Winter Fuel Economy

[shiftydave]

Hi all, quick question which I'm sure has come up previously but I've struggled to find anything. How much do you find your fuel economy is affected over the winter months? I have a Peugeot 206 1.6 HDI GTI and found over the summer I was getting up to around 50mpg, whereas my most recent fill-up was just under 40mpg. I've had a few issues with the DPF recently which may be affecting things, and just wondered if 20% less economy is normal or if there might be more of an underlying issue affecting it. Assuming the link to Fuelly works you can see the difference here: https://www.fuelly.com/driver/shiftydave/206/fuelc...

[10 Pence Short]

You're driving in dark and cold more often. This uses more fuel, as you've got the heater and the lights on.

[andy-xr]

Engine takes longer to warm up in the winter, and there's allegedly an additive to winter fuel for lower temps

[FlauM]

My fuel is shocking at the moment. My MPS was getting 35mpg but now it's around 27mpg

[BFG TERRANO]

In theory cars should be more economical in colder weather. Colder air burns with fuel more efficiently. Hence intercoolers.

However with longer warm up times, more load on the electrics this is offset so generally speaking most cars are slightly down on mpg.

[wemorgan]

On long journeys ~5%, which is about the same % of error in how it's measured.

[xRIEx]

BFG TERRANO said:

In theory cars should be more economical in colder weather. Colder air burns with fuel more efficiently. Hence intercoolers.

I wasn't aware that colder air burns with fuel more efficiently, I thought it was down to colder denser air taking up less volume, so more oxygen is present in each cylinder charge allowing more fuel to be burnt and therefore more power, not more efficiency.

[BFG TERRANO]

xRIEx said:

I wasn't aware that colder air burns with fuel more efficiently, I thought it was down to colder denser air taking up less volume, so more oxygen is present in each cylinder charge allowing more fuel to be burnt and therefore more power, not more efficiency.

Bit of both really. I work with numerous stationary 10 to 100kva diesel generators. Most of which run 24/7. Fuel economy marginally improves when temps are single or minus figures, this is displayed on the stats board on each machine. The bigger the engine the more this is evident.

[Allyc85]

My 2.0 Focus loses around 3-4mpg in the winter under normal driving. Enthusiastic drives can be quite alarming for the cars lack of performance...

[BFG TERRANO]

xRIEx said:

I wasn't aware that colder air burns with fuel more efficiently, I thought it was down to colder denser air taking up less volume, so more oxygen is present in each cylinder charge allowing more fuel to be burnt and therefore more power, not more efficiency.

If you get more power from the same volume of fuel this results in better economy?

[kambites]

BFG TERRANO said:

xRIEx said:

I wasn't aware that colder air burns with fuel more efficiently, I thought it was down to colder denser air taking up less volume, so more oxygen is present in each cylinder charge allowing more fuel to be burnt and therefore more power, not more efficiency.

If you get more power from the same volume of fuel this results in better economy?

Why would you get more power from the same volume of fuel when the air is colder? You get more power from the same volume of air because it can burn more fuel because it's denser, AFAIK.

As it happens you can theoretically get more efficiency if the intake temperature is lower because the maximum possible efficiency of a heat engine is determined by the ratio of intake to exhaust gas temperatures (see Carnot's Theorem). I don't claim to understand why, though.

Edited by kambites on Monday 31st December 13:11

[BFG TERRANO]

kambites said:

Why would you get more power from the same volume of fuel when the air is colder?

Edited by kambites on Monday 31st December 13:11

Because its denser and contains more oxygen per litre = better/cleaner/faster combustion.

[kambites]

BFG TERRANO said:

Because its denser and contains more oxygen per litre = better/cleaner/faster combustion.

I don't understand that logical step. Why does more oxygen per litre of air equal cleaner combustion? It means that the air can burn more fuel, but I see no reason it should burn it more efficiently.

[shiftydave]

Cheers for the input here folks. Sounds like any potential MPG issues caused by the DPF on mine should be fairly marginal, will see whether things improve over the coming months.

[jimbobsimmonds]

Don't forget that a litre/gallon is a measure of volume and not mass therefore a litre of cold will have more stored energy than a litre of 30C petrol.

[inman999]

BFG TERRANO said:

Because its denser and contains more oxygen per litre = better/cleaner/faster combustion.

No. It just means you can burn more fuel.

[kambites]

jimbobsimmonds said:

Don't forget that a litre/gallon is a measure of volume and not mass therefore a litre of cold will have more stored energy than a litre of 30C petrol.

Liquid petrol doesn't change volume significantly with temperature, does it? Liquids don't tend to.

[xRIEx]

BFG TERRANO said:

xRIEx said:

I wasn't aware that colder air burns with fuel more efficiently, I thought it was down to colder denser air taking up less volume, so more oxygen is present in each cylinder charge allowing more fuel to be burnt and therefore more power, not more efficiency.

If you get more power from the same volume of fuel this results in better economy?

As kambites explained, my understanding was the ECU would inject more fuel for a denser air charge to maintain its programmed air-fuel ratio.

And just to ensure we don't talk at cross purposes: when I talk about fuel efficiency I mean the chemical potential energy converted to work; to me, fuel economy is about the amount of fuel burnt, not how efficiently it was converted to work.

So to answer your question above in isolation: no, it results in better fuel efficiency.

[Mr Gearchange]

Lights on more, heating on more, heated seats on more, wipers on more, headlight washers used more etc etc - all adds up.

[McSam]

xRIEx said:

BFG TERRANO said:

In theory cars should be more economical in colder weather. Colder air burns with fuel more efficiently. Hence intercoolers.

I wasn't aware that colder air burns with fuel more efficiently, I thought it was down to colder denser air taking up less volume, so more oxygen is present in each cylinder charge allowing more fuel to be burnt and therefore more power, not more efficiency.

xRIEx has summed it up - the denser air allows you to, at full load, get more power from a given design because the same volume will have more mass of air in it, which means you can mix and burn more fuel.

But this doesn't given you any economy (in terms of miles per gallon) benefits at all, because the air/fuel ratio doesn't change. In "normal" running the only difference would be a slightly smaller throttle opening to achieve the same power output, not the same as using less fuel!

The inlet air temperature being lower can give you a marginal gain, due to thermodynamic laws, but it isn't due to the higher density somehow "allowing" higher efficiency.

Anyway, in almost every single road-use case, you use a lot more fuel in cold temperatures because your engine starts off a lot colder, takes longer to warm, and then takes longer still to warm because you're using your heater, and then you load the alternator with lights, heated screens, seats, etc etc. 20% seems steep, but if you do a lot of short trips, not unfeasible.