plonkers
New member
Air conditioning on will knock up to 10% off mpg
General Panda 4x4 twin-air mpg
- Thread starter Bix67
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General Panda 4x4 twin-air mpg
So they say, never experienced that in any car though, never seen a noticeable difference on a brim to brim tank calculation of MPG.
babbo_umbro
Established member
My lamented Panda 100HP seemed to show no difference with climate control on or off - you could hear the clutch cutting it in and out quite frequently. On the Panda 4x4 TA, initial impressions are that it makes no difference - steady speed, level road, on/off makes no difference to instantaneous mpg; on tick over the 100HP's revs would dip a fraction as the climate control cut in and then pick up to normal, 4x4 TA doesn't do the same when switching aircon on. We'll see.
i dont know what you are all talking about, crusiong on the freeway at about 110 km/h the fuel consumption is 5.6 l/100 km, but if i turn the AC on the fuel consumption jumps to 6.1 l/100 km so yeah, there is a difference. It is common knowledge of thermodynamics. The amount of energy required to cool the air is at best equal (at best because it can only be greater) to the energy the ICE has to provide.
babbo_umbro
Established member
That suggests 10% - assuming the engine's generating 40HP at this speed on a light throttle - say 30KW - that would seem to suggest that the aircon consumes about 3KW - I don't know if that's reasonable or not thermodynamically but it sounds like a lot. Does the compressor run all the time on simple aircon (as opposed to climate control, where it cuts in and out frequently)?
On a light throttle at normal cruising speeds, the engine is generating nowhere near 40HP. At a steady 60mph on level ground, it's more like 15-18HP.
With simple aircon, the compressor will still cut in and out as required to maintain the required refrigerant pressure, so will have a greater effect on economy on hot days than on cold ones.
If you are into serious ecodriving in this current weather, running with the A/C on full blast can lose you at least 10mpg. If you can't see a difference in the Ic reading, it just shows how useless Ic readings are for making mpg comparisons.
10% is probably a realistic mpg loss for most folks, but running with the windows open above 55mph will also cost you at least that.
The rule of thumb for best economy without risking death by heat exhaustion is windows open, A/C off below 50mph; windows shut, A/C on above that.
Aircon uses a lot of power; anyone running a home or small office system will see a big jump in electricity usage just now.
Edit: A quick bit of googling turned up this interesting article, which suggests we are all (me included) underestimating the mpg impact of running with A/C on. Quoting from the article (bear in mind this is a US study): "Current air-conditioning systems can reduce the fuel economy of high fuel-economy vehicles by about 50% and reduce the fuel economy of today's mid-sized vehicles by more than 20% while increasing NOx by nearly 80% and CO by 70%".
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babbo_umbro
Established member
I'm not disputing anyone's opinions/figures (makes a change), I'm just trying to find out some facts, so let's go round again.
The poster suggested (near enough) 70 mph - say 15% quicker - so (I seem to remember that power required goes up with the square of the speed) your figure would suggest 20-24HP - let's call it 17KW, so aircon is absorbing 1.7KW.
Googling "fridge kw" brought up a table showing "room air conditioner" using 1.3KW - which is a bit less but not massively so. The Panda may be a little big car inside but the volume must be a fraction of an average room - about a fifth I should guess at most - room 1000 cubic feet, Panda can't be 200 cubic feet - so, even allowing for relative insulation values, I can't see how the Panda could use more than a quarter of the domestic a/c - say 0.3KW.
I'm convinced that running the climate control on the 100HP made no noticeable difference - having tried it on and off for a substantial distance to check it. Apart from that check - not scientific enough to be called a test - the car averaged 39 mpg for 49000 miles with the climate control on all the time. Is anyone averaging 43 mpg with the cc off? I very much doubt it.
There are loads of facts in that report I linked to.
The thermal characteristics of a car airconditioning system are completely different to those of a domestic refrigerator or a room. There is almost no thermal insulation in a car & the solar heat gain is orders of magnitude greater.
Running at full chat will typically require 3-5 kW. Some of the systems fitted to American cars are reputed to sap as much as 30HP of engine power.
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babbo_umbro
Established member
So the system in a car is about one tenth as efficient as a domestic aircon - frightening - and surely an area where there should be some significant developments.
Moultoneer
Member
I quoted the actual figures in my post a day ago: an actual loss of 6.4% to
my Trekking TA's average MPG with air-conditioning on. This figure seems
quite plausible to me, even if others suggest a bigger hit is possible
It's very difficult to devise a repeatable test, so I'll be updating as my car
accumulates more miles. What's indisputable is the extra comfort provided
by the AC during the current hot spell
Chris
It's not that the A/C itself is less efficient, just that it has a much harder job to do. The system needs to be vastly overspecified by design in order to knock the temperature down quickly when you get into a hot stationary car. Also a car can reach far higher temperatures sitting in the sun than you'll ever see in a room. A car is by design a greenhouse, and you can't get anywhere near the levels of insulation you could put in a house.
Fitting glass with a better thermal reflectivity would help, but then you run up against needing to meet minimum light transmission requirements, for the front windows at least.
Steady state, once the car has reached a comfortable temperature, the load should drop down to around 1kW.
