A/C effect on mpg

[Trebor77]

13 minutes ago, KenONeill said:

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

Does this not mean that the a/c is turned off entirely ?

[Trebor77]

15 minutes ago, KenONeill said:

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

Does this mean that it just turns the a/c off ??

[Trebor77]

10 minutes ago, Trebor77 said:

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

Correction, should read 'about 5 or more mpg worse' (sorry)

[Geek42]

53 minutes ago, Trebor77 said:

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

 

I was referring to the driver mode settings, not sure what the official name of it is, but you can have eco, normal and sport. In a manual it affects the amount of steering assistance, the throttle response (misleading labelled 'engine') and the A/C settings. In a DSG it also affects the gear change points and the coasting feature and I think there are other options included (cornering lights maybe?) that I don't have.

 

I have mine in custom mode with engine (throttle linearity) = normal, steering = sport (means least assistance) and A/C = eco.

 

The compressor still runs, but on the full coldest setting it's not quite as cold as the 'normal' setting, so maybe it runs the compressor at low speed or something.

[KenONeill]

1 hour ago, Trebor77 said:

Does this mean that it just turns the a/c off ??

It turns off the chiller function, but it will still try and deliver your demand cabin temperature as best it can with no dried and below ambient air available.

[Trebor77]

37 minutes ago, KenONeill said:

It turns off the chiller function, but it will still try and deliver your demand cabin temperature as best it can with no dried and below ambient air available.

I know quite a lot about Thermodynamics. I do not understand this, sorry. How can it possibly do so??

[Warrior193]

2 hours ago, Trebor77 said:

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

My Octavia does not appear to give quite as large a discrepancy as that Trebor - 44 MPG on the computer averages around 41 MPG fill to fill (on level ground and usually at the same pump stand) for me.

[Trebor77]

21 minutes ago, Warrior193 said:

My Octavia does not appear to give quite as large a discrepancy as that Trebor - 44 MPG on the computer averages around 41 MPG fill to fill (on level ground and usually at the same pump stand) for me.

But 3mpg difference is enough to cloud the results from testing the A/C effect

[Warrior193]

22 minutes ago, Trebor77 said:

But 3mpg difference is enough to cloud the results from testing the A/C effect

Hi Trebor, but I use the same data from the MaxiDot for Climate Control on or off for comparison - so the same error applies for both conditions.

[Gerrycan]

6 hours ago, Trebor77 said:

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

I would not worry about it as you now know the variance and can allow for it as it will be consistent across your returns.

Same as your analogue speedo is almost guaranteed to be optimistic and once you determine the variance with a GPS you allow for it.

i regularly travel at illegally indicated speeds legally.

[Geek42]

16 hours ago, KenONeill said:

It turns off the chiller function, but it will still try and deliver your demand cabin temperature as best it can with no dried and below ambient air available.

 

Are we discussing two different functions? Setting AC to eco in the driver mode settings definitely still produces below ambient temperature, the compressor is not turned off.

[SWBoy]

12 hours ago, Gerrycan said:

Same as your analogue speedo is almost guaranteed to be optimistic and once you determine the variance with a GPS you allow for it.

i regularly travel at illegally indicated speeds legally.

The speedo is legally required to be optimistic in the EU.

 

It is not allowed to under-read but must not over-read by more than 10%, so most modern cars (with electronic speedos) over-read by about 5% on new tyres.

[KenONeill]

18 hours ago, Trebor77 said:

I know quite a lot about Thermodynamics. I do not understand this, sorry. How can it possibly do so??

If you have to ask how it can try and maintain a demand temperature of, say, 18C with an ambient of, say, 16C then you clearly don't know as much about thermodynamics as you think!

[pist0nbr0ke]

1 hour ago, KenONeill said:

If you have to ask how it can try and maintain a demand temperature of, say, 18C with an ambient of, say, 16C then you clearly don't know as much about thermodynamics as you think!

 

To be perfectly honest, when I read that statement from the OP originally, it made me wonder about their original question - surely they would be better qualified to calculate the effect than anyone on here? 

[Trebor77]

On 2/14/2017 at 17:08, bertJ said:

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? 

This is interesting.   In what ways have modern a/c systems (with climate control) become so much more efficient than older versions?  They both use  a volatile refrigerant fluid, a compressor driven by the engine, a condenser, a throttle valve to drop the pressure, and an evaporator? Which of these has become so much better? I'm very interested in these things

[Trebor77]

On 2/15/2017 at 08:34, Wino said:

"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"  )

Hi & sorry I only just picked this up!   I've looked through it and I find it difficult to interpret.

The Introduction states that the average vehicle doing an average 12000 miles per year uses an additional 62 gallons of fuel because of its a/c.  If I do 12000 miles at 50 mpg I use 240 gallons. 62 is about 25% of 240 ??

The report is USA and 'averages' are messy to interpret, but what conclusions did you draw?

[Gerrycan]

3 minutes ago, Trebor77 said:

Hi & sorry I only just picked this up!   I've looked through it and I find it difficult to interpret.

The Introduction states that the average vehicle doing an average 12000 miles per year uses an additional 62 gallons of fuel because of its a/c.  If I do 12000 miles at 50 mpg I use 240 gallons. 62 is about 25% of 240 ??

