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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old September 2nd, 2013, 10:31   #1
Diesl
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Default MPG increase with temperature: 1.4% for every 10F

My fuel efficiency went up from 35.5 mpg in February to 37.5 mpg in August, for the same commute. Here is the graph of five-tank average mpg vs average temperature. Until about March I extracted the weekly averages from the 'weatherspark' history plot, then I got lazy and just took the monthly average temperature. For the last few weeks in August I computed the average temperature for each tank from the daily observed averages.

The outliers (high points around 37 mpg/30-40 F) are from December 2012.
Using the five tank average gets rid of fillup variations. Maybe I should use daytime temperature averages.

The overall trend is a 1.4% increase in fuel efficiency per ten degrees F temperature increase, a 5.5% effect between winter and summer.
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Old September 2nd, 2013, 10:46   #2
MikeMars
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Yes, I see something similar (haven't got the exact figures with me). Basically, for me, MPG improves as the temperature increases (... until the climate control needs to be turned on).
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Old September 2nd, 2013, 12:11   #3
Diesl
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AC on/off doesn't seem to make much of a difference for me. I have been driving with the AC on (recycling on, 10-11 o'clock temperature setting) for July and August. Air recycling on vs off might make a big difference in how hard the AC has to run.
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Old September 2nd, 2013, 12:15   #4
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Fuel Economy: 35 Winter/41 Summer
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How are you accounting for winter fuel additive package impact?
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Old September 2nd, 2013, 17:40   #5
Diesl
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Default MPG as function of air temperature, speed etc.

The pattern doesn't fit winter diesel. It's a steady increase, not a sudden step:


It could be though some sensor drifting. [I guess I'll find out when it gets cooler again, which should be soon:]
Update 2016-12-31: avg. speed added

Added 2019-6-28: mileage vs date

I think these two plots show the best correlation; the annual speed maxima and minima seem to roughly match the mileage variation.

Overall my MPG is going down ;-(


(old) Update:
I had never looked at how much the drag loss actually changes due to air density change with temperature. Guess what: going from 30F to 70F lowers air density by 8%! Density enters into the drag power linearly. I do about 70% of my driving on the highway, where air drag dominates. 0.7*8%~5.5%! That almost fits too well....
Update Sep 5: Added a graph that shows five tank average mpg versus five tank average temperature, and hopefully updates automatically from the spreadsheet. (The graph in the first post won't update, and uses the average daily temperature for the middle tank.) - 2019-6-28: now deleted, since it shows no correlation when all years are plotted on top of each other; for individual seasons, see plots below.

Update Nov. 8: it is getting cooler, and the diesel milage is dropping. Temperature vs time chart added.
Dec 16: use http://forecast.weather.gov/product....n=1&glossary=0 average for days actually driven.
Dec 21: Correcting some of the earlier temperature averages; e.g. Nov 30 - Dec 11, 2012 went from 48F to 41F.
Update 2014-2-28:
Disentangling the last plot by season:
First the falling temperatures of winter 2012/2013:

Maybe the initial rise (from right to left) is the running in of the engine??

Then the rising temperatures of spring and summer 2013:

The falling temperatures of fall and winter 2013/2014:

The drop in MPG follows a higher MPG curve than the preceding rise during spring and summer.
Spring/Summer 2014:

Fall/Winter 2014/2015:


Summer 2015:


Update 2016-12-31: I added average speed. Unfortunately there are two competing factors at work: the ever increasing highway speeds in the Chicago area, and a change in this car's mix of highway vs. city driving. It looks like city driving is winning, and dragging my average down.
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Last edited by Diesl; June 28th, 2019 at 14:39. Reason: add mileage vs date; remove 11th plot
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Old September 2nd, 2013, 20:15   #6
Killabee228
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I like this thread... Science= Fun!
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Old September 3rd, 2013, 01:10   #7
Ultrasonic
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Fuel Economy: Best 59 USmpg, average 47 USmpg, and improving...
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Interesting data, thanks for sharing.

As you've concluded, I believe that for longer runs it is the change in average air density that is the dominant factor. (For short runs, the prolonged engine warm up time in cold weather will have a bigger impact.)

Any idea what happened in December and January though? Was your driving pattern different perhaps?

Edit: This is a post from the Ecomodder forum that attempts to calculate the relative effects of air resisitance and the use of winter diesel, which may be of interest:

http://ecomodder.com/forum/showthrea...tml#post273835
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Old September 3rd, 2013, 07:14   #8
03_01_TDI
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Tire air pressure changes with air temps. This and quicker warm up time would account for most of the gains.

