Gather that cold air con reduces mpg – is it the same if warm air is being pumped out?

Just interested!

There is a mini radiator under your dash. When you request hot air a valve diverts some of the engine coolent through this radiator.

air con can’t deliver warm air really – it’s just a clever fridge to cool the air down.

Key thing is if the compressor is running. Normally, unless you switch the a/c on, you’re just blowing ambient or warmed air so there’s just the fan to consider.

As an aside the radiator for the heating was an engine saver on my Triumph Herald on hot days. heating and fan full on and it would stop it boiling in traffic. Cooked the occupants of the car, of course…

Depends.

Some older cars, the AC switch just switches on the compressor. The compressor then cools the air and the engine heats it back up.

More modern cars switch the compressor on and off as the temperature demands it. This means that in our moderate climate it’s not on very often and therefore doesn’t use up much fuel.

If you have climate control, it should monitor temp, sunlight and humidity and choose appropriate steps – again, doesn’t use up much fuel.

Plus modern compressors are pretty efficient. I’ve found that external factors (namely weather and traffic) affect MPG far more than anything you do with the car’s accessories so it’s impossible to tell. In short, if there is an effect, it’s pretty small.

Remember A/C dehumidifies the air as well as cooling it. After a spring polaris (2 hard days of riding) in horribly damp conditions I got into my car to drive home feeling absolutely terrible. I put the A/C on and within minutes I felt loads better. I think I’d overheated (despite it being 6-7 degrees C) due to the massive humidity, and dry air in the car felt wonderful.

If it’s using energy, it’s using fuel.

Interesting. It’s a ’97 Golf with factory fitted air con (basically a on-off switch) so nothing sophisticated. It’s just today, with the fan blowing and air con off, the car steamed up (not surprisingly I guess as it’s pretty wet-air!), but with air con on, it all cleared instantly.

it will do – the aircon takes the humidity out of the incoimng air so it’s ‘dry’ and works at demist a treat. Having the compressor on will affect mpg, though.

a slightly open window once you’ve defugged the car should allow a/c to stay off.

Anything will to a greater or lesser degree – lights, radio, fan etc. Like I said, if it’s needs energy, it uses fuel.

Interesting. It’s a ’97 Golf with factory fitted air con (basically a on-off switch) so nothing sophisticated. It’s just today, with the fan blowing and air con off, the car steamed up (not surprisingly I guess as it’s pretty wet-air!), but with air con on, it all cleared instantly.

Yes, that happens to me. However with a car that age, I would expect the compressor to be working so gently like this that it would cost you less than half an mpg. You wouldn’t notice, anyway.

If you’re going at any speed using the A/C can use less extra fuel than the drag you get from having windows open. However there are so many permutations of car / speed / ambient temp / no of windows open / efficiency of the system that you can’t be hard and fast. Cars I’ve driven with it you certainly feel the drag on the engine when the compressor is on, it certainly uses more fuel.

Cars I’ve driven with it you certainly feel the drag on the engine when the compressor is on, it certainly uses more fuel

What cars? My ex-gf’s Hyundai Accent with 1.6 engine and Texan compressor was ridiculously slow with it on.

However neither the Prius or the Passat show any discernible difference at all with the aircon on or off. Esp as the aircon button in both cars simply allows the possibility of using the compressor depending on conditions, rather than switching it on for good.

the prius doesn’t have a regular aircon pump. If I remember right its on it’s own electric motor (so it still works when the engines not running) – as a result it can run off the battery (for a period of time) and thus not hit the overall power too bad

But if it drained the battery that would also affect MPG. It is very efficient, but it’s the way it’s managed that is the most significant thing, and that is the same for most modern cars I think.

it would affect mpg, but not cause drag you can feel, as you asked about

I meant there’s no discernable difference in MPG in either car with the aircon on or off. As well as power.

