Timing of the valves in an engine is controlled by a number of methods, in a four stroke engine it is done by the following:
- Chain and sprocket from the crankshaft to the cam shaft, typically the crankshaft has to rotate twice to rotate the camshaft once
- Pushrods from gearing on the crankshaft directly to the valve rocker arms (gives lower RPM than chain and sprocket)
In a two stroke engine, the valve timing is controlled by the position of the piston. Concentrating on the four stroke engine, the camlobe profile will reflect how the valve is timed. The shape of the camlobe, however, does not effect the RPM of an engine.
The diagram above shows the profile of two different cams. If the cam height is increased then the valve will open deeper into the engine, if the width is increased, the time the valve is left open will increase. Both of which will affect the following settings…
Amount of degrees from when the valve begins to open to when it is fully open, happens before Bottom Dead Centre or Top Dead Centre.
Amount of degrees from when the valve is fully open to when it begins to close. Occurs after Bottom Dead Centre or Top Dead Centre.
When the valve rebounds on the valve seat in the engine head and reopens slightly after initially closing, causing a small loss in compression
The valve stays open and doesn’t return fast enough – typically the cam shaft/ cam lobe is moving faster than the valve spring/ desmondronic valve can close the valve
The total amount of degrees that both intake and exhaust valves are open – this happens only in the transition between the exhaust and intake stroke. Typical values of valve overlap for a road bike would be around 50-60° where as racing engines will have much higher overlaps at around 100-120°.
Valve lead, lag and overlap are calculated during the engine/ camshaft design where as valve bounce and float are created as a by product of poor materials used in construction, a bad engine design, damage to the camshaft/camlobes or engine head.
Calculating valve overlap and open periods
The below diagram shows the position of the camshaft and where valve lead, lag and overlap occur in the cycle, remember that the crankshaft rotates 720° to produce this full 360° movement in the camshaft, so only the first TDC and last BDC of the crankshaft stroke are included here.
On the above diagram:
- The valve overlap is 60°: E.V.C° (30) + I.V.O° (30). Remember overlap is only measured at T.D.C
- The inlet valve open period is 265°: I.V.O° (30) + I.V.C° (55) +180°
- The exhaust valve open period is 265° E.V.O° (30) + E.V.C° (55) +180°