MPG Woes

[talltimtam]

Dont know whats up with the car (W211 E320CDI) but I am now getting 24mpg on runs around town. Does the cold weather really impact fuel economy or am I just being unlucky ?

 

[evdok]

It's the weather, i've noticed poor MPG as well

 

[lynall]

I posted similiar recently and my overall mpg dropped by approx 15% since the first cold weather hit.



Lynall

 

[gIzzE]

I'm the same, I've dropped from 26 to 23.5 round town.

Cold really doesn't help.

 

[Dieselman]

Cold doesn't affect a diesel anything like as much as a petrol due to no requirement for a richer mixture when cold.

Having said that the mpg for my last tank has been slaughtered to 34.6.....

 

[st4]

Cold doesn't affect a diesel anything like as much as a petrol due to no requirement for a richer mixture when cold.

Having said that the mpg for my last tank has been slaughtered to 34.6.....

However the fuel becomes more viscous due to the weather being colder, and in my recollection it takes far longer for a diesel car to heat up and therefore run at maximum efficiency. In my case on the commute to work, about 8-10 miles. My petrol cars never took this long to warm up, usually in 3-4 miles.

 

[Dieselman]

However the fuel becomes more viscous due to the weather being colder, and in my recollection it takes far longer for a diesel car to heat up and therefore run at maximum efficiency.

But even when stone cold it only requires 10% more fuel maximum as opposed to 400% more like a petrol.
The very fact that it takes so long to heat up shows it is working effciently anyway.

What difference does the viscosity make and how much does it thicken anyway.?

 

[fuzzer]

However the fuel becomes more viscous due to the weather being colder, and in my recollection it takes far longer for a diesel car to heat up and therefore run at maximum efficiency. In my case on the commute to work, about 8-10 miles. My petrol cars never took this long to warm up, usually in 3-4 miles.

in minus weather my E320 cdi is warm (80) in about 1.5-2 miles of 30mph

 

[st4]

But even when stone cold it only requires 10% more fuel maximum as opposed to 400% more like a petrol.
The very fact that it takes so long to heat up shows it is working effciently anyway.

What difference does the viscosity make and how much does it thicken anyway.?

In truth, not much with modern additives, but in a petrol car this isn't an issue at all, hence I reckon the increased viscosity of the fuel must be an issue.

With modern injection systems, due to the extra density of the fuel as its colder, maybe more is injected. Certainly the air is cooler and therefore more dense, requiring more air to keep the stoikiometric ratio the same.

BMW owners have these woes too, and I can't find a definitve reason to why, but air density and fuel viscosity would be my bets.

Some diesels also have an additional fuel burning heater for the cabin, as the engine takes longer to warm up. This must have an adverse effect on the cars consumption.

 

[Dieselman]

hence I reckon the increased viscosity of the fuel must be an issue.

With modern injection systems, due to the extra density of the fuel as its colder, maybe more is injected. Certainly the air is cooler and therefore more dense, requiring more air to keep the stoikiometric ratio the same.

Diesels don't use stoichiometric fuel mixtures and if injecting more fuel would produce more torque so accellerate.
Stoichiometric fuel mixture is ~15:1 and diesel AFR generally varies between 100:1 maximum and 25:1 minimum.

I can't see the fuel viscosity having any bearing at all and anyway, it's possibly less viscous due to winterising distillates being added.

If viscosity of a given fuel created a performance advantage then veg oil would give much greater output, which it doesn't.

 

[Dieselman]

Anyone fancy a go on the Merry-go-round...??

 

[st4]

Diesels don't use stoichiometric fuel mixtures and if injecting more fuel would produce more torque so accellerate.
Stoichiometric fuel mixture is ~15:1 and diesel AFR generally varies between 100:1 maximum and 25:1 minimum.

I can't see the fuel viscosity having any bearing at all and anyway, it's possibly less viscous due to winterising distillates being added.

If viscosity of a given fuel created a performance advantage then veg oil would give much greater output, which it doesn't.

