Air Con/Climate control in this really hot weather

AdrianC

EdGasketTheSecond wrote: »

Mercedes C180; also notice it on my Hyundai Lantra. Some people claim (falsely) that running air-con costs nothing; I beg to differ as it is obviously loading the engine and that extra power has to be paid for somehow i.e. increased petrol consumption. The load did seem worse in the recent hot spell so I assume the compressor is having to work harder with corresponding extra load on the engine. The air is nice and cool though.

The compressor is either working or it isn't. If it's working, it's taking the same amount of load. The compressor runs then cuts out when a lower temperature bound is reached, cutting back in when the temperature rises again. That's why it might seem to be working harder in high ambients - because it's working longer.

The power used is trivial in any reasonably-powerful modern car. 5-10bhp at most. Fuel consumption effects are minimal, and may be outweighed by the aerodynamic drag of opening a window. I've had (low-powered) cars where opening the drivers window takes 3-5mph off the top speed.

EdGasketTheSecond

AdrianC wrote: »

The power used is trivial in any reasonably-powerful modern car. 5-10bhp at most.

I wouldn't call 4 to 9% trivial (think the average bhp of cars is 110) with associated petrol cost.

AdrianC

EdGasketTheSecond wrote: »

I wouldn't call 4 to 9% trivial (think the average bhp of cars is 110) with associated petrol cost.

You aren't using all 110bhp most of the time. Very few people EVER use all the power. Sitting at a steady 70mph takes less than 30bhp to overcome drag, which is very low load at typical cruise revs. At that kind of load, a little more power required doesn't require much extra throttle opening, so very little extra fuel.

facade

So if you only actually need 30hp to go along, and you are doing 40MPG, then with the aircon on you now need 35hp to go along, an increase of 17% load, so likely dropping the mpg to 34.

If my aircon still worked, I'd use it, a couple of extra pounds a week on fuel is cheaper than buying icecreams!

I see no point in buying a car with air-con and keeping it off, like buying a 2000hp car and pulling away from the lights a slowly as a 2hp one- you might as well get a 2hp engine and 2000hp badges and save the money...

AdrianC

facade wrote: »

So if you only actually need 30hp to go along, and you are doing 40MPG, then with the aircon on you now need 35hp to go along, an increase of 17% load, so likely dropping the mpg to 34.

Not that simple. If you're only using 10% throttle opening to produce 30bhp, you may only need 11% or 12% to produce 35. Very little extra fuel required.

mattyprice4004

I monitor MPG closely brim-to-brim, and actually use less fuel in summer with the A/C on.
In winter the car has to work harder to get to temperature, using more fuel - and my shorter journeys mean that on a lot of them working temp is never reached.
Low temperature = higher air density, and higher fuel usage while warming up.

There's really not much in it, A/C compressors are very efficient these days.