Safe Speed Forums

Air con is power hungry, but that assumption isn't right. Power is used to move heat from the evaporator to the condenser - it's a heat pump.

It certainly doesn't take a Kw to move a Kw, and some heat pump systems claim 'efficiencies' in excess of 400% - the 'extra' energy comes from environmental heat on the cold side of the system.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Go on! I'm curious! A something-for-nothing machine eh????

When the compessor cuts in on my car if I'm trickling along in slow traffic at a very light throttle opening, I can feel the car slow down ever so slightly. When it cuts in and the engine is idling, the revs drop instantaneously until the idle speed control valve opens and jacks the tickover back up to where it was supposed to be. If I watch the belt tensioner as the pump cuts in, it kicks round a bit as the tension on the drive belt increases.

All these things suggest to me that there is more "work" being done by the engine when the aircon compressor is running.

I have heard of heat pumps that can be extremely efficient but I don't think the typical automotive aircon system is one of them - it's having to move heat "up hill", as it were, so I can't see any way it can do that without a net requirement of energy.

Difficult to quote efficiencies here but what Paul means is that it takes only 250W of compressor/engine power per unit time to relocate 1kw of cabin energy per unit time to the radiator (along with the extra 250W - so Energy added to the system (from the engine) is 250W, heat relocated from inside to outside (lowering cabin temperature) is 1kW. All pro rated for beeifer compressors.

It's Sunday evening and I'm bored. I looked at that DfT website for some laughs.

Idling is wasting fuel

When the engine is idling you're wasting fuel and adding to CO2 emissions. If you're likely to be at a standstill for more than 3 minutes, simply switch off the engine.

Driving at an appropriate speed reduces CO2

Speed limits are the maximum lawful speeds which may be driven in ideal circumstances. Drivers should never exceed the speed limit. Staying at or within the speed limit increases driver safety. It also reduces C02 emissions and saves money in your petrol costs. At 70mph you could be using up to 9% more fuel than at 60mph and up to 15% more fuel than at 50mph.

Point 1-Let's assume I'm stuck at a junction and want to save some CO2. I switch off my engine and sit stationary in the road. Then someone shunts me in the rear as I can't move off quickly enough to avoid a collision. Oh, and doesn't your engine produce more CO2, switching it on and off than letting it run idle? And how am I going to avoid a charge of driving without due care and attention by explaining to the police that I was following government guidelines and was saving CO2? It doesn't matter that I have whiplash, amongst other problems.

Point 2 I am smelling a large government-trained rat here. Are they asking everyone to drive at 50mph (especially on motorways) or are they contemplating cutting speed limits?

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Anyone who tells you that nothing is impossible has never bathed in a saucer of water.

The best bit about the first point is that, at least, you would be able to use your hand held phone quite legally.

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Malcolm W.
The views expressed in this post are personal opinions and do not represent the views of Safespeed.

Google is your friend...

http://www.google.com/search?q=%2B%22he ... iciency%22

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Well, I had a look at a few of the links on the google search and found this on the Wikipedia entry:

"Sometimes this is inappropriately expressed as an efficiency value greater than 100%, as in the statement, "XYZ brand heat pumps operate at up to 400% efficiency!" This is inaccurate, since the work does not make heat, but instead moves existing heat "upstream"; otherwise, this would be a perpetual-motion machine."

So it could be we're talking acros purposes here. The "COP" they refer to is "Coefficient of Performance".

I don't deny there's scope for making automotive aircon pumps more efficient than they are (and indeed, this has been the case over the last 10 years or so) but they still represent (I'm told) a pretty big parasitic loss - up to 10% extra fuel consumption under bad conditions) according to one website but it didn't substantiate the figures at all. I guess that if you're driving through death valley at the car's most economical speed and you've got it set to "max cold" you might see another 10% (depending on the car)! I had a quick look round but couldn't find any figures for the power absorbed by an aircon pump working hard. I guess it would be a couple or three horsepower in an average family saloon.

Roger, do you have any more data from whence those figures came? I'd be interested. I can understand that you might not need 1kW to move 1kW and dump it outside but 250W sounds pretty low and presumably it varies dramatically with the temperature gradient it's working against?

This is very dependant on the car you drive.

It's quite hard to get accurate figures but it is difficult to keep my car in top gear below 60mph (or more in slightly hilly areas). I threfore expect that the most efficient speed is at least 60mph probably 65-70mph depending on terrain.

Mine's like that - a diesel with 6 gears. Unfortunately, it's depressingly true that it's more economical in 5th at 50 than it is in 6th at 60! Diesels don't suffer from the same part-throttle inefficiency that petrol engines do and, mine being an MPV with a big frontal area, speed makes more of a difference than revs.