Actually, as the air temperature goes up, the air becomes less dense and the engine becomes more efficient due to less pumping loss, even if it outputs less maximum HP wide open. This can be easily seen as the temperature goes up from freezing into summer heat. Now I don't know if this is true all the way to 110F, but I'm sure it is in moderate temperature range I have here of 20F to 90F. As far as this particular Phoenix/Yuma
trip I would think AC was running full steam all the time, as well as radiator fans. Add to this empty highway with high speed limit, little head wind and battery thermal protection kicking in, limiting current and I can see how you could get unusually low mpg. Few years back I rented small car to go to Death Valley in summer, the temperature must have been 120F or more, so hot the AC stopped working all together. All went back to normal as soon as we got back into more "normal" 105F-110F area, But this particular experience made me think most cars are designed with around 110F as upper limit and you were right there.

 

As a current inmate of Arizona, I would offer 2 suggestions:

1) The air conditioner is electric, so you will be expending massive amounts of energy to cool the car at 110 degrees.

2) Hot Fuel. As the temperature increases, the density of a substance decreases. Gasoline is especially sensitive to this. The energy available per gallon of gas is lower, Some truckers measure the temperature of the fuel they purchase to correct their expected mileage. Google 'Hot Fuel' for further explanation. Be warned, you may become rather upset as you research the subject.

 

The "reduced pumping losses" is a very minor improvement in engine efficiency and you have to ignore the fact that pumping losses are dependent on the cubic feet of air pumped not the actual density of those cubic feet.

The major issue is the reduction in efficiency due to a higher heat rejection temperature. Look up the Carnot cycle efficiency and see what happens as the heat rejection temperature rises.

When you couple that with the required increase in engine HP to run the A/C at a given cooling level the loss of fuel mileage is significant.

 

Interesting discussion especially the item on "hot fuel." I did a search and found the topic very interesting.

Thanks

John

 

Seems the difference is about .5 gal in 60F-90F temp increase for 25 gal tank (I don't think underground tank would heat up to 110F). So that would make about 2% mpg change, from 40MPG to 39.2 mpg, really big deal, when AC usage alone can drop by 2-4 MPG in my car, in moderate weather, I'm sure much more in 110F. I also see much bigger mpg drop when using ethanol blend fuel. You could also blame it on the full moon gravitational pull, after all whole oceans rise their level by quiet few feet. The only people benefiting from this "hot fuel" debate are lawyers involved IMO.

 

I've noticed this. As soon as the temps got into the 90s... nights in the mid 70's... the TCH's mpg when straight down the drain. (Mind... the car is in garage so sometimes it is as hot or hotter than ambient.) I think this is due to the traction battery protection systems. The ICE runs more as its not on battery and will not charge battery until they get down to the correct temperature range for charging. A/C runs more... and FWIW.... the "ECO" system is totally worthless on a 90F day in the bright sunshine. It can't cool the car down and keep it comfy... RECIRC or not... You have to run it off Eco/ on RECIRC to be comfortable.

The car doesn't cool as well and the cascade of events is that the batteries don't pick up really cool air from the interior. It takes them longer to get into the charge temperature regime.

Also, hot pavement radiates heat on to the bottom of the car.... again... sort of broiling the batteries from the bottom as well. More heat to reject...

There's another evil hand at play here as well. It's called "density altitude"... and contrary to a previous posting... it does not make the engine more efficient due to decreased pumping losses. It makes it less efficient as the engine thinks its at a much higher altitude. Now at 93.6F, DP 57, Baro 30.01, Altitude 1660ft.... the density altitude is 4317'!!!! Its as if you suddenly moved from Texas Hill Country to Colorado !!! Your lungs and your engine are going to be sucking big time.

