AlanS

Can anyone enlighten me.

Does the air con affect petrol consumption and if so why and by how much?

ZIPPY

Yes it does as the engine has to power the air-con unit, open your bonnet and you will se the air con unit is belt driven off the engine,this is on a freewheel type of set up i believe and when you switch it on the wheel then loccks to turn your air con pump, (explained as easily as i could put it)i think its around 5-10% less economy with air con switched on.

Zippy

speedking

The theory is correct but not the maths. Aircon has to sap power, just like a fridge.

But, to cool a small meeting room takes about 3kW. Think about the heat obtained from a 3kW fire, and you'll realise that this is close to the mark.

1hp = 750W, so 4hp = 3kW

Allowing for the much smaller size of a car interior, and losses, it seems very unlikly that aircon at full chat would use more than the 4 horsepower or 2% of output, <2% on an STi

AlanS, you have to balance this against increased drag if you drive with the window open. Do you worry about the extra power used by a CD player, or having headlights on unnecessarily, or brake lights, etc.?

You probably will notice the aircon cutting in, but it isn't active once the interior is down to temperature, you have to decide between comfort and 2bhp.

AlanS

Thanks guys,b oth useful comments.

Zippy: I'm going out to see if I can see the pulley working under the bonnet. That would explain why it always says in the manual "turn the aircon off if the engine is not running but the ignition is on".

SpeedKing: I can understand that the engine driving an extra optionally on/off pulley would divert power ultimately expected to go to the wheels, but the theory about the lights and music system, is this right? These are surely powered by the battery which in turn is charged by the alternator - a constant pulley powered component irrespective of whether the lights are on or off etc.

MartinM

..but the mechanical load demanded by the alternator goes up the more it has to (re)charge the battery. With very little power being taken from the battery, the alternator is a tiny load.

I remember the good old days, when the fan belt screeched when I put the lights on full beam (that was in the dynamo days too!)

Devil's Refugee

You can get undersized and lightened wheels for ancilliaries which could help with the consumption.
Seen them somewhere but I'll be b*ggered if I can remember!

AlanS

Got the lightened wheels already - Oz Supperleggeras

speedking

Do they fit under the bonnet?