Poor diesel mpg in winter

[alfaman]

ALL engines use more fuel at lower ambient temperatures :

Car engines will operate at lower temps due to cooling effect of weather and use of heaters etc , AND also take longer to warm up to operating temp.

...from what I just about remember from Year 2 engineering thermodynamics : lower engine operating temps will reduce the maximum achievable thermodynamic efficiency - something to do with enthalpy heat loss.

..converesely you will get higher mpg in hot weather

[simonej]

Deva Link said:

wobert said:

Winter diesel has a higher benzin content than summer diesel. The benzin reduces the tendancy for the fuel to wax at lower temperatures. The additional benzin content reduces the calorific value of the fuel, so for a given volume of fuel you will get less energy out of it - hence lower mpg for steady state driving conditions

^ this - although it changes pretty early so it's not the whole story.

Apart from the obvious cooler running and additional electrical loads, many diesel engines have a high power electrical heater embedded in the cooling system which assists warm up.

Well that explains why I started noticing being about 5mpg down back in October. Went from 50mpg down to about 45 and had been wondering what was wrong - I've adjusted tyre pressures, driven with minimal electrics on and adjusted my driving style, all to no avail!

Interesting, thank you.

[fido]

So in a petrol car, you shouldn't notice any differences on a long journey .. if you can live without the heater and change up earlier (to make use of the increased torque from denser fuel-air) .. right?
The heater does seem to hurt mpg alot.

[alfaman]

fido said:

So in a petrol car, you shouldn't notice any differences on a long journey .. if you can live without the heater and change up earlier (to make use of the increased torque from denser fuel-air) .. right?
The heater does seem to hurt mpg alot.

No - it will still use more : chilling effect of air will reduce engine operating temp when warmed up : will be less efficient.

air density impact is zero to negligible at 0 deg C vs (say) 10 deg C

Edited by alfaman on Tuesday 5th January 22:40

[scotty_d]

yup i would say leaving the car on tick over with heaters going full chat for long periods to defrost will affect your mpg.

[pilchardthecat]

How odd, I get several mpg more than usual when it's cold in the diseasel. I don't do short trips though so don't suffer from lots of cold starts. I had thought this was down to additives in winter fuel increasing the cetane level and also the air density making the combustion more efficient.

My e46 330d was much the same, but previous petrol cars generally suffered worse mpg in the cold. Maybe my science is all wrong and it's because i'm getting cautious in my old age and driving more carefully

[Jim Campbell]

Colder,denser air should make the engine run more efficiently and therefore improve fuel economy.

I would say the winter blend of diesel and the increased density of the air upping the aerodynamic drag are to blame.

When filling a car in colder weather you get more fuel for a given number of liters than you would in hot weather, hence its measured in KG,s in F1 to keep it constant. So even if you're MPG is down its probably the same hit to the bank balance anyway

Ta

[Pints]

Not much to add here, other than to say I'm also getting at least 50miles less from a tank.
I've put it down to the winter fuel mix and having the heating on more often.

[doodles19]

Pints said:

Not much to add here, other than to say I'm also getting at least 50miles less from a tank.
I've put it down to the winter fuel mix and having the heating on more often.

winter fuel mix?

[The Wookie]

Deva Link said:

wobert said:

Winter diesel has a higher benzin content than summer diesel. The benzin reduces the tendancy for the fuel to wax at lower temperatures. The additional benzin content reduces the calorific value of the fuel, so for a given volume of fuel you will get less energy out of it - hence lower mpg for steady state driving conditions

^ this - although it changes pretty early so it's not the whole story.

Apart from the obvious cooler running and additional electrical loads, many diesel engines have a high power electrical heater embedded in the cooling system which assists warm up.

And quite a few actually have diesel burners believe it or not

[Deva Link]

The Wookie said:

And quite a few actually have diesel burners believe it or not

Diesel supplementary heating and how it works seems to be almost some kind of secret - I have a diesel Mercedes and nobody seems to understand how the booster heater works, and why it can be turned on and off with the a/c button.

Few recent Merc's (in the UK) have diesel powered heater but apparently many BMWs do, but their existence is hidden (ie there's no way of controlling it) from owners for some reason.

[Pints]

doodles19 said:

Pints said:

Not much to add here, other than to say I'm also getting at least 50miles less from a tank.
I've put it down to the winter fuel mix and having the heating on more often.

winter fuel mix?

AFAIK, the fuels have different additives added in winter to help keep it less viscous in the cold conditions.

[The Wookie]

Deva Link said:

Diesel supplementary heating and how it works seems to be almost some kind of secret

Yes, I agree it is quite odd.

Some cars do have controls for the fuel burning heater (e.g. the old man's G55), but it only seems to be when it's a specified option, and then it's seen as a luxury for warming your car up before you get in it.

Perhaps it's because, despite being necessary, on the face of it it seems like a wasteful idea, or the system isn't designed/safe to run with the engine switched off and it doesn't require any control.

Either that or the controls will suddenly appear as a £1000 option in the near future

[bazking69]

Colder conditions/temperatures and extra electrical loads. Simple as that.

[JonnyVTEC]

Jim Campbell said:

Colder,denser air should make the engine run more efficiently and therefore improve fuel economy.

I would say t

It wont, the chilled intakeke and cooler air makes fuel atomisation less effective.

Cold air is good for power, no efficiency. More power does not mean more efficient as to get the higher power offered by cooler air you have injected more fuel.

[s2art]

alfaman said:

fido said:

So in a petrol car, you shouldn't notice any differences on a long journey .. if you can live without the heater and change up earlier (to make use of the increased torque from denser fuel-air) .. right?
The heater does seem to hurt mpg alot.

No - it will still use more : chilling effect of air will reduce engine operating temp when warmed up : will be less efficient.

air density impact is zero to negligible at 0 deg C vs (say) 10 deg C

Edited by alfaman on Tuesday 5th January 22:40

Dont believe that. The thermostat controls the engine temp once warmed up, it would have to be really really cold to prevent a petrol engine reaching operating temperature, similarly for a diesel, although it would take a little longer to reach said temperature.