Fuel consumption in the cold.

[raf_gti]

I've noticed in the 'how is your coping in the winter' thread that several owners have mentioned that their fuel consumption has dropped by a fair bit.

What would the reason for this be?

I was always under the assumption that with the colder air providing better efficiency that the opposite would be true? Combine that with likely lower speeds then surely you would be using less fuel?

[marshalla]

raf_gti said:

I've noticed in the 'how is your coping in the winter' thread that several owners have mentioned that their fuel consumption has dropped by a fair bit.

What would the reason for this be?

I was always under the assumption that with the colder air providing better efficiency that the opposite would be true? Combine that with likely lower speeds then surely you would be using less fuel?

Quoted before you realise what you've said and try to remove the evidence.

[Motorrad]

Higher anciliary load on the vehicle?

Tyre pressures lower because they haven't been checked since summer?

I'd have expected better economy as well as fuel contains more energy by volume at lower temps (I think).

[anonymous-user]

It does take longer for engines to reach their most efficient operating temp though, so people doing lots of short runs would presumably see a disproportionate drop in mpg.

Interestingly for the first time ever my mpg hasn't dropped in the cold, I get 25mpg no matter what I do

[buzzsaw]

For me its all the time spent spinning tyres and going nowhere in this bloody snow

Edited by buzzsaw on Tuesday 7th December 18:01

[Colin 1985]

The air you are driving through is denser though isn't it?

[60]

Cold air is denser so you get more of it into the cylinders, so the ecu injects more fuel to keep the mixture right.

[Wattsie]

Various heated screens/mirrors/seats on about the place wouldn't help either, I'd guess?

[Egg Chaser]

60 said:

Cold air is denser so you get more of it into the cylinders, so the ecu injects more fuel to keep the mixture right.

This.

[nammynake]

Maybe just down to slower driving in lower gears?

e.g. compare MPG at a steady 50 mph in top gear with that in crawling traffic using 1st/2nd.

[varsas]

Egg Chaser said:

60 said:

Cold air is denser so you get more of it into the cylinders, so the ecu injects more fuel to keep the mixture right.

This.

Cold air makes the engine more volumetrically efficient (it can suck in more air molecules, because the air is denser, so you get more power) but less thermally efficient, basically some of the energy in the fuel has to go into heating the air up giving less useful work done for a given amount of fuel. It's because the air is colder though, not because it's more dense (of course both things are linked, cold air = dense air and vis versa); unless you have a car which can't adapt to different air temperatures.

As others have said, the longer time spent with the engine cold (which means, amongst other things, thicker oil) will probably make more difference to most journeys.

[coogy]

Egg Chaser said:

60 said:

Cold air is denser so you get more of it into the cylinders, so the ecu injects more fuel to keep the mixture right.

This.

Yup

[raf_gti]

varsas said:

Egg Chaser said:

60 said:

Cold air is denser so you get more of it into the cylinders, so the ecu injects more fuel to keep the mixture right.

This.

Cold air makes the engine more volumetrically efficient (it can suck in more air molecules, because the air is denser, so you get more power) but less thermally efficient, basically some of the energy in the fuel has to go into heating the air up giving less useful work done for a given amount of fuel. It's because the air is colder though, not because it's more dense (of course both things are linked, cold air = dense air and vis versa); unless you have a car which can't adapt to different air temperatures.

As others have said, the longer time spent with the engine cold (which means, amongst other things, thicker oil) will probably make more difference to most journeys.

I like that answer.

Even if it's actually 100% bks it's the one I'm going to go with

[Rammy76]

Diesel also has a lower calorific value in winter due to additives requires to keep it flowing freely.

[scarebus]

yip I agree I have gone from 17.9MPG to 16.3MPG in the last 2 months, exactly the same drive to work...... and filling up at my local BP @ £1.34 per liter I cant wait for it to warm up!!

[Sam.F]

22mpg average for me at the moment compared to my norm of 26-27. Mostly I assume because the heater is on full whack all the time and the traffic hardly moves.

[Dave Brand]

60 said:

Cold air is denser so you get more of it into the cylinders, so the ecu injects more fuel to keep the mixture right.

If the mixture's correct the efficiency of the engine will remain the same, irrespective of inlet air temperature, so the only effect of cold air will be more power at a given engine speed & accelerator pedal position.

[varsas]

raf_gti said:

varsas said:

<me saying stuff>

I like that answer.

Even if it's actually 100% bks it's the one I'm going to go with

just like being a politician, being on here is not about what you say but how you say it! (and sticking with your original answer even if it is undeniably proven to be 100% bks)

[Ricky Bobby]

Fuel doesn't atomize as well when the engine is cold. A fair amount of fuel actually condenses onto the inlet track and cold cylinders and consequently does not get burned as well. This obviously improves when the engine heats up, but it can take some time and is worsened by short trips due to the number of starts without reaching a decent operating temperature. This as well as other reasons mentioned is why fuel consumption increases.

[kwakus]

Not quite as technical as some of the other answers but I leave my car running while I scrape it and shovel the drive to warm up a bit, which I dont do in the summer, using fuel but going no where.