A/C effect on mpg

[Trebor77]

Hi, The auto heating system turns on the A/C even in midwinter. How much does it cost me, in terms of mpg.

 

I have a 1.4 petrol DSG Octavia

 

Thanks

 

KK

[Ootohere]

Turn it off and compare with what you get now then you will be able to tell us.

[Isocrem]

A/C helps to dehumidify the air in the car. Useful in Winter and Summer. No idea how much extra fuel it might use, but worth it for comfort, in my book.

[JWvrs25]

Probably not of much help but my 1.2 fabia is 5-8 mpg worse with the ac switched on

With 64 bhp its really struggling so id expect it to be a smaller effect on a larger engine

[Han2u]

There's a display in the MaxiDot to show the extra consumption, including A/C. Don't know how accurate that is though

[Warrior193]

My 1.4 TSI manual economy drops from around 41MPG to 36-38 with the A/C on around town ( on Maxi-dot display ) 

[Gerrycan]

There is no simple answer as there are too many variables involved.

Single or multi point climate control, settings, weather conditions, motoring conditions, number of people in vehicle etc.

[pist0nbr0ke]

Mine is permanently on.

 

But I turn my wipers off going under bridges, and that more than compensates for any lost mpg. 

[SC03OTT]

It's not worth worrying about. Buy one less pint and year and the cost will be covered. With change. 

[jthyssen]

On a hot summer day it may be as high as 10% or more extra fuel, but I would expect it to use much less in the winter where the compressor will run less often. 

 

If you turn it off you should remember to turn in on occasionally to avoid build-up of bacteria and mould and to exercise the A/C unit (lubrication etc.)

 

I leave it in auto all year. 

[Geek42]

I leave my AC on all year round, but it spends most of the time set to eco via the driver mode custom settings. Eco seems to be more than adequate for comfort, I only switch it up to normal for a couple of weeks each summer when it gets really warm.

 

Whether eco mode is making any difference to mpg...no idea, sorry.

[Gerrycan]

It does not make much difference whether the aircon is left on or off over winter.

I'm sure I remember our resident chief minion at RRG once posted that there is a residual circulation even when off to avoid the drying out of seals mentioned above.

 

Operation in winter to reduce humidity will have minimal effect on mpg.

In summer in hot weather on a motorway run it could be 6% to 10% worse as mentioned above but in heavy stop/start traffic consumption could be up to 30% worse. Well on very hot days here in Adelaide it is.  

 

 

[bertJ]

This sort of posting always used to amuse me when the argument was something like 'should I turn the aircon off to save 2mpg'. The standard reply from most people was 'why didn't you buy a car without aircon if you want to save a few pennies?' However, very few cars are available today without aircon. There used to be some merit in the idea that aircon sapped engine power and decreased mpg - I remember an Isuzu Trooper I owned that had a very noticeable drop in power when the aircon was switched on. Modern aircon, especially climate control, has become  very energy efficient and should be left on all the time to automatically adjust temperature and humidity as well as preventing nasty smells . Also, of course, some people still have the idea that aircon is just for cooling the inside of the car in summer. As many people have said, 'just turn the aircon on, forget it, and accept the increased comfort and safety it brings'.

I think it's time that the idea of turning off aircon to save fuel should be relegated to ancient motoring history. For instance, whatever happened to the idea that, when slowing to a stop, you should go down through the gears to save the brakes? Also, what about not turning the headlights on to save the battery etc. etc? 

[Gerrycan]

You can't buy a new car in Australia that does not have aircon as a standard feature.

 

When I were a lad in the UK to have a heater fitted was a forty pound option but since you could buy a house for that sort of money you made do with two sticks and some kindling for when it got really cold... but we were happy.

Tell that to the young kids today and they'd laugh at you.

Edited 15 February, 2017 by Gerrycan

[iriches]

There is no simple answer to "How much does aircon impact fuel economy" as I'll hopefully show...

 

A typical maximum figure for my car as seen in the Maxidot is around 0.25 litres of fuel an hour on a hot UK summer day (and I can hear the Aussies laughing...).  It starts higher than this, but quickly ramps down as the car cools.  However, the impact of that 0.25 l/h varies considerably depending upon how far I drive in that hour.

 

Let's assume that I'm in horrible city traffic, and manage to drive only 6 miles in one hour.  Let's assume in such traffic that my 1.4 TSI car may do 30mpg if the aircon were off.  It would thus have used (6 miles / 30 mpg) * 4.5 litres / gallon = 0.9 litres of fuel.  The aircon for that hour has used an additional 0.25 l/h - taking the total to 1.15 litres.  The impact of the aircon is thus (0.9 - 1.15) / 0.9 = -28%.

 

Congested City

Aircon Off = 30 mpg

Aircon On = 23 mpg

 

However, if we assume that I'm cruising on an free-flowing motorway, and manage to drive 70 miles in one hour, then things are different.  Let's estimate that at this speed, my 1.4 TSI could do 50mpg if the aircon were off.  It would thus have used (70 miles / 50 mpg) * 4.5 litres / gallon = 6.3 litres of fuel.  The aircon for that hour has used the same additional 0.25 l/h - taking the total to 6.55 litres.  The impact of the aircon is thus (6.3 - 6.55) / 6.3= -4%.

 

Free Flowing Motorway

Aircon Off = 50 mpg

Aircon On = 48 mpg

 

(All based on assumptions rather than measured values - but tallies with what I see in what passes for real life these days...)

