Relationship of Ambient Temp to Fuel Economy 1

[zdas04]

I'm currently on a road trip in a V-8 SUV. Similar trips in winter averaged between 17.2 and 18.1 mpg. Today through two tanks of gasoline (91 octane) I averaged 22.4 mpg. The last trip in winter, the ambient temperature was close to freezing. Today it was close to 100F all day. Both trips were similar duration and similar changes in elevation. The available power did not feel any different today and on both trips I tended to drive the speed limit (i.e., 75 mph on the interstates) with cruise control engaged.

The vehicle is a stock Land Rover LR3 V-8 with 22,000 miles on it and a curb weight of about 6,500 lbm (with my tools on board).

While combustion is not my field, my Masters emphasized thermodynamics and fluid flow so I'm reasonably familiar with what I think of as the underlying physics and I can't come up with a reasonable explanation for improved fuel economy at higher ambient temperatures.

Has anyone noticed this phenomenon (I can't find a reference to it in eng-tips, so I'm assuming it is so well known that only a neophyte would ask or it is "new") and is there a physical explanation for it?


David

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[GregLocock]

Air density is one thing. That will directly affect the drag you get. Also, the less dense air will force you to drive at a higher throttle opening, reducing your pumping losses.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[drwebb]

Fuel formulations get adjusted seasonaly- and higher octanes possibly more. Not sure how this would affect, but its another variable. For the magnitude you got, suspect a combination of factors.

 

[stout1]

I like the point about the fuel formulations, I notice a diiference in fuel mileage depending on which state I fill up in (MN has a different formula than WI). The other concern I have are the other variables (there are many); with such a big swing in temp you need to be aware that your differentials, u-joints, CV axles are requiring much more energy to operate. I have seen this at the track, when all other conditions are set equal a car will run better times when towed with a tow bar vs. putting it on a trailer, the tow bar allows the components to turn and warm up on the way to the track resulting in faster times.

Michael

 

[zdas04]

From a thermodynamic standpoint, the cooler-air-higher-density theory makes a lot more sense than anything I can come up with for what I've observed yesterday and today (485 miles averaged 22.0 mpg, it has never been better than 18.1 before).

Yesterday I bought gas in New Mexico and Arizona. Today I bought gas in Arizona and California.

David

 

[crysta1c1ear]

At high speed, air resistance is significant and the denser air will provide more resistance, particularly for a vehicle with a coefficient of drag of 0.41 or so (depending on model year, source

http://www.theautochannel.com/news/2004/09/23/233791.html)

and a large frontal area.

However, I don't see how the air density could change enough to make that difference in milage. However if you had the wind behind you on one journey and the wind against you on another it might make a significant difference. A 5MPH wind might go unnoticed, but with you or against you can make a 10 MPH difference. Call it 9 MPH in 75 MPH = 12%. Air resistance is say a cube law with speed, but a square law with journey distance. 1.12*1.12 = 1.2544 so just a 5MPH wind can make a real difference to the fuel required to overcome air resistance, 25%-ish.
At say 8 kW rolling resistance and 32 kW air resitance that would reduce to say a 20% difference overall.

I don't see air density making all the difference, but I think a 10 MPH or more difference in wind speed on the 'today' journey could help explain things if applicable.

 

[zdas04]

Both trips were at the speed limit. Yesterday and today the speed limit was mostly 75. On the trip in winter the first tank (in New Mexico) was at 75, the next (in Texas) was at 70 (I think) and there wasn't any real difference from tank to tank. I don't recall wind velocity in the winter trip, but today I think I would have noticed a 10 mph wind blowing sand and I didn't see any.

David

 

[GregLocock]

High humidity air is less dense than dry air, and the water apour may help with combustion efficiency.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[firefrog]

Hi zdas
One thing that hasn't been mentioned yet is the viscosity of the lubricants in your vehicle. Everything is a bit 'stiffer' when the ambient temperature is lower. Ever notice how many grades of oils and greases are on the market? Most of us don't have the time or inclination to switch over to accommodate the seasons. Another compromise we have to live with.............

 

[CCycle]

All the above is good but the most important is lubricant viscosity. Anybody remember the old VW bugs, the ones with 30 hp or so? Top speed on them was down 10 mph on cold days. We used to put engine oil in the transmission during winter to get performance back up.

A couple more things;
Boats are slower in cold water. Their speed difference matches the difference in water viscosity not the difference in water density. The difference in water density is negligible over this temperature range. I suspicion that something similar is occurring with air drag.

Fuel formulation is important. Not only the seasonal difference but area and brand differences.

 

[macmet]

Greg,

I'm just curious as to why increased water vapour increases fuel efficiency.

 

[macmet]

Greg,

Alright, I just looked through the site and saw a few threads already on this subject. I will just read through them and I'm sure it will become clear. I don't want to steal this thread.

 

[gpara]

I have noticed the same thing in driving at colder temps/highway speeds. The dense air does indeed create lower fuel economy. On one trip at 32 degrees I could only get 38mpg and when returning at 57 degrees the economy was back up to 41mpg, same day, basic calm air, same fuel in tank, the only difference was the outside air temp. In summer, on a very hot day, I can get 44mpg, this only occurs in hotter temps, the car is a 2005 chevy malibu v-6 with onboard MPG meter. The accuracy is about 3% from actual tank fill calculations.
Gary

 

[zdas04]

Air density is possible since my vehicle has all the aerodynamic styling of a brick and I am talking about 60-70F difference between the two tests (13% increase).

David

 

[circuit1952]

Did you run the heater/defroster in the winter? Are you using air conditioning now? Both can impact fuel economy.
Did you pack more/heavier clothing for your winter trip? Look at

http://www.fueleconomy.gov/feg/factors.shtml

 

[zdas04]

For both trips I had my tools in the back (about 1,200 lbm), so I'm not sure that my clothes were a major factor. In fact, for the summer trip I packed more clothes because I was staying longer. Yes I ran the heater in winter and the air conditioning in summer--but the economy was much better in summer.

David

 

[Grouser]

What about tire pressure and temperature effects? Were tire pressures the same? Tail winds/head winds?

Auto engines seem to run better if ambient air is at 50-65 degrees rather than 75-90 degrees, I suspect because of increased fuel/air density. The fricion increases becauses of lower temperature lubricants postulated above makes sense.

Cheers

 

[zdas04]

I check the tire pressure weekly and keep it dead on the manufacturer's recomendations.

David

 

[patprimmer]

If you are seriously looking for fuel efficiency, up the tyre pressure considerably.

To go one step further, use the narrowest tyres that you can fit and retain load carrying capacity.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[zdas04]

We've wondered afield here. I'm trying to figure out why with all other things nearly equal I got a 30% fuel economy buzz when the only difference I can see is the ambient temperature went from about freezing (17.2 mpg) to 100F (22.4 mpg).

I know there are many things I could do differently if I was serious about fuel economy, but I work in Oil and Gas and drive a V-8 Brick on Wheels, it is hard to assume I care a lot about fuel economy except as a mental excercise.

David