E220 CDI (W212) MPG variation with temperature

[Pottsy]

I have now done 40,000 miles in my W212 E220 Auto. One thing I have noticed is that is has a huge variation in MPG according to outside air temperature. All my journeys are long, so this is not to do with the extended warm up time - the journey MPG settles down after about 50 miles and the figure it settles down at is proportional to the temperature.

Anyway, I thought I would plot some graphs, to make sense of this.

Here is the MPG (all measured at the pump, not on the on-board computer) for each tankfull:

I got the car in Oct 09, and you can clearly see the winter dip at the end of 2010.

So, I decided to plot MPG against date, and add the monthly mean temperatures from the met office:

The blue temperature line clearly follows the red MPG line, and you can see that the car does around 55 MPG when the temperature is above around 12C, and drops down to about 48 MPG in winter.

Furthermore, at the left hand side of the graph, the red and blue lines do not match (as the engine is new), until the engine is run in (when they join). So this answers the question "how long does it take to run in a diesel" - for my car it was about 5,000 miles.

Thought this may be of interest to someone...

 

[artyman]

A very interesting analysis, I'm surprised at the temperature variation given that you state you do long runs.

 

[tmaoahthfte]

What temperature does the engine run at when warmed through? My 210's fuel consumption varied like that until the thermostat was changed.

 

[fred_raven]

I drive 35 miles everyday into london , generally before any traffic and notice that my w211 320 cdi varies between 40 and 45 mpg and seems to be not only temperature related but also seems to be humidity ! on a wamish damp day i appear to get best performance !

 

[artyman]

Humidity may well affect power, if moisture is drawn in along with air this then turns to steam, so effectively a steam engine in addition to ICE, I vaguely recall some experimentation done some years ago about water mist being injected into engine intakes.

 

[Mercsys]

Superb data!

I guess the air density and drag has much more influence than I thought possible, average speed per tank would be an interesting addition. Perhaps you wash and polish the car more in the summer months, a clean car always feels faster, everyone knows that!!

Any one got any info on charge density and EGT V ambient temps?

On a recent 200 + 200 mile trip, the early morning out leg I made 42 mpg (computer) easily but on the but struggle to get to 40 on the mid day return, I put it down to the ambient temps, but your date runs against this, maybe the bug build up and perhaps Birmingham is down hill.

 

[stevesey]

Humidity may well affect power, if moisture is drawn in along with air this then turns to steam, so effectively a steam engine in addition to ICE, I vaguely recall some experimentation done some years ago about water mist being injected into engine intakes.

Keeps the intake charge cooler as well (higher latent heat etc).

After market water injections kits are available Coolingmist Water Injection, alcohol injection and Methanol Injection systems

My mpg figures (both petrol and diesel) show a 2+ mpg seasonal difference.

 

[Dieselman]

Great data, thanks for posting. My own graph is nothing like as variable due to ambient temps, but it does make sense for a couple of reasons.
1. The whole drivetrain will be cooler so take more energy to turn until hot and

2. The engine is losing a lot more heat due to the cold air rushing round the engine drawing heat away and the intake air being cooler, so cooling the cylinders.

Try blanking the radiator grille next winter to see if that helps. I'm sure it will.

 

[Mercsys]

Engine oil change points, and viscosity will also play a part.

 

[Pottsy]

Superb data!

I guess the air density and drag has much more influence than I thought possible, average speed per tank would be an interesting addition.

Average speed is always about the same - I do the same 160 miles every day.

Great data, thanks for posting. My own graph is nothing like as variable due to ambient temps, but it does make sense for a couple of reasons.
1. The whole drivetrain will be cooler so take more energy to turn until hot and

2. The engine is losing a lot more heat due to the cold air rushing round the engine drawing heat away and the intake air being cooler, so cooling the cylinders.

I wonder... The car must be in a steady state by 50 miles, and certainly 80 miles, but the MPG always settles to a figure that is related to the temperature. I strongly suspect it's software.

Try blanking the radiator grille next winter to see if that helps. I'm sure it will.

