Valves and Valve Mechanisms

If you've got this far down the page, hopefully you understand that the valves are what let the fuel-air mixture into the cylinder, and let the exhaust out. Seems simple enough, but there are some interesting differences in the various types of valve mechanism.

Spring-return valves.

Spring return valves are about the most commonly-used and most basic type of valvetrain in engines today. Their operation is simplicity itself and there are only really three variations of the same style. The basic premise here is that the spinning camshaft operates the valves by pushing them open, and valve return springs force them closed. The cam lobes either operate directly on the top of the valve itself, or in some cases, on a rocker arm which pivots and pushes on the top of the valve. The three variations of this type of valve-train are based on the combination of rocker arms (or not) and the position of the camshaft.

The most basic type has the camshaft at the top of the engine with the cam lobes operating directly on the tops of the valves.

The second more complex type still has the camshaft at the top of the engine, but the cam lobes operate rocker arms, which in turn pivot and operate on the tops of the valves. With some of these designs, the rocker arm is pivoted in the middle (as shown below) and with other designed, it's pivoted at one end and the cam lobe operates on it at the midpoint. Think of a fat bloke bouncing in the middle of a diving board whilst the tip of the board hits a swimmer on the head and you'll get the general idea.

The third type which you'll find in some motorcycle engines and many boxer engines are pushrod-activated valves. The camshaft is actually directly geared off the crank at the bottom of the engine and the cam lobes push on pushrods which run up the sides of the engine. The top of the pushrod then pushes on a rocker arm, which finally pivots and operates on the top of the valve. The image here shows the three derivatives in their most basic form so you can see the differences between them. Note that the pushrod type shows the camshaft in the wrong place simply for the purpose of getting it into the image. In reality the camshaft in this system is right at the bottom of the engine near the crank. The rocker arms shown here are also called fingers, or followers depending on who you talk to.

Tappet Valves

Tappet valves aren't really a unique type of valve but a derivative of spring-return valves. For the most part, the direct spring return valve described above wouldn't act directly on the top of the valve itself, but rather on an oil-filled tappet. The tappet is basically an upside-down bucket that covers the top of the valve stem and contains the spring. It's normally filled with oil through a small hole when the engine is pressurised. The purpose of tappets is two-fold. The oil in them helps quiet down the valvetrain noise, and the top of the tappet gives a more uniform surface for the cam lobe to work on. From a maintenance point of view, tappets are the items which wear and are a lot easier to swap out than entire valve assemblies. The image on the left shows a simple tappet valve assembly. I've rendered the tappet slightly transparent so you can see the return spring inside.

Desmodromic Valves

Desmodromic valve systems are unique to Ducati motorbikes. From the Ducati website: The word 'desmodromic' is derived from two Greek roots, desmos (controlled, linked) and dromos (course, track). It refers to the exclusive valve control system used in Ducati engines: both valve movements (opening and closing) are 'operated." Classy, but what does it mean. Well in both the above systems, the closure mechanism on the valve relies on mechanical springs or hydraulics. There's nothing to actually force the valve to close. With the Ducati Desmodromic system, the camshaft has two lobes per valve, and the only spring is there to take up the slack in the closing system. That's right; Ducati valves are forced closed by the camshaft. The marketing people will tell you it's one of the reasons Ducati motorbike engines have been able to rev much higher than their Japanese counterparts. The idea is that with springs especially, once you get to a certain speed, you're bound by the metallurgy of the spring - it can no longer expand to full length in the time between cylinder strokes and so you get 'valve float' where the valve never truly closes. With Desmodromic valves, that never happens because a second closing rocker arm hooks under the top of the valve stem and jams it upwards to force the valve closed. In fact, the stroke length, rods, and pistons all play their part in valve timing and maximum engine speed - it's not just the springs and valve float. This is why F1 cars use such a small stroke and pneumatic valves springs. In truth, both systems, spring or Desmodromic only work well up to a limit. Newer Japanese bikes have engines that can rev to the same limit as a Ducati just using spring-return valves.

You can see the basic layout of a desmodromic valve on the right. As the cam spins, the opening lobe hits the upper rocker arm which pivots and pushes the valve down and open. As the cam continues to spin, the closing lobe hits the lower rocker arm which pivots and hooks the valve back up, closing it. The red return spring is merely there to hold the valve closed for the next cycle and doesn't provide any springing force to the closing mechanism. This is a fairly simple layout for the purposes of illustration. The real engines have Desmo-due and Desmo-quattro valve systems in them where pairs of valves are opened and closed together via the same mechanism.

Quattrovalvole, 16v and the other monikers you'll find on the back of a car.

In the 80's, the buzzword was 16-valve. If you had a 16-valve engine you were happening. You were the dogs bollocks, the cat's meouw. In Italy, your engine was a quattrovalvole. So what the heck does all this mean? Well it's really, really simple. "Traditional" 4-cylinder in-line engines have two valves per cylinder - one intake and one exhaust. In a 16V engine, you have four per cylinder - two intake and two exhaust. (4 valves) x (4 cylinders) = 16 valves, or 16V. It follows that a 20V engine has 20 valves - 5 per cylinder. Normally three intake and two exhaust. Unless you've got a 5-cylinder Audi or Volvo in which case you've still got 4 valves per cylinder. If you're in America, the thing to have now is 32V - a 32 valve engine. Basically it's a V-8 with 4 valves per cylinder. See - it's all just basic maths.

And what do all these extra valves get you apart from a lot more damage if they ever go wrong? A better breathing engine. More fuel-air mix in, quicker exhaust. When you get further down the page (and if your wife / husband hasn't come and complained to you about spending so damn long reading this stuff so late at night), you'll find some more information on why this is A Good Thing.