ambient temperature: something I never would have guessed

[stringsattached]

I just read an article published by Cummins that was recently posted inside a thread in the "Interesting Articles" section. Page 27 of the publication states (verbatim):

Ambient Temperature
Air becomes more dense as temperatures drop, which increases air resistance. For every 10 degrees (Fahrenheit) drop in temperature, aerodynamic drag increases by 2%. Thus, fuel efficiency will drop by 1%. Overall, fuel economy tends to be higher in the summer than in the winter. According to North American Truckload Fleet Data, driving in the summer increases fuel mileage by 8-12% over driving in the winter months. Temperature also affects the tires' inflation pressure. Tire inflation tends to fall when the temperature drops.... blah blah blah

I thought this was pretty neat. Everyone probably knows that their mileage goes down when it is cold, but this would not have been one of the things I would have blamed. I knew that colder air is more dense, but I guess I never put two and two together. It is worth noting that this publication was specifically written for semi drivers, but I would think that the same general principle would hold true for smaller cars/SUVs. (Then again, maybe not.)

 

[mechtech]

A semi truck isn't very aerodynamic. There will be a little more drag on the highway with denser air.
A normal car that is driven in city conditions won't be affected nearly as much. It probably can't be measured.
Low tires create more drag whether it is summer or winter.
The choke or fuel enrichment is on much more in cold weather, and the thermal effeciency of the engine is less in winter, as well.

 

[Shannow]

stringsattached,
everybody knows that their mileage improves in cool weather...at least downunder, where cool is just cool enough to get a denser/cooler mix into the chamber, and not enough to drag heat out of the block.

Aerodynamic drag is proportional to densiyy, and inversely proportional to viscosity...and proportional to frontal area, and Drag coefficient.

Therefore big, unaerodynamic trucks show the biggest losses.

 

[Palut]

This does hold true for passenger cars, but with a less pronounced effect. For example:

I started commuting to a new school in the fall, and we had a very nice indian summer. The drive is 30 minutes each way on the freeway. I was averaging about 30.5 mpg to 31.5 mpg.

Suddenly, temperatures dropped from the 60's and 70's to the 40's. I added air to my tires to keep pressure where I want it, but mileage still instantly dropped. In the colder weather, I'd average 29.5 mpg to 30.5 mpg.

I'm back up to 31.5 mpg with my new intake. I can't wait to see what kind of mileage I can get in the summer!

 

[Jonny Z]

Where I am, summer means 100+ heat and AC and lower milage because of AC...

 

[Titan]

Don't forget more dense air holds more oxygen per unit volume (at any given pressure). So, your car can have a bit more oomph when sucking in cold air. Not necessarily good for fuel mileage, especially if you use that power when accelerating.

 

[UncleS2]

quote:

---

Originally posted by Jonny Z:
Where I am, summer means 100+ heat and AC and lower milage because of AC...

---

Don't forget the humidity & the summer formulated gas! High humidity seems to hit from two sides- paper air filters absorb & swell(& restrict airflow a bit), & air partly-to-fully-saturated with moisture seems to accept fuel spray less readily & burn less efficiently. And the lower-volatility summer gas drops mileage too- at least in my car it does.

In spite of what I've read many times, my cars have always gotten best mileage with temps in the 60s to just under 80* F range.

IMO, right *Now* is prime time for gas mileage here in NETx. Warmer temps, relatively dry air, no summer-formulated gas for a little while longer(about a month)- ya can't beat it with a stick!

 

[MADMIKE]

I agree--my car (1995 Maxima) definitely has much more snap and much crisper throttle response in the winter when it's breathing cold, dense, dry air and burning the more volatile winter gasoline blend. The heat and humidty really soften the throttle response.

 

[rpn453]

Interesting; I've never thought of that as a factor. I think it would have a significant effect on cars too. Semis aren't aerodynamically ideal, but they're also carrying a lot of weight and have a lot of rolling resistance, so the percentage of losses due to aerodynamic drag may be similar.

 

[Gary Allan]

(visions of Ice Station Zebra -paraphrased since memory is getting dim)

"A bullet travels just as fast up here."

"A bullet in this air loses 10' per second per second"

 

[oilyriser]

The effect is actually stronger for passenger cars, at least at highway speeds. According to that article, a truck will put about 1/2 its power to rolling resistance, and the other half to air drag. A car puts maybe 2/3 of its power to air drag, since it is so light, and has very little rolling resistance.

High altitude driving gives better MPG for the same reason. The air is thinner there.

