While the inlet air is in the intake port, yes it is heated by the cylinder head.  Not much you can do about this.  while going past the intake valve it is heated more and will still be heated during the early portion of the compression stroke.  Later in the stroke the heating from compression takes over and then the charge will actually be rejecting heat to the cylinder surface, cooling the compressed charge.  The rule of thumb for air is that for adiabatic compression (no heat transfer) the pressure and volume will follow the equation P*V^K=const.  K is very close to 1.4.  Heating with some heat rejection will lower the effective K and it could be as low as 1.35 for the case of an engine.  Someone mentioned "efficiency", but compression in a piston/cylinder if very efficient and the "inefficiency" that is present doesn't add heat to the air.  The air at the end of the compression stroke will actually be a little lower than predicted by an efficient adiabatic calculation.  Regardless, it will be very hot and as I recall a temperature of 1100F or maybe higher is typical.  The temperature, combined with the pressure, is certainly ABOVE the autoignition temperature of the fuel.  So why doesn't it spontaneously ignite?  It takes time for the reaction to occur and at a reasonable engine speed the flame front has already arrived and the piston is on its way down before the fuel has a chance to autoignite.  Of course, while the flame front is arriving terrible things are happening - the pressure and temperature are rapidly increasing much higher values and there is significant radiation energy arriving from the flame front.  It is a race against time to get the fuel lit before it lights itself.  Temperature and time are the enemies.

Gary Casey