[Gerrycan]

Those figures pretty well tallies with my experience even though yours would probably be climate controlled aircon whereas mine is standard aircon.

I have my consumption display on L/100 so when stationary (ery?) ok, not moving, and ticking over then it shows consumption at 0.5L/hour and when the aircon kicks in this rises to 0.8L/hour.

 

In winter if your aircon was on then it would only kick in to control humidity, which would be infrequent and of shorter duration, so the overall effect on consumption should be negligible.

[KenONeill]

I've experimented on a stretch of dead flat road near work, and at ~40mph in top switching the AC in or out makes about 0.1mpg difference.

[Gerrycan]

7 minutes ago, KenONeill said:

I've experimented on a stretch of dead flat road near work, and at ~40mph in top switching the AC in or out makes about 0.1mpg difference.

Better or worse? 

 

Ignore that question. I have complete confidence in your experiment

Edited 15 February, 2017 by Gerrycan

[Breezy_Pete]

"The air conditioning system is the single largest auxiliary load on a vehicle by nearly an order of magnitude."

 

From here. (Some actual science in there, avoid if you prefer waffle like "modern air conditioning systems are very efficient"  )

Edited 15 February, 2017 by Wino

[Gerrycan]

12 minutes ago, Wino said:

"The air conditioning system is the single largest auxiliary load on a vehicle by nearly an order of magnitude."

 

From here.

 

That would be right. The cooling capacity and current/power demand of and automotive air-conditioner is roughly equivalent to a small single phase domestic split system which can service one or two rooms, or a small flat/unit. The current drawn is from 2 to 3 kW.

My 1.4tsi has a maximum of 103kw output but at low speed cruising probably less than 15kW produced so the figures we are bandying around all stack up.

 

Modern domestic units are usually inverter based so can vary the load and current drawn and our Octavia aircon can vary the load on the engine so in my tick over figures what I forgot to mention was that under full load (just switched on for the first time on a very hot day) then the consumption can rise to 1.2L/hour. It settles to the lower figure when the cooling condenser has reached its optimum temperature just above freezing.

Edited 15 February, 2017 by Gerrycan

[Trebor77]

On 2/13/2017 at 21:09, jthyssen said:

On a hot summer day it may be as high as 10% or more extra fuel, but I would expect it to use much less in the winter where the compressor will run less often. 

 

If you turn it off you should remember to turn in on occasionally to avoid build-up of bacteria and mould and to exercise the A/C unit (lubrication etc.)

 

I leave it in auto all year. 

Agreed

[Trebor77]

On 2/14/2017 at 09:29, Geek42 said:

I leave my AC on all year round, but it spends most of the time set to eco via the driver mode custom settings. Eco seems to be more than adequate for comfort, I only switch it up to normal for a couple of weeks each summer when it gets really warm.

 

Whether eco mode is making any difference to mpg...no idea, sorry.

This is a new one on me . I can change the a/c settings ? ??

[Trebor77]

On 2/15/2017 at 08:02, iriches said:

There is no simple answer to "How much does aircon impact fuel economy" as I'll hopefully show...

 

A typical maximum figure for my car as seen in the Maxidot is around 0.25 litres of fuel an hour on a hot UK summer day (and I can hear the Aussies laughing...).  It starts higher than this, but quickly ramps down as the car cools.  However, the impact of that 0.25 l/h varies considerably depending upon how far I drive in that hour.

 

Let's assume that I'm in horrible city traffic, and manage to drive only 6 miles in one hour.  Let's assume in such traffic that my 1.4 TSI car may do 30mpg if the aircon were off.  It would thus have used (6 miles / 30 mpg) * 4.5 litres / gallon = 0.9 litres of fuel.  The aircon for that hour has used an additional 0.25 l/h - taking the total to 1.15 litres.  The impact of the aircon is thus (0.9 - 1.15) / 0.9 = -28%.

 

Congested City

Aircon Off = 30 mpg

Aircon On = 23 mpg

 

However, if we assume that I'm cruising on an free-flowing motorway, and manage to drive 70 miles in one hour, then things are different.  Let's estimate that at this speed, my 1.4 TSI could do 50mpg if the aircon were off.  It would thus have used (70 miles / 50 mpg) * 4.5 litres / gallon = 6.3 litres of fuel.  The aircon for that hour has used the same additional 0.25 l/h - taking the total to 6.55 litres.  The impact of the aircon is thus (6.3 - 6.55) / 6.3= -4%.

 

Free Flowing Motorway

Aircon Off = 50 mpg

Aircon On = 48 mpg

 

(All based on assumptions rather than measured values - but tallies with what I see in what passes for real life these days...)

This is an important point, thanks. The impact of a/c depends on time & speed. For a short local journey the impact is significant, for a longer trip it is much less important. A good point

[KenONeill]

6 minutes ago, Trebor77 said:

This is a new one on me . I can change the a/c settings ? ??

Well sort of; with Climatronic there's an "Econ" mode but all that actually does is disable the AC compressor.

[Trebor77]

This all very helpful and interesting, but I worry about relying on the Car info display for mpg data. I routinely calculate my mpg by dividing my mileage by the amount of fuel I put in at the garage, over a significant period of time. Sadly it is about 5 or more mpg lower than the Car's calculated average.   KK