VIDEO: TECH UPDATE: Why Variable Valve Timing?

In this short video above (which I hope you’ve watched by now), Ducati shows how it uses a pair of small-angle rotary actuators to vary cam phase (the angular relationship between cam and crankshaft) as a means of solving some important problems of long standing.

In 2003, I asked Ducati’s Claudio Domenicali if variable valve timing (VVT) schemes had any interest for the company. His reply: For racing engines, the powerband and gearbox are well enough adapted to each other that VVT was unnecessary.

But now Ducati is having some very sporting engines carry out other missions with broader requirements, such as in the Diavel cruiser. In this application, the engine is called upon to pull from much lower revs than a sports or racing powerplant. While a sporting rider or racer has little concern with ide quality, modern big bore/short-stroke engines can have special problems there.

Here is how it works. As the engine performs the four functions—intake, compression, power, exhaust—satisfactory power cannot be obtained by opening the intake at Top Dead Center (TDC) and closing it at Bottom Dead Center (BDC). The same is true of the exhaust valve. So, to have valves open when they need to be, they must begin to open early and close late (valves have inertia, and take time to start and stop moving). One result is a period called overlap, which occurs around TDC at the end of the exhaust up-stroke, when the exhaust valves are in process of closing and the intakes are beginning to lift.

Overlap is important because it is the window through which acoustic waves moving in the exhaust pipe can influence gas exchange in the cylinder. Some engineers have called overlap “the 5th cycle” because of its large importance in engine performance. A high-performance exhaust pipe is tuned to send back a negative (suction) wave to arrive during overlap. This wave draws out exhaust gas remaining in the clearance space above the piston, then enters the intake tract where it causes fresh charge to begin entering the cylinder, even before the piston has begun its intake down stroke. This boosts torque because it increases charge purity and starts the intake process early.

It’s not all gravy. Negative and positive waves alternate in the exhaust pipe, just like sloshing water in a bathtub. So, at some lower rpm, it is a positive wave that arrives during overlap. It stuffs more exhaust gas back into the cylinder, then back through the intake valves and ports, into the airbox. Thus, when the intake stroke begins, all the piston can draw in at first is inert exhaust gas. As a result, torque falls; this is the dreaded flat spot.

Here’s the rub; you can diminish the flat spot by making the overlap period shorter, but doing that also keeps out the beneficial negative wave that boosts torque at higher revs.

High-performance two-valve-per-cylinder race engines have as much as 80 or even 100 degrees of overlap. But the Diavel, to quell the flat spot and broaden power, has only 11 degrees of overlap. Having that little overlap is part of the reason for the Diavel having less power than sportier versions of the same engine. Dirt-trackers and other practical persons have for years varied overlap to tailor power to purpose; more overlap boosts peak power but kills midrange by deepening the flat spot. Less overlap keeps the rider from “falling into the flat spot,” so power becomes wider—but its peak value falls.

This is the major purpose of VVT, to have the best of both worlds. When the engine is revving above the flat-spot rpm, the rotary actuators on the cams of Ducati’s new Testastretta DVT (Desmo Variable Timing) engine open up overlap to let in that beneficial negative pipe wave, boosting torque. As engine revs approach the flat-spot zone, the actuators close down overlap to keep out the torque-sapping positive pipe wave.

Now apply all this to idle quality. With pipe waves bouncing back and forth, alternately sucking out and blowing in exhaust product, overlap can be the key to a stable idle and quick off-the-bottom throttle response. Close down the overlap to keep out wave action, and all the piston can suck into the cylinder at idle will be fresh mixture that ignites easily and reliably.

In highly oversquare (large bore, short stroke) engines, it's easy for the small volume of idle mixture to get lost on all that vast piston acreage, so give it a break by keeping out exhaust gas backflow by closing down the overlap window. After all, dilution by exhaust product is what gives two-strokes their irregular idle; the piston takes in a sip of fresh mixture, but it just happens to be exhaust gas in the plug gap when the spark fires, so you get a misfire. The engine rotates another time and—same result—no ignition. Then, the third or fourth time—Pow!—there's finally enough mixture to fire.

This can occur in a cyclic way, making the engine eight-stroke, which is probably what’s happening in the dreaded “Ducati shudder” mentioned by my colleague Bruno dePrato in his good story on Ducati’s new DVT engine. Naturally, VVT can banish such small irritations.