Put that alongside the 18-20kW you need to cruise at a reasonable speed, and you're not so far from the 6% or so drop that other folks are posting here.
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Just completed 450 miles yesterday, filled up today and got 59mpg (although that is the Bravo and not a small car with little engine). AC (climate) on LOW for about 90% of that, and no impact on MPG to what I normally get tbh.
People forget when a car is cruising at a steady momentum the load of an AC compressor against the general momentum of the car is minimal.
If it really causing a drag capable of up to 50% increase in fuel in some cars as some people claim then you'd notice this on acceleration, and even engine braking which would increase on over run.
Unlike a house, when in motion a car doesn't need to use a fan to run air over the exchanger in front of the radiator, I still don't see how people come up with half of the figures that they do tbh.
People forget when a car is cruising at a steady momentum the load of an AC compressor against the general momentum of the car is minimal.
If it really causing a drag capable of up to 50% increase in fuel in some cars as some people claim then you'd notice this on acceleration, and even engine braking which would increase on over run.
Unlike a house, when in motion a car doesn't need to use a fan to run air over the exchanger in front of the radiator, I still don't see how people come up with half of the figures that they do tbh.
Ok all of you. Go to your car, turn it on. Turn of the radio so that the only thing you hear is the engine, let the car ran for 20 sek and listen to the engine, turn on the ac. What do you hear? You hear that the moment you turn on the ac, the engine works harder. It is producing more energy, the fuel consumption is rusing.
True, but the percentage of power taken from a small, low torque engine is far higher than from a larger high torque (Diesel) one.
When I switch on the A/C in my Diesel Doblo I can hear the compressor kick in, but it's hardly noticeable and it doesn't seem to affect performance in any way.
I have had small petrol engine cars which actually drove more sluggishly when the A/C was on.
But A/Cs are more efficient these days anyway.
And remember, if you don't use the A/C the seals will eventually dry out and you'll end up with a useless system and a car that is devalued.
When I switch on the A/C in my Diesel Doblo I can hear the compressor kick in, but it's hardly noticeable and it doesn't seem to affect performance in any way.
I have had small petrol engine cars which actually drove more sluggishly when the A/C was on.
But A/Cs are more efficient these days anyway.
And remember, if you don't use the A/C the seals will eventually dry out and you'll end up with a useless system and a car that is devalued.
babbo_umbro
Established member
I think the discussion's gone past this point - nobody is suggesting that aircon uses no energy - the question is "How much?".
Opinions vary from "so little that it's negligible" through "it's significant but justified when set beside the gains" to "it's considerable and can add ten percent to your fuel consumption".
The point of the debate is to find out where the answer lies.
It's very early days in my 4x4 TA but so far I'm inclined to think it's between the first and second of the above.
I think a lot may be to do with just how you drive your car.
If you're forever caning it around the place then diverting power to the aircon isn't going to make much difference.
If you cruise along at 50 in 6th with very low load, then quite percentage of power will be diverted to the aircon.
I'll use my aircon today on my usual route home using my usual driving style and post results. It's a 1/2 hour journey in which I would normally get over 50 mpg according to the comp.
If you're forever caning it around the place then diverting power to the aircon isn't going to make much difference.
If you cruise along at 50 in 6th with very low load, then quite percentage of power will be diverted to the aircon.
I'll use my aircon today on my usual route home using my usual driving style and post results. It's a 1/2 hour journey in which I would normally get over 50 mpg according to the comp.
Surely the point is either one drives in relative comfort and perhaps incurs a marginal cost for doing so or one drives in in an uncomfortable environment and maybe has the grim satisfaction of having saved a few pence.
Just don't get too hot under the collar about it.
Regards
Just don't get too hot under the collar about it.
Regards
Who wants to be in a hot car anyway?
The extra cost is probably no more than the price of the Sunday paper, or a cup of coffee, so it hardly matters.
And don't forget; don't use it and you lose it.
The extra cost is probably no more than the price of the Sunday paper, or a cup of coffee, so it hardly matters.
And don't forget; don't use it and you lose it.
And on that point, if people are really that MPG fixed they'd rip the whole system out if they never use it, the extra weight of a lumpy compressor etc probably uses a few more Litres of fuel annually, even when not in use vs a model without AC.
Ok, so just got home.
Here's the result
Outside temp according to car 25°c and I set the air con at max cool, max fan speed.
This route as I said I would normally get over 50mpg. My best is 62mpg from what I remember but that was near perfect conditions.
This is the second time then that I've taken an approx 10% hit when using air con
I'm with the others in that its there to be used and yes it costs me money, but that's fine. I'd rather be cool and spend an extra 20p.
For me though I don't think my results will change.
Air con at full cooling costs me approx 10%
Here's the result
Outside temp according to car 25°c and I set the air con at max cool, max fan speed.
This route as I said I would normally get over 50mpg. My best is 62mpg from what I remember but that was near perfect conditions.
This is the second time then that I've taken an approx 10% hit when using air con
I'm with the others in that its there to be used and yes it costs me money, but that's fine. I'd rather be cool and spend an extra 20p.
For me though I don't think my results will change.
Air con at full cooling costs me approx 10%
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