Are we paying too high a price for aircon in terms of fuel consumption? I acquired a 1999V Volvo S80 SE(170)automatic in January and have been really pleased with it - fast, comfortable and never missed a beat. Naturally I did not choose it to save on petrol bills and following HJ's advice - and what I have seen in The Back Room - I run it solely on Optimax.
However, HJ's other advice re aircon in FAQ 20 comes at a price - HJ recommends leaving aircon on all the time and running it hot and cold from time to time etc - advice which I have been following. Average mpg around the New Forest where I live (40 mph limit on most roads)is around 25 - 28 BUT increases to 34 - 38 with aircon off, on the same routes and trying to keep to the same driving style. The lower figure is verified by my log book figures as well as the car's computer; the upper figure by the car alone so far.
My garage (Volvo specialists Country Cars of Brockenhurst) recommend running the aircon once or twice a week. At the above mpg figures this would save about £400 a year on fuel. Most of the time in this country aircon really is not necessary - but I don't want to knacker the system.
With more and more cars with aircon this could be costing us and the environment a packet. And just why does aircon have to cut consumption by almost 30% - can't someone improve the systems used in cars?
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Interesting. I have A/C in both my cars: MB E320 Coupé and VW VR6 Highline. I use it all the time. I have experimented from time to time and have found no significant difference in fuel consumption when A/C is switched off for a prolonged period. Is it that the engine size is a factor, both cars being reasonably powerful (3.2 and 2.8 respectively) and therefore not suffering so much from the tax that A/C imposes on them? My long-term figure for the MB is around 26 mpg and for the VW 30+. Given my experience to date (based on a continuous record of fuel consumption), I am inclined to persist with always-on A/C to ensure that it continues to work properly. When was your A/C last serviced and is it working properly?
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keb
i question your figures, 25-28 mpg round town for your car is about right, a heavy car with auto box and 170bhp? at hand equals low mid 20mpg. With out question aircon uses power, the hotter the day the more power to run the aircon is required. personally i ensure the aircon is used a lest once a week to keep the system healthy, but other than when it is needed i turn it off, what's the point of running a system you don't need? do you leave the heated rear screen on constantly?.
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Some people have their headlights and fog lights on continuously! Mostly for no good reason too! Look I've got foglights - can you see them?
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Interesting figures, KEB, but are you sure that use, or not, of a/c is the only factor?
Although I have not yet covered sufficient mileage in our new V70 2.4T to have gathered representative figures (beyond proving that it's several MPG juicier than the Vectra GSi Estate that preceeded it when wearing identical lead boots), over the three years each that I drove them, at various times I conducted fuel consumption measurement with both previous cars (Vectra SRi V6 Estate and aforementioned Vectra GSi V6 Estate) for exactly the purpose discussed in this thread.
The result?
Nothing more than a maximum of two MPG variance over an extended distance, and even that was questionable given the impossibility of conducting exactly the same journey twice on public roads. Given that an air conditioning compressor takes just a couple of horsepower to run, and that any given point, the engines would have been producing horsepower varying from a handful, to their maximum (168 and 192 respectively), this small increased sounds about right to me. Infact, when running with instantaneous fuel consumption displayed, it was often hard to see the points at which the compressor engaged at all.
(Honest question) - Are you sure that you're not subconsciously driving even more gently, to eke out fuel, when trying to prove to yourself that 'without a/c' is better?
For the record, having paid handsomely for full climate control instead of air conditioning, we leave it switched on fully automatic, all of the time, in the V70. Most impressive it is, too, switching automatically between recirc and fresh depending on humidity, pollution, and temperature, as well as switching off momentarily all together when full steam ahead has been rung down to the engine room.
Glad you're enjoying the S80.
My father loves his (same engine as yours), and has so far had five years extemely comfortable motoring with it.
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SjB.
Do you get any problems with misting on your V70? Like you I leave the CC on auto ( I have a V70 D5 SE) but sometimes the screen mists up and I have to use the de-mist button for a few minutes. This also happened on my previous Series II V70 2.4T.
VD5D.
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I too find this number way too high
On my arthritic 1.6 90bhp Scenic the aircon only takes at most about 3-5 mpg off the consumption despite the fact you can hear and feel the strain on the engine when the aircon kicks in.
