QJB:
The goal is to get maximum amount (charge) of air/fuel mixture into a given combustion chamber. That is, basically, volumetric efficiency.
Therefore you compress the air to get more in. You also need more fuel to maintain the correct air/fuel ratio (roughly 15:1), so the fuel metering system has to do its bit too.
In a normally aspirated piston engine you merely suck the air in, hence inferior VE. Obviously you still compress the charge once it's in which leads nicely onto...
...the compression ratio in a piston engine: the goal there is to achieve a nice tight charge, evenly mixed at the desired air/fuel ratio where all the fuel molecules are close to the air molecules they've waited so many million years to react with. Hence the use of compression. This helps the so-called thermal efficiency (ie engine efficiency) right up to the point where you go too far with the compression and run into the phenomenon of detonation. But that's another topic.
Last edited by oggers; 11th Nov 2011 at 10:54.