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.
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.
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.
