[peter messenger]

does anybody know how much power the air con takes to run. i really feel the difference in terms of power loss and smoothness of the engine when it is on.

 

[Mark R]

What year and model are you running ?

Acc kicking in will drop the revs slightly (best noticed at tickover). But it shouldn't have any noticeable performance effect when driving.

 

[mark e]

Not quite true I'm afraid Mark in the case of the 9000 at least. Running the aircon will lead to an ovreall increase in running temps partly due to increased load, but also because the air con rad being on will mean the ambient air is a little warmer before it gets to the intercooler and subsequently radiator. Thus the intake air wil also be warmer, meaning it's less dense and at the extreme also more likely to suffer from pre-detonation leading to a reduction in power by the APC.

IIRC under high engine load, the aircon compressor is automatically disconnected.

Maybe it's an interesting test to try at the next RR day?

 

[john]

Originally posted by Mark E:
[qb]Maybe it's an interesting test to try at the next RR day? [/qb][/b]

The first rolling road I went to I left the ACC on by mistake

 

[sgould]

I did two successive trips from Watford to Devon and back last Winter/Spring. Both non-stop each way on motorway with cruise, set at "normal motorway speed", on as much as possible.

The first with the aircon on ECON and the ambient temp near freezing, so compressor stayed off: 35mpg

The second a month later, same conditions except temperature around 16deg C, causing the aircon to switch on: 32mpg.

 

[steisan]

I think I red somewhere that a compressor steals, just to be driven by the belt, around 15hp.

Don't know about the loss due to the condersor in front of the IC, I'm afraid it's much more difficult to estimate because it depends on outside temp, engine temp, stage of tuning of the engine...

 

[mark e]

Hmm... I'm not sure exactly how much Stefano, but if we take the 15hp , that's roughly 11kW. Heat pumps normally run at around 3:1 output for work input, so that would equate to a cooling capability of 33kW or so, which seems a little high! I'd estimate that an aircon system can provide about 2-3kW of cooling. In power terms this would require an input at most 2kW, which leaves about 9kW to be lost in the rest of the system, which is an awful lot- presumably most of it has to go to the condensor, which would explain why there is a significant rise in the other temperatures...

 

[murphwiz]

AC compressor has a 'swept volume' of approx 160 cubic CM..
I'm sure some clever bod will be able to work something out from that!
15Bhp sounds a little high, I was always told around 3-4bhp?

Edit>

Found some more info, not sure if it applies to 9k, but 9-5 and probably 9-3 have variable displacement compressors..
The displacement and thus power used is variable dependant on how much cooling is required.
Very clever

 

[ms08]

I would have said about 3 or 4 bhp also not to mention the fan and should you be driving at night - headlights with their load on the alternator which again will sap some power. It does seem to make a noticeable difference but is it likely to be greater than changes in air temp/density?

 

[Mark R]

Originally posted by Mark R:

But it shouldn't have any noticeable performance effect when driving. [/QB][/b]

I perhaps wasn't as clear as I'd intended to be in posting this quote. I agree with Mark E to a point that there will be an effect, but I perhaps diverge with his opinion on the extent of the effect or how noticeable it is (or should be if your car/acc is fit).

There is an effect on the engine loading - most noticeable at tickover - hence the engine hunts when acc kicks in as it draws extra power to run the compressor. But on a properly sorted car/acc system then though there might be a small mpg penalty and a very slight performance issue, normally there should be little effect in real world driving - eg the commute or a motorway cruise at within limit speeds.

As ever there's a trade off between wringing out the last mpg drops or wringing out absolute top end performance, and no doubt on a hot day you wouldn't hit the peaks of performance or mpg with it on.

And on a track day - or a quick cross country hustle where you're up the revs to keep in the power band (but within the speed limits natch ) - I do agree with Mark E - switch it off and have some fun.

 

[mikehunter]

On my '97 2.0lpt there is a very marked loss of acceleration when the aircon is on. Also I've not seen any sign that the compressor disconnects when the engine ***load*** is high (though it might at high revs - I'm not sure).

I don't think I'd notice a change of 3 or 4 bhp - seems to me that the 15 or so estimate is more likely.

I will be having the car rolling-roaded soon so will try to remeber to get it tested with aircon on and off and post the results.

 

[peter messenger]

i have a 2.0lpt 1997
i agree with mike, if i want more power overtaking i switch off the aircon the difference is easy to feel. i would say 10/15 hp

 

[cdcarlsson]

I'm pretty sure the A/C is switched off (by disengaging the clutch electricaly) when W.O.T. is used.

Nick.

 

[Mark B]

The 9000 owners' manual states that the compressor is declutched "at full throttle", regardless of engine speed.

Trying to measure the power difference on a rolling road will therefore be a bit of a challenge. Even if you prevent the compressor disengaging at full throttle, the power consumption of the compressor depends primarily on the temperature and above all the humidity of the air that it is trying to cool. So if you find a nice hot sweaty day, and turn the recirc off and set the internal temp to LO, the compressor should be working hard.

I still would be surprised if it is more than 5kW though.

 

[markcanderson]

The effect is a heck of a lot more noticeable on lower torque cars under acceleration.

I used to have a normally aspirated 2.3 GM900 with aircon... as has been said, Aircon disengages on hard accleration. When I used to put foot down, the effect of the aircon disengaging on the acceleration was noticable, it was like throwing a passenger out the window.

On the higher torque cars.. such as my TiD or the 9000 you don't notice it at all.

 

[Mark R]

Having read the posts since my last I'd say that a 15 HP difference (eg 10% for a standatd 2.0 lpt) is so big as to be unlikely. I can't imagine Saab would have thought this within acceptable design tolerances.

I would agree 3 - 4 HP would be the more likely normal effect of acc operating, which I still say is not especially noticeable in day to day driving. If it's operation is having a very marked effect a service of the vehiche/acc should be considered or pursued.

It wouldn't be a design feature for normal operation to cause such marked difference bearing in mind you might have an accident if the reaction times or power was taking a 10% hit when acc kicked in each time and the difficulty of predicting exactly when this would be. Overtaking springs to mind where a sudden 10% loss output could have immediate and possibly fatal consequences.

However, noticing Mark A's post (and agreeing) - that on higher output vehicles the difference is even less noticeable (probably because the power drain is proportionately that much less in comparison with a standard car) - I should have borne in mind that my car's been abbottised - so puts out an extra 65 BHP/50lb ft over the standard 150 bhp, so that'd explain why I don't notice a whole lot of difference.

 

[billj]

Originally posted by MarkA:
[qb]When I used to put foot down, the effect of the aircon disengaging on the acceleration was noticable, it was like throwing a passenger out the window.[/qb][/b]

Presumably your passengers noticed the effect even more than you did?

 

[markcanderson]

Not that I condone that sort of behavior... he he unless it's one's mother-in-law

 

[ms08]

[/QUOTE] When I used to put foot down, the effect of the aircon disengaging on the acceleration was noticable, it was like throwing a passenger out the window.

Must remember that ploy when I am trying to outdrag someone with passengers on board.

 

[mark e]

Hmm... I think we've probably missed something along the way here folks... namely that I reckon it's the torque, not the hp taken to drive the compressor that's the problem... if we assume that the compressor has some sort of clever internals to vary the load according to speed, then at lower revs, it will proportinately be robbing the engine of a lot more torque than at higher revs, and thus the effect will be more noticable?