[calex59]

Try an experiment. Sit in an old (1964) Volkswagen with the engine at a low idle and turn on headlights. You can audibly hear the pitch of the engine change.

Or just turn the air conditioning on in any car with 4 cylinder engine and see what happens, even driving down the road you will feel the power drop and have to compensate by pushing down on the accelerator, which is why you get less gas mileage(regardless of engine size)with an air conditioner on than when it is off.

[Lonesome in Massachussets]

The air conditioner is more dramatic, but most of the additional load is the compressor, which is purely a mechanical load.