Air is surprisingly heavy stuff. The air under an average coffee
table weighs about 1kg, which means when we push large
quantities of it through the small pipes of an engine at high
speed it has a significant amount of momentum. Engines breathe
in huge quantities of air, each 100bhp consumes about 35 pints
of air per second and expels about three times that volume in
exhaust gas. All this flow needs to be carefully controlled, the
flow into the cylinders is not constant but stops and starts
many times a second with each cycle, inertia has to be allowed
for so that enough time is available for the gas to speed up or
slow down. This is all achieved with the camshaft, a long steel
rod with lumps on that pushes the valves up and down. It's spun
round in synchronisation with the crank and pistons so that the
lumps, called lobes, hit the top of the valve stems via the
follower or other links and force them open at just the right
moment. The valve is gradually opened and shut to avoid
excessive forces which would in