’t new by any means – BMW has been offering the tech since the Eighties – but only now, with the help of improving technology and legislation to force them on new bikes from 2017 onwards, are they reaching ubiquity.

Variable valve timing (VVT) falls firmly into the second camp.

As a technology, it’s been around for decades but while car makers have long since embraced it, motorcycle manufacturers have shunned it. But, just as they’ve been obliged to take on ABS, the tightening clench of emissions regulations will soon force companies to look again at VVT.

But don’t for a moment think that’s a bad thing; properly applied variable valve timing on future bike engines promises to improve tangibles including their performance and economy, as well as ensuring cleaner exhausts.

So what’s it all about?

The internal combustion engine is a long series of compromises. These largely stem from the fact that we demand so much flexibility from them. One moment we’re expecting them to start easily and settle to a low-revving, smooth tickover, the next we’re calling on the same motor to hit 13,000rpm and pump out arm-stretching amounts of power.

What’s more, we all expect the transition from meek to mad to happen seamlessly, without so much as a hitch or hiccough on the way. And that’s before we even start to consider emissions or economy.

Engine designers are pulled in every direction, and as customers we’ll walk away the moment they fail to meet our lofty expectations.

The problem is that each of these goals requires a different set of attributes from the engine itself. Valve timing in particular needs to be compromised.

Valves are an essential element of the four-stroke engine. During those four strokes of each piston – suck, squeeze, bang, blow – there are several key moments for each valve. In simple terms, the intake valves open during the intake (‘suck’) stroke, as the piston goes down and draws a mixture of air and fuel into the cylinder. They close for the compression (‘squeeze’) stroke and the ignition (‘bang’) stroke. Then, for the exhaust (‘blow’) stroke, the exhaust valves open to allow the spent gasses out and allow the whole cycle to start again.

To open and close, the valves are opened by lobes on one or more camshafts, which geared to turn at half the speed of the crankshaft because it takes two complete revolutions of the engine to make one four-stroke cycle.

Simple, right?

Unfortunately, while the basics are straightforward, the details become a little trickier.

The problem is that to make an engine work best, the valves can’t simply open and close at the moment that their relevant stroke starts.

Instead, the exhaust valves start to open fractionally before the end of the ignition stroke, before the piston reaches bottom dead centre. They then remain open fractionally after the end of the exhaust stroke and into the start of the intake stroke.

Similarly, the intake valve will actually start to open before the end of the exhaust stroke and won’t close until after the compression stroke has begun.

There are several reasons for this, some rather too complex to enter into here, but among them are the facts that the valves don’t open instantly – it takes a while for them to reach their fully-open positions – and that the gasses they’re shepherding in and out of the cylinder also don’t react instantly.

Engine designers will often talk of the movement of these gasses in terms of pressure waves. If you imagine the gasses going through the engine behaving a bit like water it’s easier to envisage. As valves open and close to allow it in and out, and the piston moves, it sloshes around. Time the valves just right and those waves will help force the gas through them.

The problem is that timing which works perfectly at 13,000rpm probably won’t be ideal at 2000rpm, and vice-versa.

In particular, the period where both the intake and exhaust valves are open simultaneously is absolutely vital. Called valve overlap, having the intake valve start to open before the exhaust valve closes helps encourage intake gas into the cylinder. The wave of the exiting exhaust gas creates a low pressure area behind it that, with the intake valve open, sucks intake mixture in.

The problem is that the amount of valve overlap that’s chosen only tends to work at a specific engine speed. At high revs, lots of overlap is good; the pressure waves from the exhaust help suck more fresh charge into the cylinder. Working perfectly, it will actually help suck in more air/fuel mixture than would normally fit into the cylinder, so it gives a supercharging effect that basically results in performance that you’d normally need a larger cylinder to achieve. Because we’re talking about incredible speeds – at 15,000rpm each valve is opening and closing 125 times every second – there isn’t time for the unburnt fuel in the intake charge to escape through the exhaust valve even when there’s significant overlap.

But at low revs, the same amount of overlap will lead to terrible emissions; at a 1200rpm idle the valves are only opening 10 times per second, and if there’s lots of overlap that gives plenty of time for unburnt fuel to escape into the exhaust. On a racing engine that’s no problem, but road engines need to meet emissions rules and throwing unburnt hydrocarbons out is very much frowned upon.

In its simplest form it’s here that variable valve timing comes into play. Engines with the ability to alter the timing of their intake camshafts or both intake and exhaust camshafts are likely to become ever more common over the next few years.

Kawasaki VVT
VVT: a brief history

The sort of VVT that’s starting to reach production bikes now, where the camshafts are ‘phased’ – have their timing advanced or retarded – is essentially similar to the most common design seen on cars.

