An internal combustion engine functions by inducting a mixture of air and petrol (or diesel) into a cylinder, sealing it then compressing it before igniting it with a spark. The resulting explosion, really a very fast burn, pushes the piston down which provides the power to move the car. In its simplest form, there would be one inlet valve and one exhaust valve.
Effectively, the more air and petrol you put in to the cylinder the bigger the bang and the faster the car goes.
The way you do this is by putting as much as you can through the inlet valve. Once you get to the stage where you can get no more in because of the diameter of the valve you have to put in a bigger one; then a bigger one and so on until you can't get a bigger valve into the cylinder head because of the diameter of the cylinder. Not forgetting of course, that if you put a lot of air and petrol in, that's a lot of exhaust gas which needs to be evacuated, so, a bigger exhaust valve is the order of the day. When you get to the stage where you can't get two bigger valves in, you simply put four smaller ones in.
You could even get five even smaller ones (Audi, Yamaha) in as four or five smaller valves take up less space than two big'uns.
'fraid that went on a bit, as do most of my replies. Hope it may have answered a few questions. If not, well I had nothing else to do.
