Invariably measured in crankshaft degrees, the duration of any camshaft is generally accepted to be the number of degrees the valve is lifted off its seat to the instant that it shuts again. Ideally it should open instantaneously to its maximum lift and then remain open until it would be closed again equally quickly. This would produce a square shaped valve motion diagram and cause the maximum amount of charge air or exhaust gas to flow. In real life however, the opening and closing are constrained by the laws of physics and valves have to be carefully lifted off their seat, accelerated to maximum velocity before being slowed down again to zero speed when fully open. To close the valve again, all this has to be repeated in reverse with the forces involved having to be well within the physical constraints of the design and the materials used.
Thus in the traditional suck-squeeze-bang-blow cycle, the intake valve has to start moving long before top dead centre and will not be fully closed again until well after bottom dead centre. Likewise the exhaust will open well before bottom dead centre and be fully closed again after top dead centre. Often expressed as something like Inlet - 36/64 and Exhaust - 76/24, the numbers refer to the timing in degrees in the case of the inlet valve as - opening 36 degrees before top-dead-centre and closing 64 degrees after bottom-dead-centre. For the exhaust valve in this example the timing would be opening 76 degrees before bottom-dead-centre and closing 24 degrees after top-dead-centre. The duration of the intake valve opening therefore will be 36+180+64 = 280 degrees as indeed is that of the exhaust (76+180+24).
But many people think that the longer the duration, the more powerful the engine. While undoubtedly in many instances this might be the case, it doesn't necessarily always follow. It all depends on the timing of these events relative to the piston position. And the most important of these is the closing of the intake valve.
Having reached full lift when the piston is something like just over half way down the bore, the intake valve will begin to close again. At the bottom-dead-centre when the piston has come to a temporary halt, the valve will still be almost half open (see diagram). At this point when the piston is starting to move back up one would expect the charge to start flowing back through the open intake valve. However, near to bottom dead centre the piston is moving very slowly and the inertia of the incoming charge generated from the pressure pulsations in the intake port will continue to fill the cylinder. When the pulse inertia of the incoming charge is brought to a halt by the velocity of the piston opposing it and preferably when there is also a positive pressure wave just coming through, the valve should be closed trapping all the incoming charge. The higher the engine speed, in general the greater this pulsation effect. Early intake valve closing will therefore make the engine run better at low speed and conversely the later the closing, the better the engine will run at high speed - but like anything in the engine world, only up to a point!
Finding this optimum position is what engine 'tuning' is all about and while computational methods can be a great help, the final solution is best left to the engine dynamometer.
Fig. 1 - Typical intake and exhaust valve motion with intake valve almost half open at Bottom-Dead-Centre (BDC).
Written by John Coxon