# Diesel Car, worse fuel efficiency in Winter than summer



## LouisCribben (9 Dec 2009)

In my 320d car, I get about 4.6 litres/100km in summer (over 95% of all my driving is motorway/dual carriageway).

However, when the weather gets colder, my figures get worse, I'm always about 5.1 litres/ 100km in winter. 

I turn on the aircon occasionally in summer, but never in winter.

I wonder why the fuel efficiency gets worse in winter, have other people noticed a similar pattern with their car ?

The only possible reason I can think of is that somehow there is more friction when the road is colder...........

But I'm sure there are other reasons.


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## onq (9 Dec 2009)

I presume you use the heater in winter, Louis...

ONQ.


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## 26cb (9 Dec 2009)

I seem to remember reading that the diesel mix is different in Winter to cater for freezing temps....


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## LouisCribben (9 Dec 2009)

onq said:


> I presume you use the heater in winter, Louis...
> 
> ONQ.


 

I thought the heater was a freebie, it just blows in heat produced from the engine, heat which would otherwise just be lost to the outside air.
I could be wrong though.  

I use the lights in winter a lot more than in summer, that uses 3% extra fuel apparently.  That would increase the fuel burned by say 150ml in a 100km journey.


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## galleyslave (9 Dec 2009)

air con uses fuel... 
engine heat would be stinky and dirty, not to mention unreliable


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## LouisCribben (9 Dec 2009)

26cb said:


> I seem to remember reading that the diesel mix is different in Winter to cater for freezing temps....


 
Yeah I read that too in some Canadian forums, where the temp gets to minus 30 or minus 40 , the diesel there is definitely a different mix in winter

But in Ireland it doesn't really get below zero very often, although I guess the diesel would have to cope with the rare situations when it gets to minus 5.


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## onq (9 Dec 2009)

In winter there are several additional electrical loads which cause drag on the rotor and increase fuel consumption:

 You heat your car, and the hot air is blown around the car by a fan, which uses electricity.
 Even if the heat is from the engine, its not direct, its probably via a heat exchanger and I expect that is a pumped system also - more electricity.
 You use the aircon and rear windscreen heater to de-mist the front and rear windscreens and side windows.
 You may have automatic mirror defrosters.
 You use the headlights much more.
 Slippery roads may use the anti-lock system more.
 You spend more time listening to your radio in the heavier traffic.
 You use your heated seats [assuming you have them] which are electric and separate from the aircon system.
 
There you go!

ONQ.


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## tosullivan (9 Dec 2009)

onq said:


> You use your heated seats [assuming you have them] which are electric and separate from the aircon system.




 Well worth 0.5l/100km...I can't imagine life without them now


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## PyritePete (9 Dec 2009)

heated seats in the winter, piles in the Spring according to one Top Gear quote


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## mathepac (9 Dec 2009)

LouisCribben said:


> ... The only possible reason I can think of is that somehow there is more friction when the road is colder...


No, less friction. Hot tyres, hot road surface = more friction, more grip and higher fuel consumption.


LouisCribben said:


> ... But I'm sure there are other reasons...


Lots.

Internal combustion engines, petrol or diesel, have a relatively narrow temperature band where they can operate at peak fuel efficiency. As diesel lumps tend to have more old-fashioned "iron" in them than petrol engines of similar vintage, the lower ambient temperatures are,  the longer it takes for them to reach optimum operating temperatures, hence fuel consumption rises. This temperature is around 90 - 95 deg C (the Temp needle pointing to the mid point on the Temp Guage). In summer this will take 5  - 8 kms driving time; in winter it could be 3 times the distance.

The shorter the journeys and the more frequent the starts from cold, the more fuel consumption will suffer (this will also effect engine wear).

"Aha, caught you", I hear the scientific community shout, "we know that cold air is denser than hot air, therefore combustion is more effective in winter because the same volume of air contains proportionally more oxygen". Top of the class I say except that in order to make use of the additional oxygen, the engine has to reach its peak fuel-efficiency temperature.

Due to emissions requirements, while an engine is below its peak fuel-efficiency (for temperature or other reasons) it will emit via the exhaust system unburned fuel and other noxious gases. These must now be recycled through another combustion cycle in the engine to detoxify them, thereby effecting the engine's fuel efficiency further. In other words the less fuel-efficient an engine is, the less fuel efficient emissions requirements force it to be. This situation tends to be exacerbated in winter.

