Alternative Valve Operating Systems - Poppet Science

For the most part, engineering is a pretty conservative enterprise. Whether it's building computer chips, gas turbines or weather satellites, the guys and gals designing them tend to go for tried and tested methods. If you have a design that works well, is reliable, and cost-effective, then why look elsewhere?

That's why so many bikes have aluminum frames, inline-four engines and cast aluminum wheels. It's why those engines use forged steel conrods and plain crankshaft bearings. They work well, don't cost the bank, and let the designers work on developing other areas like traction control, better fuel injection and improved suspension performance.

The valves inside bike engines generally fall into this category too. Since the very earliest engines built at the start of the 20th century, four-stroke bike motors have used poppet-type valves that are opened by a rotating camshaft and closed by springs. Sure, we now have four of them per cylinder, and they're operated at sky-high revs, but if you showed the valve gear design on a modern sportbike to an engine designer from 1909, he'd instantly recognize what was going on.

Standard valve setups work well enough to give power outputs of 180 hp per liter from the aforementioned sportbike. They have massive service intervals--you only need to check the clearances on Yamaha's R1 every 26,000 miles, which is incredible considering how many times they operate over that period. Why would you want to try anything else?

But several firms have done exactly that, developing their own slightly different take on engine valve systems. And as designers look to get even more performance from engines while keeping emissions low and efficiency high, we can expect to see even more designs in the future.

Desmodromic
The first special valve setup is also, perhaps, the most successful. Since the 1960s, every Ducati V-twin engine made has used desmodromic valve operation--a clever system that uses a camshaft to pull the inlet and exhaust valves closed as well as push them open. Ducati engines have unusually shaped camshafts, and two rocker arms on each valve. As the cam turns, it presses on the opening rocker, pushing the valve off its sealing seat, and allowing gas to flow (either air/fuel mix into the engine, or exhaust gasses out). Then, as the cam continues to turn, the valve gets to its fully-open position and is held there for a short period of time. By now, the cam is starting to push on the closing rocker arm, which begins to pull the valve back onto its seat. It's a simple idea, and while the engineering in the desmo system is more complex to manufacture (and thus more expensive), it's proved to be a very reliable, high performance setup.

The advantages are several over a conventional valve setup. The lack of powerful closing springs means reduced friction and wear in the engine. And since the camshaft positively closes the valve rather than relying on a spring to do so, very high engine revs are possible without risking valve `float.' Float occurs when the spring can't pull the valve back onto its seat quickly enough, and it can result in the valve hitting the piston, causing serious engine damage. Desmo valves can also use more radical valve timing, because the positive closing system gives more control over the speed the valve closes at.

Ducati has a lot of patents over desmodromic valves (although it didn't invent them--they first appeared on a Mercedes race car in the 1950s) so it's not an easy option for other firms to use. In addition, the marketing implications of apparently `copying' Ducati's technology would surely put off proud engineers at companies like Honda. Finally, Japanese firms simply haven't needed to use desmodromics-their valve spring technology and cylinder head design has allowed them to produce incredible performance with simple, basic valve design.