Summer/Winter Gas, Not that Simple

[edhackett]

We talk about summer/winter gas formulations and their effect on mileage. Here in Reno we are required to use oxygenated fuel from October to the end of January. On top of that there are vapor pressure and vapor lock protection specifications that change almost monthly. Below are the requirements for the Reno area. These are ASTM D 4814 specifications. The alfabetic character is one of the 6 vapor pressure designations. The numeric value is one of the 6 vapor lock protection/distillation characteristics designations.

January E-5
February E-5/D-4
March D-4
April D-4/A-3
May A-3(C-3)
June A-2
July A-2
August A-2
Sept 1-15 A-2
Sept 16-30 A-2/B2
October B-2/C-3
November C-3/D-4
December D-4/E-5


With that kind of variablilty it makes it hard to sort out milage changes from oils or other modifications from the effect of changing fuel composition.


Ed

 

[GMorg]

I completely agree that gas formulations are variable and should be considered when examining factors affecting mileage. However, at least in areas of the country that have hot/cold seasons, the density of the air is THE major factor affecting mileage in the winter. If you want to observe this affect yourself, compare mileage for night driving (or very early morning) with day driving on the same tank of fuel. Where I am at, we can routinely see 40 F differences between the daily high and low in the spring and fall.

 

[Craig in Canada]

GMorg:

Thanks for writing about this, I was starting to think I was nuts...

I see people touting on the forum that mileage is worse in the winter because of the fact that cold air is denser and contains more oxygen therefore you need more throttle to get enough air. Wrong! It means you need less intake air charge volume (hence throttle angle) to properly burn the amount of fuel required to generate the power you need to keep moving.

However, the air you're moving through is much denser in the winter meaning you need more energy to move through it at speed.

If the density change is enough to affect engine output, it's enough to increase aerodynamic drag.

Craig.

 

[GMorg]

Craig:
Drag is a function of the square of air density. It is much worse than just a linear relationship.

 

[Craig in Canada]

I hate to drag wikipedia into this, but http://en.wikipedia.org/wiki/Drag_(physics)#Drag_at_high_velocity indicates that the power required to overcome drag is proportional linearly with fluid density and varies with the cube of velocity.

Translation: slow down to save fuel

 

[GMorg]

Craig, don't apologize, I was thinking backwards and should be corrected. Drag is linear with density and is an exponential function of velocity. However, I still think that drag is function of the square of velocity, not the cube.

Where "X" equals the multiplication symbol (times), the following equation applies to an object's motion relative to a gas(air).

D(drag) = Cd(coefficient of drag) X 0.5A(area divided by 2) X p(density of gas) X V^2(velocity squared).

Below is link from NASA in support of the above equation. It is arranged a little differently from the one above, but it is mathematically the same.

http://www.grc.nasa.gov/WWW/K-12/airplane/drageq.html

On the other hand, "power" is a function of the cube of velocity. And for a discussion on mileage, "power" is the the important metric.

 

[GMorg]

Sorry for the equation, Craig's link says the same thing as what I just listed above - "Drag" is a square of velocity, "Power" is a cube of velocity.