Living la vida loca!
AO Silver Member
Join Date: Jul 2005
It definitely makes a difference. The air con unit needs a lot of electric current, and to get this the alternator (may be a secondary alternator specifically for the air con actually) pushes onto the belt to draw more power. It's a subtle change but easy to demonstrate as follows:
You've got it all wrong I'm afraid. The only extra current used when operating a/c is the low speed fan to cool the condensor. The a/c runs a COMPRESSOR not a second alternator. You're thinking about industrial or domestic air con which needs a motor to drive the compressor. In a car, there is obviously no need for an electric motor to run the compressor which is powered by the engine. The compressor itlsef does not actually move or press against the aux belt. The aux belt runs around the compressor spindle which has a built in clutch. When you select the a/c, the clutch engages and the compressor begins to work building up preasure in the a/c system. The refrigerant gas travels up to the bulkhead and passes through an evaporator which extracts heat from the air thus cooling it down. The now heated refrigerant gas tmakes it's way down to the condensor (in front of your radiator) where again now the heat is extracted via the exchanger/condensor and cools it. This temperature drop is normally in the region of around 25 degrees C. An anti frost switch ensures the gas does not freeze and there is a host of other goodies in there too which I will not go into now. But as you can see, there is no current or electric motor used in running the compressor which also acts as a pump to move the gas around the circuit.
The use of this compressor takes power out of the engine and your right foot hence the need to press down harder in order to maintain speed. Ergo less fuel economy when measured at the same speed with a/c on compared to off.
Hope this clears it up for you.
164 V6 24v "Zender Alfa Sport"
Past glories: Sud 1.5ti, Alfetta GTV, GTV6, 33 1.5ti, 33 1.7ie, 164 3.0 V6, 164 3.0 V6, GTV 1.8TS, 156 1.6 TS, 159 2.4 JTDm QTronic.