Now for a gasser engine a hot air intake can really aid in mpg.
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Old September 3rd, 2013, 07:20   #9
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Quote:
Originally Posted by 03_01_TDI View Post
Tire air pressure changes with air temps. This and quicker warm up time would account for most of the gains.
Tyre pressure changes would only have a small effect, and the only way the effect you describe would be a factor at all is if Diesl didn't regularly check and adjust their tyre pressures. I could be wrong, but my bet is that someone who logs mpg data like this probably checks tyres pressures now and then .
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Old September 3rd, 2013, 10:05   #10
03_01_TDI
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The rule of thumb is for every 10 Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi (up with higher temperatures and down with lower).

Plus the summer sun warms up the tire and road surface.

In general for every 1 psi in tire pressure you can raise your gas mileage by .4 percent. Depending on tire type and sidewall flex.

Last edited by 03_01_TDI; September 3rd, 2013 at 10:08.
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Old September 3rd, 2013, 13:21   #11
powerfool
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Quote:
Originally Posted by 03_01_TDI View Post
The rule of thumb is for every 10 Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi (up with higher temperatures and down with lower).

Plus the summer sun warms up the tire and road surface.

In general for every 1 psi in tire pressure you can raise your gas mileage by .4 percent. Depending on tire type and sidewall flex.
That's all great and is good to know, but it can be a non-factor if he checks his air pressure and adjusts his tire inflation accordingly, as was previously mentioned. (EDIT: and usage or not of LRR tires).

Last edited by powerfool; September 3rd, 2013 at 13:40.
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Old September 3rd, 2013, 13:36   #12
piotrsko
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Air density (density altitude) is a big factor to drag resistance. Flying calculations for take off change like this, too.
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Old September 3rd, 2013, 13:49   #13
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Quote:
Originally Posted by piotrsko View Post
Air density (density altitude) is a big factor to drag resistance. Flying calculations for take off change like this, too.
It certainly does. Think of air resistance like running the gauntlet in football. The spacing of the gauntlet is the air density. Your speed is just the same.

So, let's say running at 12 mph, you come into contact with 3 opponents per second. You have to overcome all of that force to continue your forward motion. If you double your speed (24 mph), you will come into contact with 6 opponents per seconds, which requires double the force to overcome and continue your forward motion. This is where lowering your speed comes into play... every bit of air you displace requires force and you "accumulate" for more friction. This is just like impedance in electricity... if you increase the speed of the electricity, it increases the resistance... and it is observed by the increase in heat generated... or just like water pressure... if you double the length of a hose, you reduce the exiting water pressure.

So, air density works exactly the same way. Let's say you double the density of the gauntlet. Now, at 12 mph, you come into contact with 6 opponents per second, which is the same amount of force required to overcome half the density at double the velocity. So, with more dense air, you can overcome the increased drag by slowing down (or by drafting). If the roads are icy, it would also behoove you to slow down. Other than that, you could improve your aerodynamics.
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Old September 3rd, 2013, 14:44   #14
Ultrasonic
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Quote:
Originally Posted by 03_01_TDI View Post
Plus the summer sun warms up the tire and road surface.
You have a point there. In that I think the peak pressures obtained in the tyres whilst actually driving will likely be highest in summer when it is sunny. That's certainly my obvservation based on a few measurements. That is, the difference between the 'cold' pressure that I set and the actual in-use pressure is likely to be largest in summer.

At one time I wondered if this accounted for the higher mpg I got on long runs in hot weather, and that if so I could match this all of the time by simply running with much higher tyre pressures . But when I looked at what the pressure differences actually were I realised that the rolling resistance difference was nothing like big enough. I then had a mini Eureka moment when it dawned on me that changes in air density with temperature could be the dominant factor. As posted above, when you look at the numbers it becomes apparent that this is the case.
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Old September 3rd, 2013, 20:47   #15
Diesl
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I do check my tire pressure. :-)

I have a cheap pressure gauge I keep in the car. I corrected the tire pressure four or five times since last November.

If the tire pressure slowly crept up with ambient temperature the effect would go in the right direction, but as somebody already pointed out the rolling resistance is most likely only a small part of the overall drag. (How to check: There are posts detailing the 'butt dynamometer' method, and how to extract drag from a coast down. It should be possible to fit air drag and other losses separately. Maybe somebody with a VAG-COM has already done that?)
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