Does driving with the A/C on use more fuel?

[zlcrazy]

Hi all,

As the title says i want to know "Does driving with the A/C on really use more fuel?

Any information, real life stories/tests etc would be appreciated.

Thanks in advance.

[SensationFG8]

Yes, the air con consumes energy. Energy in a car is gained by burning fuel. Therefore to supply energy to the air conditioning you have to burn more fuel.

How much depends on the power consumption of the unit and the efficiency of the engine.

[GasoLane]

Yes. As mentioned above, using the Aircon means the engine has to work another ancillary device, hence needing more power from the engine and using more fuel.

[zlcrazy]

I wrote my question wrong sorry.

What i ment also was does it make alot of difference to fuel consumption?

[EB#]

Yes, I believe running the AC does use a little more fuel, but negligible perhaps?

A bloke I know reckons that it is good to leave the AC on all the time, as a running compressor may last longer that one that goes start/stop etc. I have taken his advice and the AC on my '03 BA has been on for the 6 years I have owned it, no probs!

My BA returns around 580-600km per tank of fuel, with a mixture of urban and country hwy driving. A good freeway stretch may return 700km+.

[zlcrazy]

Originally Posted by EB92

Yes, I believe running the AC does use a little more fuel, but negligible perhaps?

A bloke I know reckons that it is good to leave the AC on all the time, as a running compressor may last longer that one that goes start/stop etc. I have taken his advice and the AC on my '03 BA has been on for the 6 years I have owned it, no probs!

I have always done the same with my cars. Regardless of what temp i set the AC is always on.

[danzvtil]

Its academic that an ac compressor will draw power and increase fuel use- I believe 1-2 kw of power? Whether you can tell any difference in l/100 in a modern car is another thing.

[Ford_The_Win]

Originally Posted by zlcrazy

I have always done the same with my cars. Regardless of what temp i set the AC is always on.

I do the same also. It dries out the air coming into the car and I believe it is better for the longevity of the compressor.

[Gothefalcon]

Yes ! How much extra fuel depends on many factors though.

Older cars tend to have much larger compressors which require more energy to drive. If the condition of your A/C is poor.. the compressor will cut in a lot more. Running the A/C in something gutless will make a bigger difference in fuel economy than in cars with more powerful engines. The ambient temp of the day may determine how long the compressor stays engaged. and so on.
I reckon it makes about .5L/100 (ish) difference to my BF2 4.0... and even less to my NL 5.0... But My Mum's 98 Pulsar SSS 2.0 would use about 1.5L/100 more with the A/C on !??

D

[99AUXR]

A/C compressors ideally should be run all the time. O rings/seals can dry out if they aren't run for long periods of time.

[Raptor]

I seem to remember a Mythbustres episode where they demonstrated that an aircon uses less fuel than the other option, which is driving around with the windows open, at least at highway speed. The open windows upsetting the aero flow over the car and causing more drag which results in more fuel used.

[Nanoraptor]

I went on a total fuel/distance consumption recording kick for a few months with my AUII a while back. It's used exclusively for the same 102km round-trip for work on weekdays. I found aircon on 100% in warm weather made less difference than whatever margin of error was giving me a .5L/100km variance week to week.

[Davehoos]

once the car is operation and moving it should use very little fuel. the 5 KW often quoted is maximum at maximum working RPM.

short trips or idling without air flow obviously will use fuel.
with a scan tool you see fuel rate increase as much as double but its been a long time since air conditioners caused weekly change like that.

some people argue that turning lights on used lots of fuel-thats not my experience.

[TRAU BLAU]

WOW! I'm gonna use my aircon more often now, after reading that some people have them on permanently!

[poppa smurf]

I wondered about this on a recent trip across the nullabor, (boring old fart with too much time on his hands)......air con on used a consistent 2 ltrs/100 klm more than when it shut off.

according to the onboard computer readout.

I've since found identical usage many times on long trips........with van on and van off.

I watch the fuel pretty closely when towing, I only get around 250 to 300 klm (if the gods are looking down favourably) per tank without any wind.

[GasoLane]

I found that with our old AUII Fairlane the Climate control cut the aircon in to keep the cabin temp at the nominated temp.

As others have said using it all year round keeps the seals from drying out, must work as we never had to re-gas the aircon in the 7 years we had it.....and it gets hot around here

[aussiblue]

I seem to remember a Mythbustres episode where they demonstrated that an aircon uses less fuel than the other option, which is driving around with the windows open, at least at highway speed. The open windows upsetting the aero flow over the car and causing more drag which results in more fuel used.

Wrong; see http://www.discovery.com/tv-shows/my...en-windows.htm

Finding: BUSTED
Explanation: When the weather warms up, is driving with the windows down or with the air conditioning running more fuel efficient? Auto experts have claimed that AC is the wisest way to go, but MythBusters Jamie Hyneman and Adam Savage fueled up two identical SUVs and took them to the tracks to truly solve this roadside riddle.

With five gallons of gasoline in each tank, the MythBusters saddled up their SUVs and circled Northern California's Altamont Raceway at 45 miles (72 kilometers) per hour to see which one ran out of fuel first. Before starting their engines, Jamie cranked up the AC, Adam rolled down his windows, and the pair set off on their race to nowhere.