As above, if you have a decent air con system with humidity sensing, you can have it on hot and still be running the air con. Likewise some will disable the cooling above a certain demanded temp as it makes reaching the final temp difficult.

You shouldn’t notice the extra drag of the AC as the car should have a secondary throttle that opens slightly to counter-act the load (like adjusting base idle) but not all do.

Even my old ’91 toyota counters the air con with a secondary throttle and even cuts the air con compressor above 50% throttle so you still get full power. I suspect this is why most people don’t notice much “drag” from their air con when giving it some beans – that and the slight improvement in pump design.

There was an old episode of top gear where they tested two (same) cars against each other, each with same amount of fuel – one with the windows down, the other using the A/C – trouble is I can’t remember which came out on top, but obviously at m/way speeds the A/C is the comfortable option.

Not really valid to the O/P but there you go 🙂

Old cars – yes. Newer (less than 5 years old) no

As an aside the radiator for the heating was an engine saver on my Triumph Herald on hot days. heating and fan full on and it would stop it boiling in traffic. Cooked the occupants of the car, of course…

We employed the same method when driving through Death Valley in a 400 dollar car, 40 degrees outside, heating on 🙂

as the car should have a secondary throttle that opens slightly to counter-act the load (like adjusting base idle)

Or just a switch in the ECU 🙂

My Passat had a couple of switches on the injector pump to boost idle and turn of AC at full throttle.

I’d say AC is definitely better on fuel than having the windows ALL the way down, but you don’t do that, do you? You just crack them a bit.

Chase that was Mythbusters, not Top Gear.

Conclusion was that for best fuel economy under 50mph open a window, over 50mph use the A/C.

More of an impact on smaller engined cars than larger engined cars.

I can’t really tell the difference in my E39.

I usually just keep it on all the time as I have read that turning it off for long periods can cause problems with the pipes and seals.

If it cost’s me £2 every 400 miles to always have demisted windows, or not to arrive everywhere as a angry sweaty maniac, it’s worth it. The kind of people who turn their air-con off probably shouldn’t be allowed to.

(Also – if it’s turned off for a while – the compressor gives out a awful mildew smell when it’s turned back on, like winter riding shoes)

OK, I understand why it could work like this, but I’m surprised… Mainly I’m a motorbike guy, new to cars but with bikes your electrical system is producing charge regardless- you have your alternator spinning away at all times, and the bike basically takes what it needs and the rest is wasted (largely, in my experience, the rest becomes heat and destroys your reg/rec) . So running an electrical appliance such as headlights etc doesn’t impact econony. Are cars different then?

With any generator, car and bike alternators included, they are harder to turn if you take more current from them. So the more current you take off with lights and heated seats etc, the harder the engine has to work. Otherwise, you’d be able to draw an infinite amount of power from a generator 🙂

Of course, this is a moot point in the case of air conditioning compressors since they are usualyl driven from a belt off the crankshaft.

Mine is set to 21c and always on, why would you switch it off?

I notice absolutely no difference in fuel economy with the climate control on or set to econ.

The other halfs old Ford Ka was horrid with the a/c on. Lumpy throttle response & ridiculously sluggish. Apparently on a heavy throttle it stopped the a/c compressor, but it wasn’t noticeable.

I’ve been told if you a/c smells a bit damp, stick it on re-circ with the fan on full whack and spray in a load of neutradol or similar. Not tried it myself, but will do at some point.

Modern, big engined car, leave the air con on, it makes no odds.
Older or smaller engined car, turn it off.

“With any generator, car and bike alternators included, they are harder to turn if you take more current from them. So the more current you take off with lights and heated seats etc, the harder the engine has to work. Otherwise, you’d be able to draw an infinite amount of power from a generator”

Well, the last comment doesn’t fit- it’s not neccesarily a case of getting more power for the same work, but a case of using more of the output rather than wasting it.