Does your car have an auxiliary fuel burning heater to warm the cabin on cold mornings. Some diesels have this and this would have an adverse effect on the consumption. I don't know if mine had this feature, but if it did would put this problem to bed.

All I know is that on a gentle run from Helensburgh to Edinburgh I can have my car @ 50mpg plus, and last weekend despite driving like a nun it returned 48mpg despite my best efforts

When its really icey here, and when the days are milder its the icer days that the car (on its trip) records a noticeable drop in MPG. Remember the fuel in the tank is unchanged, so the additives may not be as effective for the really cold days.

There are 3 possible variations @ work that I can think of if the fuel remains constant (I fill every 2 weeks but the temps day to day vary massively here):

Fuel viscosity, denser air being intaken, and the air temp dropping causing tyre pressures to drop.

The car takes longer to warm up and on a 16mile commute thats fairly significant if for 8-10miles the car isn't @ optimal temp.

 

[gIzzE]

Why does a diesel always take more of a battering in winter than a petrol then??

My last cars summer and winter mpgs...

Diesels
320d 40 33
535d 29 24
2.0tdi vag 44 36
2.5tdi vag 37 31

Petrols
E320 26 23.5
M3 21 19
335i 27 25


The petrols may use alot more when getting to temp but they are to temp within a mile, two at most, where as the diesel is not quite to temp by the time I hit the first roundabout on the dual carrigeway which is exactly 8 miles from home.
When I used to live and work in the city it was 4 miles across town which took around 15 minutes and it was not quite to temp. on -ºc days when I arrived, never had a petrol to compare it.

I am in a real quandry at the moment, need a newer estate but not sure what to get, E320 cdi has been ruled out as it isn't quite quick enough and I think I will get bored, but can't decide on another 535d or an E500. I would only need to get 23mpg from the E500 for it to give similar fueling costs to the 535d.

 

[st4]

I'd get the E500 gizze. You know the phrase "you'll never be late with a V8".

As estate cars go, a 211 IMHO would tick more boxes as that type of car than an E60. An E500 would be an excellent buy for you and you've still got an M3 anyway.

 

[Dieselman]

What I'm trying to say is that the diesel doesn't need to be at optimum temperature to be efficent, it's efficient even when stone cold.

I don't understand your comment about the fuel in the tank being the same, it's not.
Winter diesel is blended with lighter fractions to reduce the risk of waxing. This actually makes it less dense than regular diesel, which is one reason for an economy reduction.

I think the biggest reason for increased effciiency is that the rest of the drivetrain is cold so the lubricants are more viscous.

 

[gIzzE]

I'd get the E500 gizze. You know the phrase "you'll never be late with a V8".

As estate cars go, a 211 IMHO would tick more boxes as that type of car than an E60. An E500 would be an excellent buy for you and you've still got an M3 anyway.

I think you're right, and this sounds stupid, but after having the E320 estate I think I would miss not having a Merc to waft round in.

 

[gIzzE]

What I'm trying to say is that the diesel doesn't need to be at optimum temperature to be efficent, it's efficient even when stone cold.

I see. sort of....
I still don't get why I see a bigger percentage drop with the diesel than the petrol, on shorter runs anyway, do 100 miles and it seems to claw it back, this is why I thought it must be because the derv takes longer to get to temp.

 

[Dieselman]

I still don't get why I see a bigger percentage drop with the diesel than the petrol,

That's the bit I don't understand, I don't get that.
Just checking my records gives no clear evidence of consumption increase that corresponds with temperature year on year.

I've just plotted a graph of my consumption and cannot see any relationship between temperature and consumption.

 

[spock500]

The drop in mpg during winter is probably down to the additional draw on the battery caused by ancillary items.

Glow plugs in some models will stay on longer, you may keep your headlights on along with heated rear and front windows/side mirrors all of which will tax the alternator.

Some of the CDi models may even have fuel heaters which stay on for a while.

I have also heard that tyre pressure are lower when cold causing extra drag.

 

[gIzzE]

Maybe the cooler air lets the turbos breathe more and thus you tend to enjoy the right pedal more than usual!


The tyre thing is a good point.