Very early in the development of the four-stroke engine it was understood that if the valves were to only start to open as a stroke began and to be fully closed as a stroke ended, engine performance would suffer. That’s because it takes time to open and then to close a valve, and there wouldn’t be enough time at the fully open position to adequately fill or empty the cylinder during the stroke.
The obvious solution would be to begin the opening before the start of the stroke and to complete the closing after the end of the stroke. But this created a potential problem at one specific, unique time during the four strokes: the end of the exhaust stroke, which is then the beginning of the intake stroke. With the intake valve opening early and the exhaust valve closing late, there would be a time when both valves were partially open simultaneously. This situation is called valve overlap, and its length is measured in degrees of crankshaft rotation.
With both valves open, exhaust gases can contaminate the clean intake charge, and some fuel/air intake mixture can exit into the exhaust. Various strategies can minimize these problems, but the situation is complex, as it depends very much on engine rpm. In general, a relatively small amount of valve overlap works well at lower revs, while more valve overlap is best for high revs.
With an engine with fixed valve timing, as we see in most motorcycles, designers and tuners must therefore compromise on valve overlap, choosing a setting that gives preferred advantages at certain revs while sacrificing performance at other rpm. In the motorcycle world, high-rpm performance is often optimized while at least to some extent sacrificing low- to mid-rpm rideability, power, fuel economy, and ideal emissions.
If we could change valve timing, and thus also valve overlap, as the engine gains and loses revs, we wouldn’t have to compromise so much. Serious ideas and patents for variable valve timing began to appear in the 1920s. Some hardware showed up on aircraft shortly thereafter, but it wasn’t until 1980 that a production-ready VVT (variable valve timing) system was introduced on an Alfa Romeo automobile.
Motorcycle manufacturers began experimenting with VVT shortly thereafter, but the technology made only limited appearances due to expense and packaging issues. The intensely competitive Japanese market for 400cc sportbikes in the 1990s saw both Suzuki and Honda offering VVT (’91 Suzuki GSF400V, ’99 Honda CB400SF). The CB used Honda’s VTEC system and the Suzuki a similar design. The goal here was to improve low- to midrange power on these peaky high-revving fours.
Honda used the VTEC system again on the VFR800 beginning in 2002, and Kawasaki used a cam-phasing system on the intake cam on the Concours 14 from 2007 on. Ducati has recently introduced cam-phasing VVT (called Desmodromic Variable Timing or DVT) on the Multistrada and XDiavel models. Yamaha’s used a system much like VTEC, but using solenoid operation, on its current NMAX 155 scooter (not a US model). For 2017 Suzuki will implement a centrifugal VVT system on the GSXR 1000’s intake cam.
So how do these mechanisms work? What do they do to alter valve timing? We’ll take Honda’s VTEC first, as it was the first motorcycle VVT system for worldwide sale. VTEC stands for Variable Valve Timing and lift Electronic Control. There are many VTEC variations, but the one on the VFR800 motorcycle is the one we’ll look at. This engine has four valves per cylinder, so there are two intake valves. There are two rockers and two cam lobes for those intake valves. At lower engine speeds (up to about 6,500 rpm) one cam lobe operates one rocker and one valve. The second valve remains closed. At a pre-determined engine speed an electronically controlled oil valve opens, and oil pressure slides a pin sideways, locking the two rocker arms together. The second rocker is now depressed by the second cam lobe, and since the rockers are now locked together, both valves are now operated by the second cam lobe. The second cam lobe has different timing, lift, and duration from the first lobe. VTEC isn’t a progressive system—the change is discrete, so it’s all-or-nothing, leading to a noticeable and sudden difference in performance.
The Kawasaki Concours mechanism is a phasing system, which means that the cam (in this case the intake cam) is rotated or phased relative to the crankshaft. This is a common automotive-type system (from Mitsubishi in this case) in which the cam-drive sprocket is divided into two parts, outer and inner. The sprocket itself (the outer half) doesn’t change rotational position relative to the crankshaft, while the inner part is fixed to the camshaft. One part has radial vanes while the other has corresponding openings much wider than the vanes. At prescribed engine rpm, engine oil is routed into the openings by electronically controlled valves. Depending on the side of the vanes that gets the oil, the sprocket and the cam are rotated clockwise or counterclockwise relative to one another, advancing or retarding the valve timing. The alteration is progressive, not discrete, which gives the advantage of continuous alteration as rpm rise and fall. Kawasaki’s goal here was to strengthen midrange torque, and it worked.
The Ducati DVT system is very similar to the Kawasaki’s, with the major difference being that the Ducati has phasing hardware for the exhaust cam as well. Phasing on both cams gives twice the total range of variable timing, and these engines, which were previously tuned for midrange torque, have gained both significant extra top-end power and low speed smoothness with the DVT double-cam system.
Why, then, have most examples of motorcycle VVT been for the intake cam only? Two reasons: First is cost. Two cam applications cost about twice as much to produce. Second is cost effectiveness. If you can only manipulate one cam, VVT for the intake cam gives better results in terms of power because it has a predominant effect on how much intake charge fills the cylinder. But VVT on both intake and exhaust clearly is to be preferred if the cost increase can be managed.
Suzuki’s soon-to-be-released centrifugal system, called SR-VVT, has an interesting background. It’s also a phasing system, but MotoGP racing has banned hydraulically and electrically driven VVT systems. To be able to use VVT in racing, Suzuki came up with a two-part sprocket similar in some ways to the Kawasaki/Ducati types. But instead of being rotated by engine-oil pressure there is a series of steel balls in curved grooves in the outer section, ramped straight grooves in the inner. As the assembly gains rpm, the balls move outward against spring pressure, rotating the two parts relative to one another.
So this is where motorcycle VVT stands now. Where is it going? Automotive systems like BMW’s Valvetronic and Fiat’s Multiair, both of which vary valve timing and lift to such an extent that engine speed is controlled by the valves themselves rather than by a throttle plate, show that there are exciting possibilities waiting to be explored. Fully cam-less engines with valves operated electromagnetically, hydraulically, or pneumatically are under development but not production ready.
Hardware that opens and closes valves at the precise times and distances necessary to provide optimum torque and power, best fuel consumption, and minimum emissions at every engine speed is not a distant dream. It’s within sight. Will it be impossibly expensive? Not if the leading-edge automotive systems are any indication. We are already seeing the end of some of the compromises we’ve lived with since the dawn of the internal combustion engine.