I'm new to air con. General advice?

BeenThroughItAll

n217970 wrote: »

Now is that from the computer or actually calculated from fill ups? As I have found the computer to be wildly over optimistic; mine is currently showing 64.2. With the AC on it would be around the 58-59mpg mark. 55mpg on the computer in my car would be around 47mpg calculated.

Brim to brim, calculated properly as all my MPG figures are - although I found the fuel computer was actually extremely accurate on the S80, never more than about 1-2mpg out. V70 wasn't hugely out either, but was less close to reality than the S80. If yours is 8-9MPG out I'd suggest there is something not quite right somewhere in general, possibly with ECU calculation of load resulting in injector trim changes out of spec. That could account for a large change in consumption for a specific load factor like AC being turned on.

n217970

BeenThroughItAll wrote: »

Brim to brim, calculated properly as all my MPG figures are - although I found the fuel computer was actually extremely accurate on the S80, never more than about 1-2mpg out. V70 wasn't hugely out either, but was less close to reality than the S80. If yours is 8-9MPG out I'd suggest there is something not quite right somewhere in general, possibly with ECU calculation of load resulting in injector trim changes out of spec. That could account for a large change in consumption for a specific load factor like AC being turned on.

Nope afriad not, take a look on Volvo owner forums, computer is well know for being 15-25% out.

BeenThroughItAll

n217970 wrote: »

Nope afriad not, take a look on Volvo owner forums, computer is well know for being 15-25% out.

Well, none of mine:


2002 S40 1.9D
2003 V40 1.8BiFuel
2003 S80 2.4 Auto
2004 S80 D5 Manual
2002 V70 D5 Manual


have ever been anywhere near that far out.


I still maintain that a 10% decline in consumption is indicative of an ECU controlled fuelling-trim issue. Modern AC systems simply are not that inefficient.

Gloomendoom

Presumably, using aircon has a proportionally greater effect on fuel consumption the slower the car is driven.

That may explain why there is such a variance in additional fuel consumption experienced.

Apodemus

n217970 wrote: »

So, without quoting posts, to answer some of the 10% sceptics - it is a 2005 Volvo S60 D5 (so plenty of power) that spends nearly all its time wafting along at 50-60mph. It regularly returns 55mpg brim to brim on this kind of commute with the AC off, with it on it manages around 50mpg. Given I am an incredibly boring individual I have tested this theory many times and apart from small variances I find the results conclusive.

I presume it has an impact of around 10% as at 50mph the engine has very little load on it and even a "small" amount extra is quite large proportionally. Maybe if I were stop/start 30mph driving it would account for a smaller % but I can only comment on what I have seen.

My failure to show impact is on a 03 Volvo V40 1.8 GDI petrol, calculated on 5,000 miles of mainly motorway from North of Scotland to South of France and back, at an average of 42mpg.

Apodemus

BeenThroughItAll wrote: »

Well, none of mine:


2002 S40 1.9D
2003 V40 1.8BiFuel
2003 S80 2.4 Auto
2004 S80 D5 Manual
2002 V70 D5 Manual


have ever been anywhere near that far out.


I still maintain that a 10% decline in consumption is indicative of an ECU controlled fuelling-trim issue. Modern AC systems simply are not that inefficient.

Again, my trip computer mpg has always been pretty accurate on the 03 V40.

Carrot007

I genrally leave my aircon on all year round. the mpg drop is greater when opening your windows (but sometimes that's fun!).

On my current I turn it off when cold be only because there is something odd in the heat exchanger (at a guess, i does become hot!) that is not easily fixed and I get no hot air with it on.

Mercdriver

n217970 wrote: »

Nope afriad not, take a look on Volvo owner forums, computer is well know for being 15-25% out.

Proportionally, on a forum, how many people express how happy they are with things rather than unhappy with things?

I'd suggest that it is probably 1 in 10 at best, so your sample is unlikely to be representative of actual experience.

On a Volvo owners forum you are less likely to have people saying things in a defensive manner...

thescouselander

An AC compressor can't use much more than 2 or 3 horsepower and the duty cycle is generally less than 50% unless it's a really hot day. The effects on MPG are going to be pretty negligible.

Also on modern cars many of the ancillaries are switched in and out at the right time to maximize fuel economy. So, for example, the AC compressor will disengage under a high load scenario and engage when the car is slowing to recover energy from the cars momentum.

sheramber

n217970 wrote: »

Now is that from the computer or actually calculated from fill ups? As I have found the computer to be wildly over optimistic; mine is currently showing 64.2. With the AC on it would be around the 58-59mpg mark. 55mpg on the computer in my car would be around 47mpg calculated.

But i the computer was 10% out it would be 10% out on both with and without AC.

One time I checked without air con on the journey to a place, then on the way home with air con I used less fuel on the way back.