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

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[crysta1c1ear]

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.

 

[Zapster]

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.

 

[zdas04]

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

 

[patprimmer]

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

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[zdas04]

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

 

[patprimmer]

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
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[OP800]

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.

 

[GregLocock]

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



Cheers

Greg Locock

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[mattsooty]

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

 

[lewtam]

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.

 

[waross]

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