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Hi VD5D,
Yes, though I'm sure I know the cause in my case:
The windscreen misted up very heavily whilst driving the car back from Folkestone, having just imported it. It was late at night, and totally pitch dark out in the coutryside when it happened, with the degree of misting up so quick and severe that I had to stop and park the car within a few seconds. It was total white-out, and would have been interesting a few minutes previously on the motorway.
When I looked at the outside temperature gauge, I was amazed to see that it had dropped from eight degrees earlier on, to just +1 degree C, and whilst I looked, continue to drop to -2 degree C in the next few seconds.
The misting up has never happened again, including at times of high ambient humidity, so I therefore put it down to the outside temperature falling below the (4 degree, I think) threshold needed for a/c to work, worsened by the temperature fall being so abnormaly fast that the system didn't have time to react and dry itself out (as it is designed to do in the case of Volvo CC).
HTH.
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Hi SjB,
Yes, I have had the same type of white-out happen a few times in both V70's, I think it maybe due to the re-circulate/fresh air button. I find it's worse on re-circulate, CC set on auto and the car is full of people and it's raining therefore high humidity. It happens extemely quickly for no apparent reason, so as I said before I hit the de-mist button and the screen is clear again in a few seconds. Very odd. I'll have to re-read the manual!
Cheers
VD5D.
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KEB,
I have found with the V70 D5 there is no significant rise in fuel consumption with the the aircon switched on all the time. I did over 2000 miles in France last year with the aircon on all of the time and averaged about 44 mpg. Perhaps it's something to do with the high torque of the D5 engine. I find during hard acceleration the aircon stays on, unlike the 2.4T which momentarily switched off.
VD5D.
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At 3000 rpm an a/c compressor takes about 8bhp, but it doesn't run all the time. If it's cold outside it barely runs, if hot, it runs a lot more. Therefore there's no way it can account for those differences in fuel consumption. Assuming the compressor isn't siezed (and you'd probably know if it is), then the figures don't add up.
So why run it all the time? Quite simple, 'cos it makes no difference at all. The power steering pump runs all the time, even when going in a straight line. The alternator runs all the time, even during the day. The fuel pump runs all the time, even when idling.
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Dave N, agreed the compressor only loads up when required, but will the cooling fan be running all the time?
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Whether the cooling fan runs all the time will depend on the design of the system. On a friends Rover 200 the simple on-off type system on his car runs the fan all the time. On my 827 fitted with ATC (Rovers climate control) the fan only ran when the compressor ran.
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On some cars with CC the compressor runs all the time unless the cooled air output drops below 5deg. To regulate the temperature hot air is then mixed back in with the cold!!
Some normal air-cons are fine and only kick in and out the compressor as required to set the temperature to a pre-determined level, my Saab 9000CS and Fiat Punto behaved like this. On both these cars its was hard to spot the difference if fuel economy. I usually drive with the air con all the time in the Punto and still average 49 MPG.
Some seem to keep on cooling until very cold before kicking out, I found the Mondeo to be like this . Air con on - fridge, air con off too warm. Fuel consumption badly affected!
The air-con control on my Octavia Clasic is attached to the keep fit window winder!
The new Polo looks interesting, is that control a thermostat that operates the compressor? or does it simply still always add hot air to cold?
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Dave N,
Of course it make a difference! If you know of a design of refrigeration plant which requires no power input to run, then patent it very quickly and your family will never have to work again for many generations to come. Just how measurable the difference is will vary considerably from car to car and with prevailing conditions.
The reason PAS pumps run all the time is that if they had a clutch like a/c compressors they would be unable to react instantly to driver steering inputs. If you could eliminate the need to run the pump when driving straight or at idle then the load on the engine would be reduced and fuel economy improved. This has in fact been achieved by EPAS - Electric Power Assisted Steering. The system is lighter than an hydraulic one and the motor only runs when needed, contributing to improved emmisions at idle and improved economy when driving.
The alternator must run all the time. When you start the engine you remove energy from the battery, which must be replaced. Also, when running, the ignition system, various ECUs and the fuel pump all comsume electric power all the time. If the alternator were not running this would eventually drain the battery. A major advantage of alternators over the old dynamos is that they are load sensitive, ie they only generate the power required by the load placed on them - consequently the power they drain from the engine (and hence economy effect) varies in proportion to electric load.
Although the fuel flow rate is low at idle (1 to 3 litres per hour depending on engine), the fuel pump must continue to run to supply this fuel. Without its fuel supply the engine will stop. Modern electric fuel pumps are much more efficient than old cam driven ones, reducing the power they sap from the engine. Also by providing a constant pressure supply they contribute to the injection systems ability to keep the engine running at peak efficiency on every stroke.