A major drain on fuel in a car is the electrical system - lights, window wipers, cabin heating, window and mirror demisters, heated seats, etc. These are precisely the accessories on the car that are used more in winter than in summer, directly effecting fuel consumption.

Are there possible solutions to the problem?


Engine pre-heaters powered from the mains that plug into and heat the engine block, coolant and oil before you start the engine, used extensively in Scandanavia, Canada and the US. Think of them as "super" glow-plugs that are fitted to diesels anyway
Block off the radiator fins with card-board until the engine heats up (old lorry driver's trick that might not do your warranty any good) and then hop out in the middle of the M50 to remove the card-board


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## mathepac (9 Dec 2009)

onq said:


> ...
> 
> Even if the heat is from the engine, its not direct, its probably via a heat exchanger and I expect that is a pumped system also - more electricity....


 Some cars use electric heaters in the passenger cabin, most use a heater matrix (additional radiator) plumbed into the cars cooling system and some expensive cars use both with the plumbing taking over from the electricity once the engine reaches operating temperature.


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## onq (9 Dec 2009)

mathepac said:


> <snip>
> 
> Are there possible solutions to the problem?
> 
> ...


AFAIK, these were first invented to prevent the cylinder heat splitting due to the extreme cold of winter in these countries, where the temperature drops below the usable range of the anti-freeze in the car cooling system.

IOW, I don't think they're used to pre-heat the engines per se, but again, this would be a definite benefit and side-effect.



> Block off the radiator fins with card-board until the engine heats up (old lorry driver's trick that might not do your warranty any good) and then hop out in the middle of the M50 to remove the card-board


Ah sure just leave the car there and pull the bike out from the boot and off ye go...



ONQ.

<feeling better now that the whole country hasn't been destroyed by a Nuclear Strike Budget>


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## breakdabank (9 Dec 2009)

The answer is that modern diesels are so efficient they don't generate enough surplus heat for a cabin heater, so they have a separate diesel-powered Webasto heater, which adds to the overall fuel consumption.

A benefit of this is that the interior of a diesel car can get nice and warm on winter mornings well before the engine has reached its normal running temperature.


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## LouisCribben (9 Dec 2009)

tosullivan said:


> [/list]
> Well worth 0.5l/100km...I can't imagine life without them now


 

I laughed when I read that someone can't imagine life without heated seats in his car !!!! 

Thanks everyone for the great answers, it has brought complete clarity to the question I asked.


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## ollie323 (10 Dec 2009)

Agreed on all the above. 
Also: The colder oil in the engine and gearbox is more viscous, along with wheel bearings etc. It all adds up and it takes time to get up to temp. In the meantime it is having a major effect on fuel consumption.
I've taken steps to offset this and it has helped reduce the hit. I have a 200W block heater and on frosty mornings it makes some difference, my engine is up to 40c after an hours heating. Windows also clear very quickly which is not a bad thing. 
I also have my radiator blocked off by 3/4 and it definitely keeps my engine a little warmer, having warmed up quicker in the first place. The fan does not come on either so i'm not overheating my engine. When the weather gets warmer i'll remove it.
Also bear in mind that a diesel engine starts to get colder as it idles. So in stop go traffic it will cool off. 
A petrol engine on the other hand will get hotter in stop go traffic so a grille block will have to be smaller than a diesel one would be.
I realise its a bit of tinkering but a 50% grille block in a diesel will cause no problems whatsoever so it could be used safely. 

The colder air is more dense too so requires more work to push a car through it. The same for standing water on the road. 
Roll on summer.


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## Ancutza (10 Dec 2009)

My fuel consumption has gone from around 11.5 litres to around 12.3 litres/100 km.

I put it down to having fitted winter tyres and them having a different compound than the summer ones.  I hadn't considered other factors such as air density etc.

I'll try your solution of blocking off a portion of the grille, Ollie.  Thanks for that one!


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## ollie323 (10 Dec 2009)

Just observe your engine temperature to be on the safe side. A grille block will limit your top speed so use your discretion. If you have a diesel and it's cold or frosty then at least half the radiator can be blocked up. I can't do more than 100kph in my yaris as i have it blocked by maybe 90%, even in the summer. Which isn't very warm here nowadays. Takes me barely 10 mins longer to do my 55km commute which is worth it for the fuel savings. It pays for my insurance!


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## butch3 (7 Mar 2010)

Well I am not allowed to post any links before 15 posts, but those below are right on the subject, you know what to do with them:

metrompg.com/posts/winter-mpg.htm
allbusiness.com/transportation-warehousing/599027-1.html


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