Cold air density vs MPG
 

Recently I drove 800 miles through 15F to 35F, dry air and was able to monitor my mileage. Instead of the usual 53 MPG @ 65 miles per hour, I was running 49 MPG or worse at temperatures below freezing. Thanks to Ken@Japan who provided the air densities at different temperatures, I was able to put this spreadsheet together:

TempF TempC density_kg/m3 %_density MPG
15 -9.4 1.34 111.7% 47.5
32 0 1.29 107.5% 49.3
68 20 1.20 100.0% 53.0
85 29 1.17 97.5% 54.4

There was good agreement with the air density changes and MFD displayed mileage. The MFD has a horizontal scale line at 50 MPG so it was easy to see the pattern.

Bob Wilson

Re: Cold air density vs MPG
 

One other thing that effects density is the barometric pressure. When I was taking flight training we used to have to calculate density for every flight. There is a formula for density altitude that pilots use to determine the performance of the aircraft. You need the altitude, barometric pressure, dew point and outside temp.

See this wiki article http://en.wikipedia.org/wiki/Density_altitude


Here is a calculator http://wahiduddin.net/calc/calc_da.htm

Re: Cold air density vs MPG
 

So then might the short air intake drawing air from inside the engine compartment offset the cold air losses over a longer trip?

Might be worth the investment over a span of years driving in the north.

Re: Cold air density vs MPG
 

Hi,
The drag has to do with the weight of the air that has to be pushed aside and pushes against the vehicle motion. Take for example, swinging you hand through water, a very dense fluid, versus air, a much less dense fluid. As the air gets colder and colder, it becomes denser and like water, becomes harder to move through.

Don't forget that the heated air from the engine compartment takes energy. So even if there were enough hot air to make a difference, the new problem would be the amount of energy it would take to heat the air.

No, the only thing that helps is either slower speed or aerodynamic improvements.

Bob Wilson

Re: Cold air density vs MPG
 

Ah, so cold air is just heavier to push into for a car. Thanks for the corrections!

Re: Cold air density vs MPG
 

I thought that reduced mpg in cold air was because the battery's electric output drops as the temperature does.

Re: Cold air density vs MPG
 

Not really since the battery also 'warms up' during operation or in the summer, may be cooled by a fan. The battery is important but it is not the primary source of motive power.

The electrical parts of our hybrids solve the problem of inefficient ICE operation regions. What this means is when the ICE runs badly such as at idle or low power regions, the battery/motor can 'fill the gap' and turn off the ICE. Later, when more power is needed or the battery level is low, the ICE starts up but runs in an efficient mode.

One of the curious aspects is that a low speeds in cold temperatures, I can get better than 50 MPG after the car is warmed up. At high speeds, the car has to displace denser air, the drag increases. But once warmed up and at low speeds, the MPG greatly increases.

Bob Wilson

Re: Cold air density vs MPG
 

There are a ton of things that the colder air/dense air can affect.

- Drag on the aerodynamic properties of the vehicle.
- Changes to fuel use due to the fuel/air mixture changes using denser air.
- The temperature of the fuel itself affects how it atomizes before combustion.
- Colder air going through the radiator and pulling more heat out of the cooling system, thus needing more heat input from the engine to maintain an optimum temperature. Normally not a real big deal on non-hybrids because they usually generate more heat than they need, but when the ICE of a hybrid kicks in, or it doesn't let you get into EV at all, only because the temperatures are too low for optimum operation - that's fuel used for things other than normal operation that would not be needed in warmer weather.

Re: Cold air density vs MPG
 

Hi,

Actually I have a Graham miniscanner in my Prius and was monitoring:

• ICE coolant temperature
• MG1 temperature
• MG2 temperature

At highway speed, 65 mph, and using a 'water noodle' lower grill block, the car reached thermal equilibrium within 30 minutes. Thereafter, the changing temperature effects were visible on in the MPG display. BTW, aerodynamic drag is proportional to the air density, which was a function of the air temperature. I was driving from one edge to the other edge of a high pressure sensor.

The fuel-air mixture is handled by mass-flow sensor and the Atkinson cycle pushed part of the charge from the hot cylinder back into the hot manifold. It is pretty well warmed up during the evaporation cycle. You're normally right about heat being needed at lower speeds but at 65 mph the energy demands provides a surplus of waste heat.

Bob Wilson

Re: Cold air density vs MPG
 

I guess I wasn't even thinking about anything at highway speeds because EV isn't even possible (at least in my FEH - is it possible to go EV at highway speeds in a Prius?) so it really becomes irrelevant in that case.

I was considering the cases where EV should be possible, but the minimum temperature requirements programmed into the system will not allow it so the engine is forced into ICE-on condition in order to maintain adequate temperatures.