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Relationship of Ambient Temp to Fuel Economy

Relationship of Ambient Temp to Fuel Economy

Relationship of Ambient Temp to Fuel Economy

(OP)
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
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

RE: Relationship of Ambient Temp to Fuel Economy

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.

RE: Relationship of Ambient Temp to Fuel Economy

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.

RE: Relationship of Ambient Temp to Fuel Economy

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

RE: Relationship of Ambient Temp to Fuel Economy

(OP)
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

RE: Relationship of Ambient Temp to Fuel Economy

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.
 

RE: Relationship of Ambient Temp to Fuel Economy

(OP)
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

RE: Relationship of Ambient Temp to Fuel Economy

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.

RE: Relationship of Ambient Temp to Fuel Economy

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.............

RE: Relationship of Ambient Temp to Fuel Economy

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.

RE: Relationship of Ambient Temp to Fuel Economy

Greg,

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

RE: Relationship of Ambient Temp to Fuel Economy

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.

RE: Relationship of Ambient Temp to Fuel Economy

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

RE: Relationship of Ambient Temp to Fuel Economy

(OP)
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

RE: Relationship of Ambient Temp to Fuel Economy

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

RE: Relationship of Ambient Temp to Fuel Economy

(OP)
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

RE: Relationship of Ambient Temp to Fuel Economy

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
cheers

RE: Relationship of Ambient Temp to Fuel Economy

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

David

RE: Relationship of Ambient Temp to Fuel Economy

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.

RE: Relationship of Ambient Temp to Fuel Economy

(OP)
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

RE: Relationship of Ambient Temp to Fuel Economy

zdas04
Can you rule out things like this .... ?

Clutching at straws idea #1.
The slightly higher air resistance in cold weather just tipped the balance between the vehicle using one gear ratio (eg 6th in the warm weather) and another (eg 5th in the colder weather), and then the difference is due to the engine running different speeds and throttle settings, even if the road speed was the same.

Clutching at straws idea #2.
The engine is so overpowered that at light loads a significant portion of the engine power is just used to overcome engine mechanical friction, driving accessories, pumping losses, etc. If you need half your income just to live, then a 10% pay increase can give you a 20% increase in disposable income. Maybe the lower air resistance with warm air is leveraged in a similar way to give a larger fuel saving than one might naively expect.

RE: Relationship of Ambient Temp to Fuel Economy

I would question the accuracy of your data.  Besides the potential error in calculations, wind, temp, humidity, tire pressure, etc. there is also an uncertainty in the honesty of the vendors selling the gas.  I recall a few years back where a number of stations rigged their pumps to short change their customers after the first five gallons dispensed.  Same guy sold the software to all the stations. The software always metered correctly for the first five gallons to make standards testing for compliance with NIST Handbook 44 difficult.  As I recall, most compliance tests for measures did not exceed 5 gallons.

I have also heard stories of folks adding gas to vehicles where they knew the owner monitored their mileage.  When they stop adding the fuel, it drove the owners crazy when they tried to duplicate the mileage.

Lastly, you may have had a whiskered spark plug that did not misfire at idle.  It is possible that your vehicle was not running properly when you had the poor mileage at speed.  Whiskered spark plugs sometime loose the whisker and start working properly again.    

RE: Relationship of Ambient Temp to Fuel Economy

(OP)
Zapster,
Any of that is possibile, but the vehicle is pretty new (2005 purchased in Feb 2005) and has had all the service that the manufacturer called for.  The first tank of gas in each case was purchased at the same place.  The second tank in each case was purchased from a stations along the interstate.

David

RE: Relationship of Ambient Temp to Fuel Economy

Sounds to me like the cold start enrichment is faulty and enriching the engine continuously when the ambient temperature is low.

30% is just way to much for different air quality.

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.

RE: Relationship of Ambient Temp to Fuel Economy

(OP)
Thank you for that, it seemed way too much to me as well.  What is "cold start enrichment", I haven't heard that term before.