In the end, Jamie's air-conditioned SUV stopped first, while Adam's window-cooled car ran for 15 miles (24 kilometers) more. Since rolling down the windows sustained Adam's SUV for a significantly longer distance, the MythBusters busted the expert advice.

For fuel-savvy summertime driving, keep your air cooling au naturel.

[aussiblue]

but from that same Mythbusters http://www.discovery.com/tv-shows/my...en-windows.htm

The Society of Automotive Engineers (SAE) did an actual scientific study of this same question and they busted the mythbusters.

“Based on a study conducted by the Society of Automotive Engineers (SAE), driving with the windows up and the air conditioning on is typically a more fuel-efficient way to drive. Twovehicles were used in the test, one was a full size SUV with an 8.1-liter V-8 engine and the other was a full-size sedan equipped with a 4.6-liter V-8 engine. Overall, both studies showed that driving with the windows down has a significant negative effect on the fuel efficiency -- more than using the vehicle's air conditioner. The study concluded that the more aerodynamic the vehicle, the more drag open windows will create. When driving at speeds of more than 55 miles per hour with the windows down, there's a decrease in fuel efficiency of 20 percent or more for a sedan.”

Their tests showed a significant loss of fuel economy at highway speeds when rolling the windows down, and the more aerodynamic the car, the more fuel efficiency was lost by rolling the windows down. So, if you are driving 50mph or more – specially over 55mph – the SAE says windows up will save fuel even with the AC
running. And these guys use far more scientific methods to answer questions rather than simply do random tests for the sake of entertainment

and see http://www.sae.org/events/aars/prese.../2004-hill.pdf for a summary of the SAE report.

[Jastel]

Around town slow driving a/c definitely uses more fuel.
Over about 80kmh the aero concerns of open windows outweigh the a/c drain.
So if you want to save fuel turn a/c off, drive slowly but DONT open the window either.

In the cabs on a cool day fuel consumption is about 10% down, but engines operate better in cooler temps as well so you sort of have a double saving.

As said cooling and a/c system health (or lack of), driving style, how cold it is set, fresh/recirc air all affect it as well.

[aussiblue]

But the debate continues see http://wheels.blogs.nytimes.com/2008...ype=blogs&_r=0 and http://www.bankrate.com/finance/auto...el-or-not.aspx

[aussiblue]

Around town slow driving a/c definitely uses more fuel.
Over about 80kmh the aero concerns of open windows outweigh the a/c drain.
So if you want to save fuel turn a/c off, drive slowly but DONT open the window either.

In the cabs on a cool day fuel consumption is about 10% down, but engines operate better in cooler temps as well so you sort of have a double saving.

As said cooling and a/c system health (or lack of), driving style, how cold it is set, fresh/recirc air all affect it as well.

It might also depend on if you have power windows and how efficient they are; some tests have taken into account the energy consumed by powering the windows down. You also have to take into account how many passengers you have and their metabolism (no ladies with hot flushes please :-) ) as it effects how hard the air con has to work to maintain a constant temp.

[Winter XR8]

I always leave my a/c running. The extra fuel use would nearly be unmeasurable.

[mik]

The short answer is if you put more load on the engine it will compensate by using more fuel .

[Grippy]

Are older tech A/C compressors less efficient than today's technology? I have always avoided using A/C where possible to keep fuel consumption down.

[NTF6]

Originally Posted by Grippy

Are older tech A/C compressors less efficient than today's technology? I have always avoided using A/C where possible to keep fuel consumption down.

They are all piston compressors, older ones actually looked like a piston compressor, the newer ones in todays modern cars are still pistons inside just actuated horizontally by the swash plate. Older ones are actaully more effective as used commercialy in r502 systems but the design is too large and power consuming for todays modern car.

[NTF6]

Originally Posted by 99AUXR

A/C compressors ideally should be run all the time. O rings/seals can dry out if they aren't run for long periods of time.

They should be run regularly not all the time.

As for the people who leave it on all the time, I can't see the reasoning behind it. The compressor cycles in and out (stops and starts) while in use anyway and when you turn the engine off the compressor stops and then starts when you turn the engine on again so the point that leaving it on is better doesn't make sense.

If you turn it on one day and leave it off the next makes no difference. Run them occaisoinaly to lube the seals is all that's required.

[xrystl]

My VW Crafter (2.5 diesel) costs me 1.5 litres per 100km to run the AC , @ 1500km weekly it's about a $40 hit - lucky I enjoy Scotch more than the AC in the vehicle.

[blackf6]

Not as much as driving with the windows down.

[PG2]

I drive with both the AC on and the windows down??

[99AUXR]

Originally Posted by NTF6

They should be run regularly not all the time.

As for the people who leave it on all the time, I can't see the reasoning behind it. The compressor cycles in and out (stops and starts) while in use anyway and when you turn the engine off the compressor stops and then starts when you turn the engine on again so the point that leaving it on is better doesn't make sense.

If you turn it on one day and leave it off the next makes no difference. Run them occaisoinaly to lube the seals is all that's required.

Sorry but I don't see why they shouldn't be run all the time, I service air con systems in cars at the workshop regularly have never seen an issue caused by an air con system being run all the time. The compressor will cycle every ~20 secs anyway if its got the temp sorted so its moot point anyway.

Just like the gas should be changed every few years but no one ever does