OT: fuel used by air conditioner

[Scott]

My car has the usual miles per gallon display. Does this take account of the air conditioning or is it only measuring the fuel used by the engine? Or does it come to the same thing?

[Jim Stewart ...]

titter titter

[mm0fmf]

Nice troll.

[charles]

The air conditioning is, usually, powered by the engine so the fuel consumption will include its load on the engine.

[alan_m]

Same thing. The engine works harder to drive the AC hence uses more fuel, and therefore fewer miles per gallon.

[Jim Stewart ...]

or a clueless wummin

[Brian Gaff]

Well since its driven by the engine, then surely it uses fuel. Brian

[Scott]

Thanks for clarifying that. The reason I asked is that the scare stories are that the aircon wastes vast amounts of fuel. It seems to me that it is fairly modest in practice. I have noticed that the car loses some power when the aircon is in use.

[Harry Bloomfield Esq]

Distance covered is worked out from the ABS pulses from a wheeel. Consumption is estimated by how long the injectors are open and the fuel pressure. The two figures allow a calculation to be made of mpg.

The a/c is driven by the engine, so will be included as part of the calculation, because the injector will be open for a slightly longer amount of time.

[Harry Bloomfield Esq]

Negated by the fact that windows can be closed, so car has lower wind resistance, net result depending on the car - it might make no difference to mpg.

[Andy Burns]

My car guestimates the fuel consumption of the A/C, it varies depending how hot is is outside, and how cold you set it, and whether you're idling, it's generally under 0.5 litres per hour.

[Brian]

I tend to run the a/c all year round. * Once or twice I’ve tried checking the difference in mpg using the built in system. I’ve never spotted a difference.

• Partly to ensure the seals etc stay lubricated etc but, in winter, it speeds up demisting windows.

[Jamesy]

Yes it is the same thing.

[alan_m]

About 3% loss and the engine may need a tiny bit more throttle when pulling away.

Opening a window at speeds over 30 mph probably uses more fuel than AC.

[Paul]

When a car is on flat level ground, external wind speed is zero, fuel is used to battle against air resistance. This is especially important at high speed, and when surpassing the speed limit.

This can amount to an engine consumption of 20HP (15kW equivalent). If you drive a big high-box van for your work, it could be using 40HP slamming through the air.

The auto air conditioner, you can tell from the width of the belt used, the energy needed is over 1kW. It could be close to 3kW or 4HP. When the cabin of the car is close to target temperature, the AC clutch disconnects the compressor from the engine, so no power is needed. It will cycle in and out, maintaining the temperature. While it is cycling, the average over time might be less than 3kW.

So yes, it does affect fuel consumption. A good consumption meter would reflect the impact of AC, every time the AC switches in and engages it's clutch plate so it can spin up. It might vary between 18 miles per gallon and 22 miles per gallon as the AC switches in and out. YMMV.

*******

Depending on your OBDII scan gauge, it may have an option to read out fuel economy. So even if your car lacks a fancy dash display, to tell you the fuel economy, with a scan gauge you can display such info. If the car simply does not collect such info, then of course a scan gauge cannot tell you what fuel is being used. Using an OBDII scanner required further research on your part, to find whether your car has the info, and what the cheapest scanner is to read it out. The scan gauge plugs into a connector in the driving area and it runs off a 12V pin inside the connector. You could leave the scan gauge running for the entire trip, or turn it off and look for a "summary" readout option if it exists.

You want to be on a flat section of road on a windless day, to reduce factors which may make things look bad. Some prairie drives are excellent for this sort of work. Hold the car at a constant speed, while the AC cuts in and out, to compare consumptions. And you don't want to be on a serpentine road either - a straight road not demanding of acceleration helps hold things constant.

But this sort of thing is if you want to pretend to be a scientist and get approximate numbers for your own beast. If you are Simon Leadfoot, then the impact of AC could be quite small by comparison to your gas-wasting driving style.

Rolling down the two front windows also wastes gas, as well as making the cabin noisy. So that's not a free lunch either. Selecting car finish and materials to reduce solar gain can help -- some drivers question the merits of modern vehicles having glass roof areas, because they can cook the driver.

I have never seen any merit to driving a black painted vehicle. When you first get into the vehicle, the cabin temp can be incandescent. Some people for example, re-paint the car to a matte black finish and I cannot think of a finish worse than that for solar gain.

Paul

[The Natural Philosopher]

Interesting factoid, the faster you drive the less impact aircon will have on the mpg.

Sad really because its when stuck in traffic you need it most

[RJH]

Depends on the way the car calculates mpg (if it's actual, metered, fuel, or an algorithm of other variables), but whatever, almost certainly yes.

The amount of fuel used will depend on the vehicle - smaller cars will use a far higher proportion of fuel driving aircon than a larger more powerful car. So you'd be more likely to notice the affect on fuel consumption in a Ford Ka than a Mondeo.

Also, ISTR reading that opening windows over a certain speed has a bigger impact on fuel consumption than using the aircon - so by that reasoning windows closed and aircon on at motorway speeds.

[John Rumm]

Yes, probably :-)

Some compute average consumption simply based on the historical average. So factors like how you drive and in what conditions will affect that. Use of air con will also factor in with time.

[Scott]

Thanks for the informative explanation. I did not realise a clutch plate was involved. This explains some of the curious noises and the lurching feeling when the AC kicks in. .

[Scott]

Mine is a Nissan MIcra 1.2 litre so I'm guessing this counts as a smaller car. The AC makes a difference especially going up hills.