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Parrot of Doom

Original Poster:

23,075 posts

119 months

[news] 
Tuesday 5th January 2010 quote quote all
Why is diesel mpg generally lower, in cold weather like what we're experiencing now?

I usually get about 450-500 miles per tank, for the last month or so I've been getting about 400-450. Driving style is completely unchanged.

Chris_w666

22,654 posts

84 months

[news] 
Tuesday 5th January 2010 quote quote all
Not 100% certain but mine is down by about the same amount too. I would imagine its to do with the fuel being more dense in the cold or because the engines of diesels are generally cooler than petrol ones the cold weather makes it work a little harder.

Dick_Phallus

1,155 posts

69 months

[news] 
Tuesday 5th January 2010 quote quote all
Parrot of Doom said:
Why is diesel mpg generally lower, in cold weather like what we're experiencing now?

I usually get about 450-500 miles per tank, for the last month or so I've been getting about 400-450. Driving style is completely unchanged.
Are you sure about that? I'd have thought your style would have changed when faced with snow and ice, but if you've got the skills to deal with those conditions you adapt instinctively, so are not consciously 'trying' and so don't think you are. If that makes sense...

Nick_F

7,194 posts

131 months

[news] 
Tuesday 5th January 2010 quote quote all
Ours is down too; not sure how much is due to the 15 minutes idling on the drive to defrost every morning, but even when up and running the trip computer shows pretty rubbish numbers.

Lost soul

7,361 posts

67 months

[news] 
Tuesday 5th January 2010 quote quote all
mine is down the same (525d) but i think its the same with petrol cars
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andyc.

1,012 posts

78 months

[news] 
Tuesday 5th January 2010 quote quote all
You know I replaced 2 really manky air filters in Novemebr on my diesel where the dash had been telling me to sort it for 6 months and MPG dropped from a regular 40+ to 30-32mpg.Ive beem trying to suss it out ever since,...maybe its the temperature as you say.

Edited by andyc. on Tuesday 5th January 21:34

Parrot of Doom

Original Poster:

23,075 posts

119 months

[news] 
Tuesday 5th January 2010 quote quote all
Dick_Phallus said:
Parrot of Doom said:
Why is diesel mpg generally lower, in cold weather like what we're experiencing now?

I usually get about 450-500 miles per tank, for the last month or so I've been getting about 400-450. Driving style is completely unchanged.
Are you sure about that? I'd have thought your style would have changed when faced with snow and ice, but if you've got the skills to deal with those conditions you adapt instinctively, so are not consciously 'trying' and so don't think you are. If that makes sense...
No, I'm plodding along at the same speeds on the motorway, and driving on A-roads if anything I'm being much more gentle with the accelerator.

RYH64E

4,913 posts

129 months

[news] 
Tuesday 5th January 2010 quote quote all
My mpg in a petrol car is down too, and the main change in my driving over the last few weeks has been to slow right down. My 10 mile B road drive to work usually gives about 35mpg, 37mpg if I drive really slowly and about 15mpg on a de-stress drive home... At the moment I am getting about 32mpg whilst driving like Miss Daisy. I have put it down to running on choke for longer.

For those who are wondering, I don't get the calculator out after every journey, it just so happens that the on board computer thingy automatically displays the trip mpg.

Superhoop

3,219 posts

78 months

[news] 
Tuesday 5th January 2010 quote quote all
A few reasons really - And petrols are no different either...

1 ~ Cold air is more dense (meaning in effect more air in the cylinders), so more fuel can be burnt - Which means you get more power wink When you least want it frown

2 ~ As the ambient temperature is much lower, it takes much longer for the car to warm up, and as a result injector durations stay longer for longer (think - having teh choke on for longer in say, an old mini) - All assuming that is, that you drive a modern diesel with common rail injection (Which are mapped in much the same way as petrol engines now)

3 ~ Higher electrical loads - heater on longer, lights on morning and evening, heated rear screen etc) meaning the alternator has to work harder, creating a slightly higher load on the engine, which requires a sloght increase in fueling

Add in the fact that yu probably also leave the car running for 5 minutes or so in the morning whilst you defrost the windows, and Bingo, higher fuel consumption.


supersingle

2,730 posts

104 months

[news] 
Tuesday 5th January 2010 quote quote all
I'll have a stab at it:

Cooler air is denser so the engine is pumping more air at any given revs. The engine has a MAF sensor which measures the airflow and the ECU chucks in more fuel. So you get more power but greater pumping losses and more fuel used.

Am I wrong?

tonker

46,961 posts

133 months

[news] 
Tuesday 5th January 2010 quote quote all
isn't there a load more resistance off the snow and slush so you have to work harder to make the same progress....... hence lesser economy....

Six Fiend

5,557 posts

100 months

[news] 
Tuesday 5th January 2010 quote quote all
Have noticed it a lot in the van - economy is down by at least 5-7 mpg. At the mileage I do it hurts at the pump to the tune of £25+ a week.


andy-xr

9,745 posts

89 months

[news] 
Tuesday 5th January 2010 quote quote all
Idle is much higher on mine than normal recently, even when warm'ish. I guess the fuelling system is compensating for "other stuff"

wobert

1,252 posts

107 months

[news] 
Tuesday 5th January 2010 quote quote all
Parrot of Doom said:
Why is diesel mpg generally lower, in cold weather like what we're experiencing now?

I usually get about 450-500 miles per tank, for the last month or so I've been getting about 400-450. Driving style is completely unchanged.
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

Nick_F

7,194 posts

131 months

[news] 
Tuesday 5th January 2010 quote quote all
tonker said:
isn't there a load more resistance off the snow and slush so you have to work harder to make the same progress....... hence lesser economy....
I'm sure there is, but here in Somerset there's been no snow since February last year...

Deva Link

26,934 posts

130 months

[news] 
Tuesday 5th January 2010 quote quote all
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.

wobert

1,252 posts

107 months

[news] 
Tuesday 5th January 2010 quote quote all
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.
In mainland Europe from mid-October through until mid to late-March.

It's only in recent years that "Winter Diesel" has been used in the UK, as it wasn't deemed to get cold enough here. From what I've heard, we now get WD from November through til the end of Feb

V8A*ndy

2,162 posts

76 months

[news] 
Tuesday 5th January 2010 quote quote all
andy-xr said:
Idle is much higher on mine than normal recently, even when warm'ish. I guess the fuelling system is compensating for "other stuff"
My Kangoo snotter has also started this. Lumpy "chug chug" at the lights as normal then at the next set of lights a smooth "thrummmm" and 1k RPM ??????????????

Deva Link

26,934 posts

130 months

[news] 
Tuesday 5th January 2010 quote quote all
wobert said:
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.
In mainland Europe from mid-October through until mid to late-March.

It's only in recent years that "Winter Diesel" has been used in the UK, as it wasn't deemed to get cold enough here. From what I've heard, we now get WD from November through til the end of Feb
It's been used here for years - unless it's changed, the start date is mid-Sept.

chr15b

3,215 posts

75 months

[news] 
Tuesday 5th January 2010 quote quote all
mine got down to 4.3 on the trip to work the other morning
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