Temperature Effects on Fuel Economy

[7speed]

I read a lot, and many people have mentioned that fuel economy in the winter is worse than fuel economy in the summer.

Question 1: Why is the above true?

Now, if fuel economy improves as the temperature increases, I ask:

Question 2, 3, and 4: Is there a temperature above which fuel economy begins to decrease? Why? What is this temperature?

[Jemdawg]

You'd have to think of this in generalities rather than specifics unless you plan to do some sort of a case study on this. I'm sure you could Google around to find a more elaborate answer than I can provide. There are a few things that can account for this significant difference in fuel economy.

1. The first, and foremost, especially for those who drive mostly at highway speeds, is the density of the air. In the winter, the air can be as much as 20 times denser than in the summer, this can create as much as a 10% loss in highway fuel economy.

2. There is also the factor of letting your car warm up before you leave. As this may not account for most of the loss in mpg, it does, at least, account for some of it. Whatever gas you've used to warm your car up was wasted, as it earned you 0 mpg, since your car has not moved.

3. Snow and ice force your car to work harder and use more fuel.

4. In addition to the aerodynamic drag there is also the tire drag, which can also be increased by 20% in cold weather conditions.

5. The oil takes longer to thin out, which causes more friction in the engine and thus worse fuel economy.

Those are only a few of the many factors which explain the effects of temperature on fuel economy.

[Legacy40k]

When gas is cold.. it shrinks.. like everything else, molecules get closer togeather. So, because of this, your car uses up more because more gas fits into the same area.

That is why gas pumps always say 'Corrected to XDegrees' so that they are accurate regardless of the temperature outside.

[CopyChief]

Truly an interesting physics lesson, if they put things in terms like this in school maybe more kids would pay attention. Anyway, another question on this topic: what effect does the change in gas formulation (in some areas) have on fuel economy?

[proxim2020]

The winter blend, depending on the specific formula, has 3% less energy than the summer blend. Less power = more consumption to retain that same amount of power. In the warmer months you turn back to summer blend and higher prices. Summer blends burn much cleaner to help reduce smog and pollution, but come at a hefty price, 10-15 cents a gallon at wholesale level. This is one of the reasons we see prices jump during the summer. The EPA regulates the specific winter and summer formula's all across the country. So they could be different from state to state.

[XterraVersa]

3. Snow and ice force your car to work harder and use more fuel.

4. In addition to the aerodynamic drag there is also the tire drag, which can also be increased by 20% in cold weather conditions.

Your other points are right on with #2 being the mpg grand champion killer. #1 is a bit overstated.

#3 Snow & ice reduce the friction between your tires & the road. Everytime a wheel slips, the power that was applied from the gas is wasted. Your car loves it since there is reduced strain on the drivetrain. This is overcome in many regions by using studs or chains or reducing tire preasure, but these lead to mpg loss, see #4.

#4 is not drag. It is an increase in rolling friction cause by a reduction in tire preasure. As your tire preasure decreases, your foot print increases, creating more friction. To overcome this loss, add more air.

When gas is cold.. it shrinks.. like everything else, molecules get closer togeather. So, because of this, your car uses up more because more gas fits into the same area.

This was true for carbed engines, but with todays modern computeriezed engines, all the fancy sensors detect what is coming out the tail pipe. If the fuel was densified by being colder, the sensors will see unburnt hydrocarbons in the emmisions. The computer then leans out the gas to compensate.

[motoguy128]

Whenever you start an engine, it will consume heat energy to reach full operating temperature. When the cylinders are cold, they soak up some of the heat energy that would normally go into making power. The engine, transmission and fluids weigh over 500lbs. All this weight must reach on average something like 300 degrees or more. Yes, some of the heat is normally waste heat anyway, but some of the heat comes from the energy normally used to make horsepower. Even backpressure in the exhaust is increased at lower temperatures.

On the positive side, in cold temps the air is very dense and dry. However, the air is probably too cool to run the engine at the most lean, optimum conditions.

[Magnes]

Your other points are right on with #2 being the mpg grand champion killer. #1 is a bit overstated.

Actually the change in air density is the biggest factor influencing fuel consumption over the seasons, but not because of the increased aerodynamic drag.

First of all, the difference in air density between room temperature and freezing is about 8% (not 2000%)! That is significant because it means every time an engine cylinder draws in air it is getting 8% more oxygen at 32deg than at 72. In order for proper combustion to take place the engine management system must inject 8% more fuel to compensate.

In effect, cooling weather is like increasing the displacement of your engine, with all the corresponding power and fuel consumption consequences. In going from 72deg to 32 degrees, it's like your Versa's engine going from 1800cc to 1944cc!

[7speed]

Thanks to all who responded.

...First of all, the difference in air density between room temperature and freezing is about 8% ...

... In going from 72deg to 32 degrees, it's like your Versa's engine going from 1800cc to 1944cc!

This brings me to a relatively new thought:

If air is less dense at higher altitudes (lower pressure) to begin with, then that would partially (or totally) negate the effect of decreased temperature.

[mkaresh]

I've never thought of fuel economy being better at high altitudes. Power is certainly lower.

One complicating factor is that fuel economy isn't simply an effect of engine size. Sometimes larger engines are more efficient than smaller ones, if the smaller one has to operate outside of its efficienty rpm/throttle range to produce sufficient power but the larger engine does not. If an engine has to work very hard at higher altitudes, fuel economy probably is worse rather than better.

[Magnes]

As I understand it a larger engine will always consume more gas, all else being equal. Larger engines have the advantage of being able to run at lower RPM, which means they may economise on fuel compared to smaller engines IF the gear ratios are modified to suit.