Aircon increases fuel usage, why ?

snowdog · 30 Jun 2008 at 03:21

Hello, as per title, how does turning on the aircon increase fuel usage between 3 and 10% ? I always thought airco just uses electric power, and the alternator spins at the revs the engine is at not ''faster'' when there's more power needed.

How come does an aircon in a car ( and other electrics) increase the fuel consumption of a car ?

BigglesPiP · 30 Jun 2008 at 03:40

The air con uses fuel 2 ways.

- The engine drive a compressor which makes the air-con work, driving the compressor requires a few bhp.
- There are fans, which use electrical power, which puts load on the alternator, which sucks engine power (albeit a lot less than the compressor).

alternator always spins at engine RPM, but the work required by the engine to spin it varies with electrical load.

snowdog · 30 Jun 2008 at 06:29

BigglesPiP said:
but the work required by the engine to spin it varies with electrical load.

Thank you ! I didn't know, I just thought it always created as much electricity as last time on that rpm, and just dumps everything into the battery, regardless how much devices in the car use it...

Also a compressor :eek:

, I thought it'd just be some very basic fan and not a compressor.

Thanks for explaining

.

BigglesPiP · 30 Jun 2008 at 06:55

The compressor is for the refrigerant liquid/gas, not the air:

Its how you can make air cooler.

Air is blown through the evaporator by a basic fan. The evaporator is very cold, just above freezing, or below freezing in dry climates in turbo mode.

Peter Diffey · 30 Jun 2008 at 09:57

So basically air conditioning is like a fridge, which consumes a large amount of electrical energy.
This work is removed from the engine which is driving your car, thus it costs fuel to power the air conditioning unit.

OllyM · 30 Jun 2008 at 10:01

Can't say I've noticed any difference in consumption with the AC on or off in my car, so I leave it on all the time. Even if it does use a bit more fuel, it makes the car a lot more comfortable

IAmATeaf · 30 Jun 2008 at 10:29

OllyM said:
Can't say I've noticed any difference in consumption with the AC on or off in my car, so I leave it on all the time. Even if it does use a bit more fuel, it makes the car a lot more comfortable

What he said, I've got climate control and it's 'climating'

the cabin all year round.

Peter Diffey · 30 Jun 2008 at 10:31

It would only reduce your MPG by like 2 maximum anyway, car and environment dependant ofcourse

dannyjo22 · 30 Jun 2008 at 10:35

Doesn't opening the window use more fuel than aircon anyway?

PiKe · 30 Jun 2008 at 10:39

dannyjo22 said:
Doesn't opening the window use more fuel than aircon anyway?

Up to 60mph it's more economical to open a window, past that, aircon is more economical.

Peter Diffey · 30 Jun 2008 at 10:39

dannyjo22 said:
Doesn't opening the window use more fuel than aircon anyway?

I guess if you are speeding down the motorway with your window fully down

jamoor · 30 Jun 2008 at 10:52

Yeah, the difference is negligble, one of my friends is convinced it uses something like 30% more fuel, so doesn't switch it on!

Hawk Tuah · 30 Jun 2008 at 11:13

jamoor said:
Yeah, the difference is negligble, one of my friends is convinced it uses something like 30% more fuel, so doesn't switch it on!

If he's driving a supermini, he might notice a fair difference. Although 30% is quite unlikely.

The bigger the engine, typically the less you'd notice it being on. Its also dependant on the type of driving you're doing. Short stop start journeys around town will mean mean the aircon is going to have a big effect on your mpg. On a long motorway journey you'd hardly see any difference.

Raymond Lin · 30 Jun 2008 at 11:31

Isn't that if you drive above 50mph, the fuel consumption is balanced out from the removal of the wind resistence by opening the window to cool down.

Hawk Tuah · 30 Jun 2008 at 11:44

Raymond Lin said:
Isn't that if you drive above 50mph, the fuel consumption is balanced out from the removal of the wind resistence by opening the window to cool down.

That, and, it takes less energy to keep a car cool, than to cool it down in the first place.

Kappa · 30 Jun 2008 at 13:13

IAmATeaf said:
What he said, I've got climate control and it's 'climating' the cabin all year round.

Pete19s · 30 Jun 2008 at 13:15

BigglesPiP said:
The air con uses fuel 2 ways.

- The engine drive a compressor which makes the air-con work, driving the compressor requires a few bhp.

This has always got me confused. The compressor is belt driven and as far as im aware is always being driven while the engine is on. So unless im being thick pushing the AC button isnt suddenly going to cause the engine to do more work?

[TW]Fox · 30 Jun 2008 at 13:18

Pete19s said:
This has always got me confused. The compressor is belt driven and as far as im aware is always being driven while the engine is on. So unless im being thick pushing the AC button isnt suddenly going to cause the engine to do more work?

It's on a clutch so it kicks in and out.

G_D · 30 Jun 2008 at 13:31

jamoor said:
Yeah, the difference is negligble, one of my friends is convinced it uses something like 30% more fuel, so doesn't switch it on!

Bad for the air con never to be turned on. Instead of a few quid more in petrol he is going to be paying many hundreds to have it put right later on.

Davey D · 30 Jun 2008 at 13:43

G_D said:
Bad for the air con never to be turned on. Instead of a few quid more in petrol he is going to be paying many hundreds to have it put right later on.

what that man said. you should at least give it a blast every few weeks, else you will find it wont work, or wont pump out cold air when you eventually get round to using it. even in the winter, just set your blower to warm, then put the aircon on to let the coolant circulate for a while