In fact, the very first production VVT car set-up, seen on the Alfa Romeo Spider back in 1980, was essentially similar to the ones that are still most common today. The idea is that the intake camshaft isn’t solidly attached to its sprocket. Instead, there’s a ‘phaser’ that allows it to rotate by a few degrees in relation to the sprocket, advancing or retarding the timing.

These phasers are usually designed to simply allow two positions. They have two main parts; an inner rotor that’s bolted to the camshaft and an outer sprocket that’s driven by the cam chain (or belt, in Ducati’s case). The inner rotor has lobes that fit into wider chambers in the outer sprocket. By forcing engine oil to one or other side of these lobes, using the engine’s existing oil system and a simple set of electronic solenoids to allow it in or out of the chambers, the inner rotor is shifted (‘phased’) in relation to the outer sprocket.

In 2014 Ducati brought this arrangement to the Multistrada with great fanfare, with phasers on both the intake and exhaust camshafts. But Kawasaki has actually been phasing the intake camshaft on the GTR1400 ever since its introduction in 2007. While the Kawasaki’s design (pictured) is only on the intake cam, and only phases by 27 degrees (compared to 45 degrees on the Ducati), it’s essentially similar in concept.

The simplicity of cam-phasing VVT, and the fact that it’s relatively easy for manufacturers to modify existing engines to accommodate it, means it’s already dominant in cars and it’s also likely to be the arrangement that becomes most common in bikes.

 

Suzuki GSX-R1000
Suzuki GSX-R1000

While it’s by no means the first bike with VVT, Suzuki’s 2017 GSX-R1000 is the first all-out superbike to be seen with variable valve timing. It’s also got a very clever, purely mechanical system that eliminates both the need to be plumbed into the engine’s oil system and the need for electronic solenoid actuators.

The elegant solution was really driven by the fact this system is already in use on the firm’s GSX-RR MotoGP bike, and in MotoGP both hydraulic and electronic VVT systems are banned. That excluded all the existing systems and forced Suzuki to think laterally if they wanted to get the combination of bottom-end grunt and top-end power that VVT offers.

The GSX-RR and GSX-R1000 system is actually remarkably like the hydraulic cam phasers used by Ducati and Kawasaki. Like them, Suzuki uses a two-part cam sprocket – one half carries the sprocket itself and is driven by the cam chain (above and top of page). The other is firmly bolted to the camshaft (inlet cam only for the GSX-R1000, but there’s no reason the system couldn’t also be applied to the exhaust.)

Instead of lobes in oil-filled chambers, the two halves of the Suzuki phaser are connected using steel balls running in radial grooves machined into both halves. On the sprocket side, the grooves are slightly angled, while on the camshaft half they point straight out from the centre and get shallower towards the outer edge. A spring plate forces the camshaft side against the sprocket side, and the balls are sandwiched between them in the grooves.

The spring pressure forces the steel balls towards the deeper part of the grooves in the centre when revs are low. As engine speed rises, the centrifugal force of the steel balls pushes them outwards, and because of the angled alignment of the grooves on the sprocket, the effect is to advance the cam timing.

The clever system means that altering the spring force will change the engine speed at which the cam phasing happens. The design also means that the changeover from retarded to advanced intake cam timing should be smooth and gradual as revs rise.

Suzuki has, unsurprisingly, heavily patented the design.

The proof will be in the pudding, of course, but the VVT system appears to work well on the GSX-RR GP bike so there’s every reason to believe the GSX-R’s cam phasing will be just as effective.

Honda VFR800 VTEC
So what about VTEC?

Ah, yes. You’ve noticed we haven’t mentioned Honda’s long-running VTEC system.

Honda was among the pioneers of VVT in the car world, introducing VTEC in the late Eighties, and unlike most systems it varies not only the valve timing but also the valve lift and duration. It achieves it by having two cam lobe profiles and two sets of rockers for each pair of valves. At low revs, the softer cam profile is used while the rocker on the more aggressive cam moves freely

At a pre-determined engine speed a solenoid valve allows oil pressure to force a pin into place between the rockers, locking the second rocker into position and bringing the more aggressive cam profile into play.

But that system is only used in Honda’s cars.

Confusingly, the Hyper-VTEC system used on bikes like the VFR800 (pictured) and the CB400 Super Four is quite different. It actually stems back to the earlier REV system used as long ago as 1983 on the Japanese market CBR400F, and works by ‘switching off’ two valves in each cylinder at low revs. Again, solenoids and oil pressure are used to force pins in or out of position, but in the case of Hyper-VTEC, they actually engage or disengage the cam lifter from the valve stem. The system is fitted on four-valve-per-cylinder engines, but only to one intake and one exhaust valve in each cylinder.

