Temperature and MPG: Wind drag experts needed... Ernie?

[Flatland2D]

It seems that temperature could play a significant role in fuel economy. Today's trial was higher than my previous trials, and it was a good deal hotter too. Refer to my results posted in the tire pressure thread in the experiments section for details.

What do you guys think about this? Can temperature play that much into MPG? I got 3.2 mpg more today and it was 16 degrees hotter. I don't believe there to be that much error in my measurements. It makes sense that denser air would take more force to push through. Thoughts?

[diamondlarry]

Denser air takes more fuel. With the air being denser, the O2 sensor picks up on the higher amount of oxygen in the exhaust and the ECU dumps in more fuel to compensate.

[Matt Timion]

Originally Posted by diamondlarry

Denser air takes more fuel. With the air being denser, the O2 sensor picks up on the higher amount of oxygen in the exhaust and the ECU dumps in more fuel to compensate.

I guess this means that theoretically I will get better gas mileage than you because I live at around 4000 feet (air is less dense). Does this make sense?

[SVOboy]

Originally Posted by Matt Timion

I guess this means that theoretically I will get better gas mileage than you because I live at around 4000 feet (air is less dense). Does this make sense?

I think this is why there is such a thing as california epa stuff and all that, but I'm not sure.

[Ernie Rogers]

Hi, Flatland, sorry, just saw your post--

Your experience is consistent with mine-- substantial improvement in mileage on very hot days. To me, 16 degrees doesn't seem like much, but okay. Getting out my phyics & chemistry handbook--

Here is the air density (dry air, normal sea level pressure):

10 deg. C / 50 F --- 1.25 kg /m^3
21 deg. C / 70 F --- 1.20
30 deg. C / 86 F --- 1.165

I put these densities into my fuel economy spread sheet, and got the following miles per gallon for my car:

50 deg. F --- 53.42 mpg at 70 mph
70 deg. F --- 54.52
86 deg. F --- 55.35

The second and third numbers are just 16 deg. F apart. Nope, you can't explain your results based on the change in air density, or on air drag. But, in reality I get bigger changes with my car too. Where to look--

Does the engine change efficiency? Maybe.
Do the tires change rolling resistance? They surely do.

Contrary to what I hear most people say, I believe it's true that soft rubber gives lower rolling resistance than hard rubber. And, rubber does soften up a lot with warming. Adding that to a slightly higher tire pressure on a warm day, I suspect the improved mileage is partly due to better tire performance.

Who knows, maybe there are some other temperature-sensitive elements we've overlooked.

Ernie Rogers

[Flatland2D]

Thanks for the good info. So the math doesn't really prove anything about drag. But from both our experiences temperature has been a significant factor.

I'm sure efficiency changes with temperature. That's the whole purpose of the IAT sensor mod. Force your ECU into reading a much hotter temperature and therefore use a leaner mix. Larry gets great results with this mod. I only got about an extra 1 mpg with it. It's possible I was using the wrong valued resistor. The engine was OBD2 converted to OBD1 and probably still uses the same IAT sensor now as before. I am pretty sure there is some consistency in all these sensors though, so I'm not sold on the idea that that is why I didn't see the huge gains.

I am not sure a change in tire performance is responsible for this either. That was the purpose of this experiment - test a higher pressure than normal. The change in fuel economy was greatly overshadowed by the error caused in temperature differences. My base mpg was 34.7 and that was when my tires averaged about 27 psi all around. I didn't get much better at similar temps than when I had 40 psi in them. I'm not sure if I made note of this in my results but the pressure on the hottest trial was 42 psi. I've measured the asfault at 140 degrees on a hot day with an infra red temp gauge. That easily accounts for the extra 2 psi and the rubber was probably a little softer. However, if the 13 psi gain from doing this experiment didn't change things, the extra 2 psi didn't do squat.

I graphed my results vs. temperature and fit a curve to it. All the results are within 1 mpg of the curve. Temperature has to be responsible for this correlation. Whether it's through reduced drag, ECU using a leaner mix, hot air simply combusting better, or all those factors and more together, there is a significant difference in efficiency at different temperatures. It might be possible to test if it has to do with the intake temp. You could use a fixed value resistor reading a normal temp (say 100 degrees) and do trials at different temps. But maybe the hot air allowing for better fuel vaporization would still give you gains. I guess it'd be too hard to hold them all constant and test one at a time.

What do you think about my idea to normalize fuel economy for uniform comparison? Would it even be possible to figure 37.0 mpg at 94 degrees would be equal to xx.x mpg at 72 degrees? This way it doesn't matter if you do your trials in the day or night, hot or cold, you can always convert your results back to a standard temperature. I suppose this would only be possible with extensive testing at different temperatures to generate an accurate curve.

[b1gmoose]

Good Morning,

Some ideas to chew on.

1) Cold air is dense. As it gets colder, density increases. Increased densisty means more oxygen can fit into the cylinder. This is the idea behind having an intercooler on your turbo. It'll cool the charge down and you can get more air in your cylinder that way.

2) Cold air is dense. The denser it is, the harder it is to move through. Ever wonder why the speed records are set at the salt flats in Utah? And not on the tundra of Alaska?

At -20 deg F, it takes quite a bit of energy to move through the dense air. Yes, your engine is slightly more powerful with the increased oxygen content. But also, remember, most vehicles coolant flowing through the throttle body to heat it so it won't freeze over.

~ryan

[Flatland2D]

I know cooler air has more oxygen per unit volume, but there is also the theory going around that a warm air intake is better for fuel economy. Refer to SVOBoy's post about this. The idea is that either warm air helps fuel vaporize better or that the warm air causes a leaner mix through the IAT sensor. Maybe both.

Ernie's math didn't directly support the idea that denser air could have caused the drag leading to the differences I got.