Not saying you’re wrong you understand, I don’t understand how alternators/generators work other than that it’s spinny magnet magic 😉 Just trying to understand it. According to the workshop manual, the generator produces a constant amount of power (relative to engine speed o’course) which the bike then makes use of as it will, and the wee thyristor in the reg/rec acts to bypass the “load” and return some of the power to the generator. At which point I have no clue what happens next

Northwind, you’ve kind of got the right idea, but as battery voltage rises/electrical load decreases, the amount of power produced by the alterntor will also decrease, and so will the amount of power required to spin the alternator.

If you want more information on how they work, google brushless alternators. The most basic ones have no regulation (typically only produce a few amps, and any lights will run of unrectified AC power), but may have some form of external regulation, while more advanced ones use exciter field coils to regulate output.

Regarding the original question. Yes.
Whenever the aircon compressor is spinning, extra fuel is being used.
And even with the heating set to hot, the compressor will still be running, unless the outside temperature is below about 4degC. Below that temp, most common aircon systems risk freezing up, which is bad for the system, and there’s no benefit to having it running anyway, as there’s very little moisture left in the air to have any noticeable effect on humidity.

Also, windows steaming up after the aircon is switched of is entirely normal. With the air con on, moisture condenses on the cold evaporator, and drains via a drain pipe (that’s why you get can get pools of water underneath vehicles in hot weather). When you switch the air con of, the evaporator then warms up, and any surface moisture left on it will evaporate and be blown into the passenger compartment, which if the air outside is cooler and windows cold, will condense onto the windows.
Strange odours that appear after you turn the aircon of, are usually caused by mould/bacteria on the evaporator that get blown in by the water evaporating of the evaporator. Best cure for this is a can of air con treatment spray (most you put aircon on full with recirculate on, stand the can in the footwell, activate the can, then shut the door and leave it running for 15min)

Well, the last comment doesn’t fit- it’s not neccesarily a case of getting more power for the same work, but a case of using more of the output rather than wasting it

Power and work are effectively the same thing. In technical terms, power is an amount of work per second.

The alternator generates the same VOLTAGE as it spins, but if you switch on the accessories you are drawing more CURRENT from it. In electricity, power = voltage x current, so the more current you draw, the more power you are taking from the crankshaft. If you don’t draw any current the alternator will be really easy to spin and won’t draw much power from the engine. If you draw a lot of current, the alternator will be really hard to spin (like pedalling up a hill on your bike) and therefore the engine will need more fuel to make it turn.

The alternator does not generate a set amount of energy. It has the ability to generate whatever you draw from it, up to a limit which is dictated by how it’s made, and how much power the engine can produce.

Like the engine on your bike. You can accelerate slowly, or a bit quickly or very quickly depending on how hard you twist the throttle, and use up more fuel. But there is a limit due to the engine’s design 🙂

And even with the heating set to hot, the compressor will still be running, unless the outside temperature is below about 4degC

Depending on the car, mate. Many modern cars are more sophisticated than that.

Ah, so I’m getting voltage mixed up with current mainly. Thanks for that! I didn’t realise the alternator would get physically harder to rotate when it’s producing more current.

Our Galaxy handbook says to leave on Auto @ 22c for optimimum performance. Obviously I knew better so ran it for a few weeks on Eco with windows open. Difference in mpg…none. On the advice of some of the very helpful chaps on the Galaxy owners club site I’ve left it on Auto, as there have problems if its left off for long periods. I think this applies mainly to cars with climate control, not just a/c & manual controls.

Cars I’ve driven with it you certainly feel the drag on the engine when the compressor is on, it certainly uses more fuel

I’ve never noticed any difference on any of the diesels i’ve driven. This problem was very apparent on a 1.8L 3 Series BMW petrol I drove 5-6 years ago, but not the little 1.4 Honda Jazz I was driving last week.

I guess thechnology has moved on a little.

Depending on the car, mate. Many modern cars are more sophisticated than that.

Very true, mate.
But I was keeping things simple, and you’ll find all modern cars are more sophisticated 😉