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KEB,
I have the same car as yours, and find no noticeable difference in fuel consumption with the A/C on or off. I had wondered about it, so did a check and found that at a steady cruise-controlled 60mph, the S80 returns 44mpg regardless of A/C setting. I would have thought that under these lightly-loaded conditions, any significant effect would be noticeable. Oh yes - the A/C does work!
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I don't know as to what effect the AC has on the MPG of my 405 but when the compressor comes in you can feel the difference, almost like a split second stutter. It also takes the edge off the performance.
Steve.
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LHM,
Thanks for this. I have just come back from a trip on a reasonably clear M4 and did the same as you, though at a higher speed - and the car computer calculated consumption in the low 30s regardless of a/c and ECC settings.
Perhaps my New Forest trips were a bit of a freak result? And I agree with the guy who mentioned that perhaps I was using a lighter touch and biasing the results - difficult to be sure. I did a site search on this and the postings in (I think) July 2002 included a good one by DaveN explaining that a/c compressors are really quite beefy, using something like 9 - 10 bhp. However, out of my total 170 bhp that should only be a small penalty.
For the sake of the system I think I will just set the ECC to Auto / air con on and stop fiddling!
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When I had a Rover 827 between '96 & 2000 I found that using the aircon cut about 2mpg off the fuel consumption. This was consistant whether the car was driven manually or cruising on cruise control.
Although some correspondents here state no noticable difference in mpg using aircon, if you think about it aircon must consume extra engine power which in turn will require the engine to burn more fuel. The exact amounts of extra fuel required will vary according to a number of factors.
Interestingly, GM conducted an experiment in the early 90s whereby they drove two identicle cars around a Michigan test track for about two weeks in mid summer. They designed a regime whereby the cars were alternately driven with aircon off & windows open and aircon on & windows shut. The experiment proved that it is more fuel efficient to drive with windows shut and aircon on - the causal factor here being that with windows open the increase in aerodynamic drag is greater than the load the aircon creates.
When I was working on Discovery II design team at Land Rover in the mid 90s (when I bought my 827) I asked the aircon engineers about how often I should run the system. The advice I was given was that I should run the system for at least half an hour continuously once (or preferably twice) a week. This was not difficult to do - in the summer the aircon is used to cool the car and in winter it can be used for fast demisting.
There are however a number of steps you can take to mitigate the extra fuel consumption by reducing the workload on the aircon:
1. Park your car in a shady spot. If you must park in full sun use a reflective screen shield. Not only will this keep the interior cooler, it will protect it from the UV radiation which fades trim.
2. When the interior is hot, vent the car initially by driving the first half mile with the windows open and aircon off. This will get rid of much of the excess heat before you close the windows and put the aircon on. You also avoid putting extra load on the engine whilst it is running 'on choke'.
3. When hot the car will cool quicker if the system is put into recirc mode. If it is really hot outside it will be more effecient to keep the system in recirc rather than constantly cooling hot incoming air.
4. When temperatures and sun load are more moderate, consider leaving the aircon off altogether - but remember to use it twice a week (see above).
5. When using the aircon try not to set too low a temperature - the cooler you want the interior the harder the aircon has to work (applicable to systems with auto temp control).
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I never noticed any loss of power or fuel consumption with the air-con on in my old M.B. but I have tried it with my new one ( its got a snazzi trip computer thing) and it makes no difference at all to fuel consumption or power.
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Well said cyd
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HJ made Cyd's aerodynamic point some time ago ie the drag and turbulence from an open window causes more fuel consumption than the drag of the a/c compressor on the engine.
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Oh you lucky people to be driving 5-cylinder S80's.
I'm running the 2.9litre six-cylinder with four speed auto. Great luxury car, very smooth and perfect for motorway trips. But......suburban fuel consumption is no better than 22mpg. Given this remarkable figure, it is not surprising that I do not notice any difference between a/c on or off.
The major factor in these cars must be the four speed box. The mph/1,000rpm figures are approx 13/20/30 for 2nd/3rd/4th gears. So the car spends most of its time in 2 and 3 and only gets to 4th at about 40mph after a distance. Now, with 5 gears, the economy, must be better. However, with all that torque in a diesel, why have 6-speed boxes? Remember the first Porsche Turbo - only four gears, when the normal 911 had five.
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