No need! Did you know the 212 already does this for you? (Link)

 

[Melk63]

For petrol engines the low winter mpg has also something to do with the intake air temperature to the engine. Cold air is denser, therefore in winter a larger mass of air enters the engine, the ECU adjusts by allowing more fuel, hence lower mpg. The actual torque output of engine increases slightly. The effect is more pronounced in forced-induction (turbo and supercharged) than naturally-aspirated engines and I think diesel engines too. In petrol engines, when the air intake warms up , the ECU changes the ignition timing to prevent knocking (pre-ignition), reducing engine torque. Intercoolers and water injectors are used in forced air systems to reduce the air intake temperature to cylinders and therefore prevent pre-ignition (knocking) and increasing torque. I have a turbo-charged MX5 (aftermarket kit) and the power noticeably increases in a cold winter day! The mpg also drops noticeably.
Mel.

 

[Mercsys]

Air density goes up by about 8% in the drop from 0° to 18° we are looking at and as most of the other factors remain constant, speed, drag, area… we are able to say with some confidence the power required will increase by the same, however the turbo, intercooler, combustion process will become more efficient, the humidity will play a part but at around 0° the air can dry out completely messing with the results. Demisters and other cabin heating systems will also add to the winter power requirement. Some diesel fuels will also change in winter, this could also need to be accounted for.

I must get out more...

 

[Tinkerer]

My 2001 E220 cdi used to give variable fuel consumption until I noticed the engine temperature dropping to 60C on a slow part of my journey after it had already reached 80C. CDi engines give their best alround performance when the engine has reached 90C which is why they are fitted with a 'cabin heater' to get the engine temp up quickly on cold days. I changed the thermostat (headache!!) and the fuel consumption settled down to 46 mpg on an average 30 mile journey every day with a consistent engine temp of 90C after 10 miles. Of course, given the ever rising price of fuel I have now taken to wearing lighter shoes ( for acceleration only!) and can now achieve a respectable 52 mpg. On the old damp air trick, consult your thermodynamics which in laymans terms states that there is no such thing as a free lunch, if water was an energy fuel hey! we would all be using it instead of petrol.

 

[Pottsy]

For petrol engines the low winter mpg has also something to do with the intake air temperature to the engine. Cold air is denser, therefore in winter a larger mass of air enters the engine, the ECU adjusts by allowing more fuel, hence lower mpg....

Mel,

The diesel is quite different - it has no throttle so always just pumps the full amount of air. You then add fuel to make a certain amount of power (in this case about 40 bhp to cruise at 70), so the density of the charge makes hardly any difference. On a petrol (and possibly a NA diesel) dense air will allow a greater maximum power (and associated consumption).

Air density goes up by about 8% in the drop from 0° to 18° we are looking at and as most of the other factors remain constant, speed, drag, area… we are able to say with some confidence the power required will increase by the same, however the turbo, intercooler, combustion process will become more efficient, the humidity will play a part but at around 0° the air can dry out completely messing with the results. Demisters and other cabin heating systems will also add to the winter power requirement. Some diesel fuels will also change in winter, this could also need to be accounted for.

I must get out more...

I make the density change about 6.6%, and given that only half the losses are aerodynamic (at 70) then that accounts for 3.3%. The difference in consumption for me is about 13%, so that's 10% missing somewhere!

The 2.2 litre Honda CRV we have (so a similar sized and format engine) varies considerably less, and individual variations are much greater than the seasonal one:

 

[oakleyboy]

why is the cabin of my e220 cdi too hot when the ac is not on?

 

[Troon]

why is the cabin of my e220 cdi too hot when the ac is not on?

Welcome to the forum. Please post this in a new topic rather than tagging it onto the end of an existing and barely relevant one. I'd suggest the Interior section: click the New Thread button.

You may also want to give a few more details on your car - age, options (2-zone or 4-zone climate) etc.

However, I suspect the answer is simple - in the summer, the ambient temperature is relatively high, and your car is a nice insulated greenhouse. If you're too hot, turn the A/C on.