At low revs, the Hyper-VTEC valves are disengaged, which means the engines run as two-valve motors with relatively small valves and conservative cam profiles. The combination improves economy and adds torque, since small valves increase the speed of the gas flow at low revs.

At higher revs, the system engages the second intake and exhaust valve, converting the engine to four-valve-per-cylinder mode. As each valve has its own cam lobe, these high-rev-only valves can use a different cam profile, increasing overlap and lift for more high-speed performance.

On the downside, many have complained that the switch-over is too noticeable and that the system adds to servicing costs. Honda seems to be equally unconvinced, since it’s never made a big effort to spread the system across its range despite the on-paper advantages that it offers.

Suzuki GSX-RR
What does the future hold?

For the last 12 months there’s been no escaping talk of the new Euro4 emissions standards and the difficulties manufacturers face in meeting them. Now they’re in force, but there’s little breathing space for bike firms as the next-generation limits are just around the corner.

Euro5 limits, which will be introduced in two steps in 2020 and 2021, are harder still to meet. For the first time it’s likely that variable valve timing will become an option that firms have to consider to keep performance where we expect it while reducing emissions further still.

It’s likely that many bikes will take on the sort of cam-phasing set-ups used by Ducati and Kawasaki. It’s proven technology that’s relatively easy to add. However, it has limitations when it comes to extreme revs, so might not be ideal for four-cylinder superbikes.

Suzuki’s all-mechanical set-up looks promising, with a race-proven background and the scope to be further modified. The firm has already patented versions with electronic controls to alter the change-over behaviour, plus versions that work on the exhaust camshaft as well as the intake. Intriguingly, one set of patents shows the system fitted to a V-twin engine, perhaps hinting at a future production model.

But at the moment every production VVT bike has just scratched the surface of what’s possible. Other ideas are out there that take the technology much further.

On the motorcycle side, one is Suzuki’s 3d cam, which again shows an elegantly simple solution. It’s based on the idea of having elongated cam lobes that vary in profile from one end to the other. At one end, they’re nearly round, with minimal valve lift and little or no overlap, but at the other end the same lobe can have a race-style profile with aggressive lift and lots of overlap. The transition from one to another is achieved by sliding the cam lobes along the camshaft. Suzuki has shown more than one concept engine with this idea, which it’s been developing for more than a decade.

In its ultimate guise, the 3d cam lobe has the scope to eliminate the need for a separate throttle. Instead, the valve lift alone can be used to regulate engine speed. This offers potential improvements in performance, emissions and economy.

It’s not a new idea, and in the car world it’s been possible to buy throttle-less engines for years. BMW is a leading exponent, with its Valvetronic set-up. This uses a system that alters the fulcrum of the rockers, so that while the cam profiles are fixed the effect they have on valve lift can be altered. Used along with hydraulic valve phasers on both camshafts to alter their timing, the result is a fully variable valve system that can be used with no conventional throttle.

Fiat VVT system

More comprehensive still is Fiat’s MultiAir engine (pictured). Its intake valves are computer-controlled, with a normal camshaft working on a hydraulic system that transfers its movement to the valve. By releasing the hydraulic pressure at specific times, the intake valves can be made to open later, to close earlier and even to open and close twice in quick succession. Lift is also controlled by releasing pressure, eliminating the need for a throttle. It’s been in production since 2010, so the technology is already proven.

The downside to all these systems is that they’re expensive compared to conventional, fixed valve timing and lift, as well as being bulkier and having more moving parts.

Perhaps the ultimate in valve timing and lift comes with the idea of removing the camshaft from the equation entirely. Several firms have created prototype engines that rely purely on electronic, pneumatic or hydraulic means to open and close valves, with computers governing their timing, duration and lift. Lotus, BMW, Renault and Ford are among the companies that have prototyped versions of the camless idea. But at the moment tiny supercar firm Koenigsegg, via its subsidiary FreeValve, appears to have the lead on the tech. It has developed several prototype engines and is working with Chinese car company Qoros to bring the system to production.

In their ultimate evolution, an engine with completely variable, camless valve control, with a computer deciding the lift and timing of each valve independently, could offer massive improvements in performance and economy. It’s even possible for such engines to seamlessly switch between four-stroke and two-stroke operation, which sounds like a tempting prospect. Of course, there’s no throttle on these, either.

The lack of camshafts also means there’s no need for cam chains, belts or gears, the cylinder heads can be smaller and lighter and the whole engine will have fewer moving components.

It’s quite possible that the future will see camless engines going head-to-head with ever-improving electric drivetrains, and there’s no reason that both shouldn’t be successful. Either way, the chances are that they’ll be better that anything we can dream of today.

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