Do hot ambient temps have a negative effect on FE?

[basjoos]

Originally Posted by Old Tele man

Cross-country race planes back in the 1930-40's would fillup with COLD fuel and takeoff in the morning because as the day warmed up, so did the fuel, which then expanded in volume...essentially allowing a 100 gallon fuel tank to contain MORE than 100 gallons.

The thermal expansion coefficient for gasoline is 0.00053, so if the pilot filled his tanks with 100 gallons of gas at 60 degrees F in the morning, that pilot's tanks would be holding 101.59 gallons of gas when the temps got up to 90 degrees F later on that day. But, pilots flying distances cross country usually climb to a high altitude for more efficient flying and the air temps at 25,000 feet certainly aren't going to be at 90 degrees F.

The temperature lapse rate is 3.56 degrees F /1000 feet. If the pilot took off from an airfield at 1000 feet above sea level on a 90 degree F day and leveled off for his cruise at 25,000 feet above sea level, his outside air temp would be 5 degrees F, so the fuel wouldn't remain at a temp of 90 degrees F for very long. Since the big radial engines used by air racers in the 1930's typically had fuel burn rates in the 20 to 30 gallons per hour range, even if that pilot had a 1.59 gallon increase in fuel on board, that increase would only translate into a few minutes of additional run time. I guess every little bit of advantage they could get helped.

[XYZ]

Originally Posted by basjoos

The thermal expansion coefficient for gasoline is 0.00053, so if the pilot filled his tanks with 100 gallons of gas at 60 degrees F in the morning, that pilot's tanks would be holding 101.59 gallons of gas when the temps got up to 90 degrees F later on that day. But, pilots flying distances cross country usually climb to a high altitude for more efficient flying and the air temps at 25,000 feet certainly aren't going to be at 90 degrees F.

The temperature lapse rate is 3.56 degrees F /1000 feet. If the pilot took off from an airfield at 1000 feet above sea level on a 90 degree F day and leveled off for his cruise at 25,000 feet above sea level, his outside air temp would be 5 degrees F, so the fuel wouldn't remain at a temp of 90 degrees F for very long. Since the big radial engines used by air racers in the 1930's typically had fuel burn rates in the 20 to 30 gallons per hour range, even if that pilot had a 1.59 gallon increase in fuel on board, that increase would only translate into a few minutes of additional run time. I guess every little bit of advantage they could get helped.

You can recite all the figures and calculations you wish to convince everyone. You may be right. Your statistical analysis may or may not be convincing to everyone reading it.

However, the important and pertinent concept involved is that gasoline expands and vaporizes as ambient temperature increases.

I guess every little bit of advantage they could get helped.

I've heard that in many ways on this list, from those who are proponents of hypermiling and seeking every advantage possible. I would think this possibility (and probability) might be of interest to those who try to conserve energy, no matter how small the gain.

Everyone can do as they please. But I will still avoid fueling up late in the day when the ambient temperature is over 90 degrees, if I can. By doing so I have nothing to lose - and possibly something to gain, no matter how small the gain.

[jamesqf]

Originally Posted by XYZ

However, the important and pertinent concept involved is that gasoline expands and vaporizes as ambient temperature increases.

Another important concept is that if you have a reasonably modern car (since the '80s, at least) the fuel system is sealed. Just for simplicity, assuming your car is parked for a few days with a full tank, around here the gasoline might warm up to 100 F in the day, yet drop to the mid-50s at night. Yet no gasoline will escape from the tank: what expands and vaporizes during the day will condense and contract at night.

Indeed, you don't even need a modern fuel system. During the '70s oil embargo, I worked in a junkyard (in California's San Joaquin Valley, where it gets pretty darned hot). Because gas was so expensive, we started draining the tanks of wrecks that had sat for years, and using that gas. Cars kept running on it.

[XYZ]

Originally Posted by jamesqf

Indeed, you don't even need a modern fuel system. During the '70s oil embargo, I worked in a junkyard (in California's San Joaquin Valley, where it gets pretty darned hot). Because gas was so expensive, we started draining the tanks of wrecks that had sat for years, and using that gas. Cars kept running on it.

That supports what I stated in the other thread (the one about "stale" gas). I too have used gas that is years old, with no problem.

[Mustang Dave]

In my experience, hotter ambient temperatures = better fuel economy. (And I have the fuel logs to demonstrate it - 168K miles w/Big Red and 59K+ miles w/White Pony.) Less engine warm-up time, lower lubricant viscosity, and lower air density (less aerodynamic drag) all contribute to better fuel economy. Summer fuel blend also makes a significant (positive) difference of about 1 MPG in my Mustang.
Fortunately, it rarely gets hot enough in this part of Arizona that I need to use A/C, (except when I have a female passenger ), so A/C's rarely a factor.