 

[Bryanccfshr]

Humidity increases air density as well. For a standard rifle a 20% increase in humidity will cause a .308 to lose 1 MOA in elevation upon impact. If it can effect a profile as small as that it can effect a giant subcompact car, motorcycle etc. Temperature also has the inverse effect.. raising the temperature by 20°F increases the impact from the same rifle with the same lot of ammo by 1 MOA of elevation. (for the non shooters 1 minute of angle is equal to 1 inch per 100 yds)
Hot dry air offers less resistance than cold damp air.

 

[Titan]

bob wrote: I am guessing that the denser air causes leaner more efficient burn - improved mpg.

Nope, that's backwards. Hotter temps cause Fuel injected motorcycles to run leaner, which should yield better mpg. My Vulcan can be fooled into sensing the air temp is colder, and the ECU responds by enriching the fuel/air ratio. It gives it more power, but drops the mpg. It can be fooled by putting a resistor inline with the temp sensor wire. OR, you can add a fuel enricher, that attatches directly to the injector wires, and this use drops mpg, although gives more power.

Leaner running engines are hotter. Hotter temps also exacerbate that condition if you don't have an oxygen sensor in the exhaust stream. HOWEVER, hot, lean engines are more efficient, unless you are getting predetonation.

 

[bob_ninja]

Cooler/denser air affects different machines in different ways.

The only machine for which I noticed a difference is my bike. As Shannow stated, I got better efficiency in the fall around 50 and 60 F. I got between 5% and 10% MPG improvement. I asked several people for an explanation and got none.

Now bike's shape has a very low drag coefficient so air drag is not an issues until you reach 90+ mph. Thus denser air = higher drag doesn't affect bikes as much as trucks.
On the other hand, the engine uses carburetor which is not as precise as fuel injection. I am guessing that the denser air causes leaner more efficient burn - improved mpg. At least that is the only explanation that makes sense.
In any case I certainly see a significant improvement. (then again perhaps my air filter was too dirty causing rich mixture ....)

 

[oilyriser]

"20% increase in humidity will cause a .308 to lose 1 MOA in elevation upon impact"

very interesting. Humid air, in bulk, should be less dense than dry air, since O2 and N2 are heavier than H2O. There must be something special happening as the bullet travels at supersonic speeds through the humid air. Maybe it has something to do with the temperature of the compressed air in front of the bullet. Maybe more heat energy is lost to the bullet when there's moisture present.

 

[Bryanccfshr]

It is still a decent combustion with a fixed amount of fuel and air resistance analogy that has stuck in my head for over 15 years.
The rule of thumb that this is, was taught to me in the Marine Corps and using military shot logs over a couple of years with one rifle in several different climates I saw that the adjustment formulas for climate worked. Elevation also had an impact but I cannot remember off the top of my head how many feet above see level equalled 1 MOA but the round did travel further at higher elevations.

As for the scientific explanation of humidity causing the round to travel less distance I may be wrong but I always thought that humid air was denser near ground level despite the chemical composition of the compuonds in the vapor it is more of a meteorological phenomena than a chemical one. But then again, I just use the tools I am given and try not to overthink what obviously works. Note that the round leaves a nifty visible vapor trail as the air becomes saturated.

 

[Shannow]

Bryan, there are guys at the rifle club who can track the path of a .308 with the naked eye (when standing directly behind the dhooter...it's incredible, they can call a shot in the black or white at 600 yards.

 

[oilyriser]

Yeah, if the air is saturated to begin with, compressing it would heat it up, and any colder surface, like the bullet, would condense water out of the air. The bullet is probably condensing water as it flies along, which slows it down a bit, since more of the work that went into compressing the air is lost as heat. I bet if you could measure the temperature, the bullet would be warmer after going through damp air than dry air at the same temperature. I'll have to think about this some more.

 

[Bryanccfshr]

Given enough time on the range and accurate eyesight anyone can do it. A good shooter can tell where the round is hitting right after the round goes off. Making the call and the spotter will confirm or correct the shooter for dope adjustment and vice versa.


Back on topic, looking up the science of air density it is true that dry air is more dense. I have no explanation for the phenomena other than factors outside of normal aerodynamics.. perhaps combustion effeciency goes down. If the same muzzle velocity is reached at the end of the barrel at the moment the ballistics go from internal to external the round should theoreticaly travel further. This leads me to believe that the internal ballistics or combustion is less effecient(perhaps the water vapor taking up space in the chamber and barrel that denser air would occupy, which is counter intuitive to me). Either that or the explanation you made above concerning a supersonic phenomena.

Either I lost elevation for a scientific reason or I lost elevation because I believed I would(Marine Corps science)

10's of thousands of rounds logged though measuring humidity with dope adjustments to compensate for it, Strange.

 

[oilyriser]

I wonder if a pre-heated bullet would go faster. Does the hot air from the rad that flows under the car help reduce drag?