MotoGP Blog

Pneumatic Valves

December 28th, 2007

Pneumatic valves were first used in modern MotoGP when the Aprilia Cube arrived on the scene in 2002 . Developed by Cosworth from their experiences with Formula 1 technology, it was a beast of a bike. Three years later in 2005, the KTM engines used by Team Roberts used pneumatic technology. Suzuki were next to go pneumatic with their 2006 bike, in a search for more power from their (relatively) weak engine in the last year of the 990cc era, and also to help with the development of their 2007 800cc bike. Kawasaki now also use the technology, and Honda have announced their 2008 bike will also have pneumatic valve springs.

After a slow start pneumatics are now the valve springs of choice in the MotoGP paddock, with only Ducati differing with their use of their desmodronic system. Why have pneumatics suddenly replaced steel springs? Before we answer this, let’s discuss how conventional steel springs are used and some of the problems with them.

(For an overview of how four stroke engines work and some further background on internal combustion engines, see this article on howstuffworks.com. The article discusses car engines with valve rocker arms, but the principles are the same for motorcycles.)

Conventionally sprung valves

four-stroke-engine As the crankshaft rotates, the stem of the valve is forced downwards by the lobe of the cam, opening the valve and compressing the spring. As the cam rotates further and the peak of the lobe passes the valve stem, the spring begins to extend and close the valve. At all times the stem of the valve is kept in contact with the cam by the valve spring. Image from Wikipedia: Four stroke cycle engine valves

Problems with conventionally sprung valves

Problems with conventional springs fall into two categories:

The qualities of the spring.
Valve float.

The qualities of the spring

In order to maintain control over the valve, the spring needs to be very stiff and durable. Not too stiff, or they will sap power from the engine unnecessarily, and not too soft or the valve will not be properly controlled. The qualities of the springs across all of the valves of the engine also need to be very similar to ensure the engine runs smoothly.

Valve float

The limits of conventionally sprung valves are at reached at around 18,000 rpm. At this speed, each valve opens and closes 150 times a second, or every 0.006 seconds. At these speeds a phenomenon known as valve float starts to happen, when the spring cannot return the valve to the closed position quickly enough so the valve does not close before the cam is trying to open it again. This results in at best a serious loss of power and at worst a ruined engine as the valves contact the pistons.

Pneumatic valves

Pneumatic valves are a straight replacement for the steel spring. In place of the spring is a sealed chamber containing pressurised gas. As the cam lobe presses down the valve, the gas is compressed. As the lobe passes the top of the valve stem, the pressure in the chamber forces the valve back into its closed position. These system run at between 12 - 16 bar pressure, usually use nitrogen gas as it is inert, and provide a naturally progressive spring. A progressive spring is one where the more compressed a spring is the harder it is to compress it further. The qualities of the pneumatic spring are such that much higher engine speeds are possible, well in excess of 18,000 rpm and the behaviour of the valves at these high revs are much more predictable.

Problems with pneumatic valves

A pneumatic system faces some different problems than a conventionally sprung system:

Gas leakage.
Additional weight.
Increased complexity.

Gas leakage.

As with any pressurised system, leakage is an issue. If the pressure within the system drops, the engine will fail. This necessitates the carrying of a supply of gas with the engine, and a system to monitor and feed gas into the system as required.

Additional weight

The gas bottle, the additional sensors and electronics to monitor and control the pressure all add to the weight of the system. In motorsport, weight is the enemy of performance and so this additional weight is a disadvantage to the solution.

Increased complexity

The system is much more complex than the alternative. Not only has the system to maintain gas pressure but also to spot if it will run out of gas and shut down the engine before there is a catastrophic failure. This additional complexity means it is harder to get right and more expensive to develop and maintain. The experiences of Suzuki and Aprillia in developing their systems bear this out.

Why are pneumatic valves suddenly so popular?

The additional complexity of the pneumatic valve solution indicates why it has been slow to catch on in the paddock. It is hard to get right, and up until recently there were many other ways to get more power from your engine that were cheaper and easier to develop. Springs were fine for 990cc bikes as there was plentiful power on tap, more than could easily be used by the riders or the tyres. With the advent of the 800cc bikes the power is less easy to find. The smaller capacity means that to get more power the engines have to rev faster, revving faster has led to problems controlling the valves and so to a different spring solution. The popularity of pneumatics in the paddock now is another aspect of the constant search for more power and speed.

References