Fig. 32.—Sketch showing Slot
in a Poppet Valve Head for
Grinding-in purposes.
Fig. 33.—Inlet (A) and Exhaust (B) Valve Cams
for a Slow-running Petrol Engine.
The Camshafts and Eccentric Shafts.—These are usually made from the same material as the crankshaft and machined from the solid bar, the projecting cams or eccentrics being afterwards cut to the correct shape. In the case of a camshaft it is very important that the shape of the cams should be such that they lift the valves quickly off their seats to the full extent of their opening (or lift), keep them open for as long a period as desirable, and then allow them to close quickly but without shock. Cams which have straight sides are more in favour than those with curved sides, but if the action of the cams is to be theoretically correct the side of the cam should be curved in such a manner that the valve is lifted at first with a uniformly increasing speed and afterwards with a uniformly decreasing speed, so that it will be at rest in its top position. If this is not done the valve tappet may jump a little above the cam each time the valve is lifted. In Fig. [33] the cam A is intended for the inlet valve and the cam B for the exhaust valve, the essential difference being that the exhaust valve must be kept open longer than the inlet valve, and therefore the exhaust valve cam is the wider of the two. The timing of the inlet and exhaust valves of an up-to-date engine may be explained by considering the crankpin circle as divided into 360 parts or degrees. If there were no lag or lead in the opening of the valves, then they would open when the crank was on its dead-centre and close when the crank was on its dead-centre. The inlet valve would open when the crank was on its top dead-centre and close when it had reached its bottom dead-centre, this representing the suction stroke of the engine. Then would follow compression and explosion, giving two strokes or one revolution before the exhaust valve commenced to open. The exhaust valve would then open when the crank was on its bottom dead-centre and close when the crank reached its top dead-centre corresponding to the completion of the exhaust stroke. It is very important that the pressure of the gases above the piston when it commences to move upwards on the exhaust stroke should be as low as possible, and this can only be secured by opening the exhaust valve towards the end of the explosion or power stroke, thus allowing the bulk of the gases to escape and leaving the piston with little resistance to encounter on its upward exhaust stroke. Therefore we give the exhaust valve a lead of about 30 degrees, which means that it begins to open when the engine crank is 30 degrees from the bottom dead-centre on the downward explosion stroke, and we give it a lag of about 5 degrees in closing. This means that we keep the exhaust valve open until the crank has moved 5 degrees over the top centre, so that we may fully utilize the momentum of the gases to clear out the cylinder or scavenge it. As the piston moves rapidly up the cylinder on the exhaust stroke it pushes the gases in front of it out through the exhaust opening, but when it gets to the top of its stroke the piston stops and then comes down again for the suction stroke, whereas the gases will tend to keep on moving if they are not unduly restricted in their passage through the exhaust system, so that we can generally obtain some slight advantage by giving the exhaust valve a small amount of lag in closing.
The pressure of the gases in the cylinder after the exhaust valve closes will nearly always be a little above atmospheric pressure, and therefore nothing is gained by opening the inlet valve immediately the exhaust closes—we generally allow an interval of 5 degrees, which means that the total lag of the inlet valve is 10 degrees in opening, or the inlet valve does not begin to open until the crank has moved 10 degrees off its top dead-centre on the downward suction stroke. At the end of the suction stroke the piston will again come to rest before moving up on the compression stroke, but the gases will continue to rush into the cylinder from the carburettor owing to their momentum if we leave the inlet valve open a little longer, hence we generally give it a lag of 20 degrees in closing, which means that the inlet valve does not close until the crank has moved 20 degrees up from the bottom dead-centre on the compression stroke.