Using an 'engine tuner's calculator' you find that the ICE is producing only 92.5% of its sea level (standard temp pressure) rating of 147 hp or about 135hp. That's a significant loss in this low hp machine. Maybe more so since its a modified Atkinson cycle and the 'calculator' is for Otto cycle engines. So... you put your foot in it a lot more to give it that same acceleration that you have under normal conditions... but remember your batteries aren't pulling as much of the load so you go into it even deeper. The result is more fuel consumed....

We've gone from 38 - 40/41 mpg down to 35 -36 ...

I hope this helps... cryptic as it is...

 

Can you explain why i'm getting 57.7 mpg with a 68% full tank of gas. (AC is off) My wife and I drive to town usually after it's cooled off or at dusk. I have my own wx history using my davis wx station.

Leaving for town at 8 pm the history came up with a density altitude of 7121 feet. The return trip was at 11pm in which the density altitude was 6573 feet. The barometer had increased may be the reason. I could tell the TCH tended to stay longer in the EV mode more than normal.

As many know I live 8 miles in the country near Alamogordo, NM (White Sands, Holloman AFB) Out here their is not that much traffic and a 4-lane almost anywhere you go. I drive at 40 mph using the cruise. My almost daily travel loop is 32.66 miles.

The density readings I show here were on 7/18 Saturday night. My mpg read 56.6 when we left for town. When I got home it was up to 57.7 That's a full 1.1 mpg gain for this trip. I'm hoping to build this tank to 58 or maybe 59 mpg. I may end up barely over 50. I will know in about 20 days.

I have owned may cars and been always been driving for top gas mileage. I have saved lots of gas over these last 45 years of driving. This '07 TCH has been the biggest jump in mpg so far. I did have a '03 Corolla that averaged 39/40 and 42/43 highway driving similar.

You mentioned mountain driving. I once owned a VW Rabbit diesel. I drove it on a vacation to Colorado. The car averaged 55 mpg in the mountains. My theory, what goes up has to come down. It was a 5 speed manual shift. I put it in neutral and let if fast coast down. Nice slow curved 4-lane highway made it easy to do. My climb speeds were around 40/45 mph so as not to crowd the little 1.8 engine. Never will forget that 65 mpg highway on the way home. I was driving 60 using the cruise. My theory on the Colorado driving is the low humidity due to the elevation.

Did I mentioned where we live it's 4050 elevation. In the north part of town were we go a lot it's 4400 feet.

Here is a current picture of my mpg and note the fuel gauge. I hope I can add to this when it reads near empty. Most times I loose the good number and end up with 50/53 mpg.

click for large view.

 

My highest ever tanks in my old Honda Accord were in the mountains, constant up and down, winding roads and you bet I was very surprised. Part of the thing was that in the mountains I couldn't drive my normal 70 MPH I would do on highways, but still climbing the hills on almost full throttle in lower gears, I was expecting much worse economy and it wasn't the only time, every time I would go into mountains it would be the same. Whatever the theories are they do not follow my experience. When your engine is not full throttle, the air flow is restricted and that would cause loss of energy, the less dense air is, the less restriction and less force needed to move the air for the same power output. I can tell you one thing, it's not hot fuel issue on his trip for sure, I would put my bet more on bad fuel or even rigged pump dispensing less fuel than shows.

 

No clue, RBurt.... low speeds, minimal traffic, few stop lights, no A/C, less fuel on board (less weight), higher tire pressures, tire brand?, driving habits??? ... and maybe some old fashioned good luck sprinkled on top.

Its an excellent result for you and your TCH experience. We've been lucky to bust 42+ on trips to SA ... but only on occasion. Down hill to there it goes up... goes away coming home and up hill.

Never the less... DA plays a role, IMHO. On a stinking hot day this car ... and our Sienna van ... is down on power. It is noticeable and the fuel consumption goes up some.

 

Hmmm...I think I also have a 0.04 GPH on my ScanGauge which would explain why my Trip Fuel Economy goes down when I turn off my car at an extended red light or if I simply turn off my car and let it sit with the ScanGauge still on.