The report is USA and 'averages' are messy to interpret, but what conclusions did you draw?

Do you really not know that US gallons are smaller than Imperial gallons?

[Trebor77]

On 2/17/2017 at 11:49, KenONeill said:

If you have to ask how it can try and maintain a demand temperature of, say, 18C with an ambient of, say, 16C then you clearly don't know as much about thermodynamics as you think!

I think we have drifted off my original Q a bit?  I was trying here to understand how disabling the a/c compressor (via the 'econ' setting)  could allow the a/c to operate., ie 'cool' the ambient air. If the ambient T is 16, and I want 18, I turn on the heater.

 

My original Q concerned the mpg drain caused by the a/c in cold weather. 

[Trebor77]

23 minutes ago, Gerrycan said:

Do you really not know that US gallons are smaller than Imperial gallons?

bigger by 83%, so reduce 25% to 21%

[Gerrycan]

4 minutes ago, Trebor77 said:

bigger by 83%, so reduce 25% to 21%

And the bottom of page 1 the average consumption for American vehicles is quoted at 8.3km/L or 19.6 miles per US gallons.

 

This is going to take a loooong time if you insist on being spoon fed every detail.

Edited 18 February, 2017 by Gerrycan

[bertJ]

57 minutes ago, Trebor77 said:

This is interesting.   In what ways have modern a/c systems (with climate control) become so much more efficient than older versions?  They both use  a volatile refrigerant fluid, a compressor driven by the engine, a condenser, a throttle valve to drop the pressure, and an evaporator? Which of these has become so much better? I'm very interested in these things

This was an assumption I made when comparing modern cars equipped with A/C with cars I owned in the 1990's. I specifically mentioned a 1994 Isuzu Trooper that had a very noticeable power drop when the A/C was turned on. You could hear a clutch engaging (compressor?) and the rev counter on tick over would drop about 200 rpm. Also there was a very marked and obvious increase in fuel consumption. The car had a 3.1 litre turbo diesel that produced only 112 bhp; with non A/C fuel consumption was about 32 mpg dropping to about 24 mpg with the A/C on. Such figures would be unacceptable today.

I can't imagine a similar A/C demand being inflicted on modern engines especially those producing less than 100 bhp so I would be very surprised if development of A/C systems hasn't kept pace with engine development. However I'm more than willing to be corrected by A/C experts on the forum.

[Nathanio]

And by leaving it off you dry out the seals and damage the system. Any savings in MPG will be eradicated by fixing the system!

[Breezy_Pete]

3 hours ago, Trebor77 said:

Hi & sorry I only just picked this up!   I've looked through it and I find it difficult to interpret.

The Introduction states that the average vehicle doing an average 12000 miles per year uses an additional 62 gallons of fuel because of its a/c.  If I do 12000 miles at 50 mpg I use 240 gallons. 62 is about 25% of 240 ??

The report is USA and 'averages' are messy to interpret, but what conclusions did you draw?

 

I didn't look at the details much, I just thought that sentence that I quoted was a good illustration of how power-hungry AC systems are/can be.  Changing the temperature of stuff takes a lot of energy, and always will do (as you appreciate, I'm confident). The exact amount of power used will vary enormously depending on the ambient temperature and humidity, and the set temperature in the cabin, as is reasonably obvious.  No-one can ever say "my AC increases consumption by x" unless they specify the exact conditions they are referring to.

 

What you could do, with reasonable ease, is find someone with - or use your own - VCDS capability to watch fuel injection quantity in real time while running with and without AC on, in whatever conditions interest you.  The ratio will tell you how much difference it's making to your fuel use.

 

 

1 hour ago, Nathanio said:

And by leaving it off you dry out the seals and damage the system. Any savings in MPG will be eradicated by fixing the system!

 

That's not been the case in Skodas for some considerable time.  Our 2005 Fabia trickles the refrigerant around at a few percent of pumping capacity even when AC is off, for just this reason.  I think all Fabias right back to 2000 are the same. I doubt that any more recent models don't do something similar.

Edited 18 February, 2017 by Wino

[MoggyTech]

The older aircon systems used a magnetic clutch to engage the compressor, when these systems were off, no aircon fluid flowed and seals would dry out and eventually fail. The system in the Octy is the newer constant flow type, and the level of aircon flow is controlled by a valve. On my 1.4TSi I can set the aircon to light, normal or heavy. I leave it on all the time set to light and do not see any noticeable MPG hit.

 

The old AC compressors where basically just a small engine with pistons inside and it took considerable engine power to drive them. IIRC the new ones are vain drive and require far less power to keep them turning.

[Trebor77]

On 2/18/2017 at 14:43, Gerrycan said:

And the bottom of page 1 the average consumption for American vehicles is quoted at 8.3km/L or 19.6 miles per US gallons.

 

This is going to take a loooong time if you insist on being spoon fed every detail.

A good point, thanks, I assumed 50 mpg, so this gives us about 40% of 21 mpg, ie 8 mpg. This sounds more sensible. It fits with my last car's a/c penalty. Thanks again. K