Air-con dilemma

[Vanny]

Aircon will have much the same effect whatever the speed.

You'd be surprised, high higher RPM the compressor can get away with applying less torque to the engine, so as a load function, will use less of the engine power at higher speeds.

Also at higher speed you can usually get a better result from ram air, so the inlet air is pre compressed and cooled, so you need less AC performance, and thus less wasted fuel.

Anything over 40 mph and your better off (on any age of car) with the windows shut. Modern cars, that is more like 25mph, mostly due to body hole leakage.

Modern light weight compressors with variable displacement and SCX use very little energy from the engine. The climate system will actually use more power from the blower motor and engine fans. It's a shame the BX AC system has the fans on continuously, its probably the most inefficient part of the design.

[mat_fenwick]

It's a shame the BX AC system has the fans on continuously, its probably the most inefficient part of the design.

But at higher road speeds surely the ram air will help spin the fans, therefore meaning they'll draw less current?

[Vanny]

the fans stall in the ram air and current draw goes up, its like putting your finger in. Not sure about BX fans, but on most cars once you get the 40mph the fans are actually restricting air flow. Could be interesting to see what current they do draw at various road speeds . . . .

[kiwi]

I think aircon vs opening a window using more fuel depends on the speed you're travelling. Aircon will have much the same effect whatever the speed, while the effect of an opening will increase with speed.

As for comparing the fuel consumption of different cars, there are so many mechanical factors as a result of wear that there are too many variables to put fuel consumption down to 1 thing.

Just to clarify something I said which I guess was aimed at me.

I have noted no differance in Fuel Consumption between driving with Aircon on or off! I keep pretty consitant records any diferrance could be explained away by the terrain/driving style but any effect caused by aircon is minimal and nowhere near the figures the fear mongering Autombile clubs like to qoute. Maybe thats just the design of the BX aircon I dont know, but I do tend to switch it off and use windows open with speeds of 50kph/30mph or below.

[citsncycles]

I've never had aircon myself (at least half the cars I've owned didn't even have heater fans!) but both my brother's Toyota Previa and my Dad's Rover Stirling used to show a 1-2 mpg drop when using the aircon - you could actually watch the figures drop on the Rover's display! A mate with a Xzara has seen the same thing, although it has to be said that driving style seems to make more of a difference in that case.

[mat_fenwick]

both my brother's Toyota Previa and my Dad's Rover Stirling used to show a 1-2 mpg drop when using the aircon

Bear in mind that a 1-2 mpg instantaneous drop only works out to a 0.5-1 mpg overall drop if the compressor cycle time is 50%. And the BX system lets you adjust that manually, so you can just have the compressor kicking in even less than that.

[MULLEY]

A/C saps power from the Mini & thats supercharged, no idea about mpg, probably worse but not noticeable as its so crap anyway, haha...

[kiwi]

There is very little point in comparing the BX Aircon technology from the 80s with that of a more modern mellenium era car!

[Wooscary]

I had a Passat where if you turned the air con on all the lights would go really dim and the engine would stall.

Something to do with the air con belt being attached to the aux belt and the air con pulley having something called
"irreparable corrosion". ???

Oh yeah, then the belt went on fire, at a petrol station, where a fire engine was filling up. They didn't seem worried.

[Vanny]

There is very little point in comparing the BX Aircon technology from the 80s with that of a more modern millennium era car!

Why not? Surely that is how developments in technology are made? And while it might be 30 years late, I'm still developing my BX from where Citroen gave up. High intensity brake lights, central locking from both doors, remote rf locking, instant open windows on both sides etc

A simple cut off for the air con engine fan relay at a certain speed threshold isn't beyond my electronics skills, so that is taking modern technology, applying it to 70's technology (when most of the BX development was done), and improving efficiency.

It is, I'll agree, completely pointless, but as an engineer it ticks boxes and makes me happy!

[kiwi]

Vanny I was refering to fuel economy impact, not the technical side of things. But if you want to go there then fitting a temperature control switch would be an advance over the on/off basic slider.

[Kitch]

Well, I think following all this I'm just going to leave it in the grey car. Trying to turn some money over on it might relieve tensions at home about how much time I spend away
It's not like it works in the grey car anyway, it's just the guts are there.

[Tim Leech]

Well, I think following all this I'm just going to leave it in the grey car. Trying to turn some money over on it might relieve tensions at home about how much time I spend away
It's not like it works in the grey car anyway, it's just the guts are there.

When I bought it originally in 1998 it didnt work then! And by 2005 it had been disconnected and the compressor was in the boot.

[Kitch]

Yeah I assumed as much it had been in bits for ages.

I *might* chav the recirc function and heater motor etc for BAH and have a non-aircon car with heater recirc. I actually find that useful....just in case you're following something that stinks!

So for now, plan with grey car is going to have to sort all the rust, sort all the suspension pipes and sort the engine out.

Nothing big then