David

RE: Relationship of Ambient Temp to Fuel Economy

On carburettors it used to be called a choke.

On mechanical fuel injected race cars, it is often a squirt bottle aimed down the inlet.

On modern EFI, I am not sure how they determine the enrichment needed for cold start, but they will need some method. I would think it is built into the ECU and works off signal from ambient and coolant temperature sensors.

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.

RE: Relationship of Ambient Temp to Fuel Economy

The major difference in fuel use is air density between the temp extremes with other influenes as well.
In cold weather the air density needs more fuel to maintain correct fuel ratios and this also oftern feel like more power.
In hot temps the reverse happens.
EFI systems make these changes so you see the result in fuel milage as long as all else is the same.
Blame it on the air twmp and oxygen sensors.
The more air that is combusted, the more oxygen shows up in the exhaust, the more the ox sensors call for riching up the fuel to maintain a 14.7 to 1 ratio.

RE: Relationship of Ambient Temp to Fuel Economy

So your theory is that David was driving faster or accelerating more quickly?

Cheers

Greg Locock

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

RE: Relationship of Ambient Temp to Fuel Economy

Thats not really EFI systems work.

There is always a method of determining the air amount that is entering the engine (often using a temp sensor as well).

The amount of fuel that is injected is calculated in a software function that uses (at its most fundamental level) air mass per stroke - this is the feedforward fuelling amount.

The O2 sensors are then used to Feedback & 'trim' the fuelling to bring the AFR to stoich. Then a lambda dither is effected to allow the catalyst to work.

It is NOT a case of the you opening the throttle plate to a given angle and the O2 sensors demand & control the fuelling.

If as you say the density is changing then the air amount per stroke is changing (as is torque), so you are not actually comparing apples with apples. In effect the BSFC will remain constant for a given airmass - with temp not being a major factor.

By the way, if you want a detailed explaination of cold start enrichment I suggest you start another thread & I will oblige.
MS

RE: Relationship of Ambient Temp to Fuel Economy

Simplistic approach to follow:

18.1MPG to 22.4MPG - 23.7% increase in fuel economy.

Drag Force = A*cd*p*v^2      Power = Drag Force * v

A = frontal cross section area
cd = drag coeff
p = density
v = fluid velocity

13.5% Temp increase = c.13.5% density decrease

All else being equal in the drag force formula, a 13.5% decrease in density would lead to a 13.5% decrease in drag force, and a corresponding decrease in power required to move the car at velocity v.  This is a decrease in NET power that the engine has to supply.  If we multiply by the thermal efficiency of an IC engine of say 30%, it should take 45% (13.5/0.3) less fuel to power the vehicle.  This only applies to the air friction component of the vehicle.

This is too large for me, so there must be some corresponding losses in the situation:
The hot (lower density) air will result in less air/fuel mix being charged into the cylinders, so the power output engine output per cycle is less while rotating losses remain the same.  Auxilliary systems (air con, cooling pump) would use more power at the higher temps.

The decrease in the drag component would be offset by the decrease in mechanical efficiency and a 23.7% increase in fuel economy sounds feasible to me.

A man is never so right as when he's talking about cars, but I'll have to admit the above logic could be flawed - the engines I have experience with are 2000hp, weigh 15 ton and are stationary when everything's going to plan, so air friction isn't something I have to contend with in a professional sense.  

LewTam Inc.
Petrophysicist, Leading Hand, Natural Horseman, Prickle Farmer, Crack Shot, Venerable Yogi.

RE: Relationship of Ambient Temp to Fuel Economy

In my area of western Canada, the service station gasoline pumps correct the volume delivered to a standard temperature. When the gasoline in the storage tanks is warm, more volume but the same mass of gasoline is delivered.
Could volume correction or lack of volume correction in either the service station pumps or the onboard measuring systems be a contributing factor?
respectfully

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