FE in the WInter

In January I had occasion to drive to South Carolina and back, ~800 miles 1,280 km total, to pickup a failed traction battery. The temperatures ranged from 15F to 39F (-9C to 4C) in low wind at my standard, 65 mph / 104 km, cruising speed. What I found was my highway mileage varied in direct proportion to the air density.

In city driving, the EBH helps but the thermistor hack works even more effectively. However, you still have rolling drag increases from cold effects on tires and other parts. Still, once the vehicle is warmed up and running at speeds that there are minimal ICE thermal losses, you can achieve nearly 90% of the summer performance. But these are very narrow conditions involving driving for at least 30 minutes to warm-up the vehicle and then keeping the speeds high enough to avoid ICE thermal running.

Does this help or do you have some specific questions?

Bob Wilson

 

In a Nutshell:

Extra fuel needed to keep the engine warm
Short drives 10 miles or less
Winter blend fuels can cause a 10% drop in mileage, even when everything is warmed up.

"Repost"

My observation @ 0 (f), no block heater, no grill blocking, 5,000 miles since the end of November.

Driving less than 10 miles gets me about 25 mpg
10-25 miles gets me about 35 average
20-40 miles gets me about 40

@ 20 (f) add about 4 mpg to the 0(f) figures

@ 40 (f) add about 6 mpg to the 0(f) figures

In a nut shell Winter sucks. Between the winter blend of fuel (good for a 10% loss) and the cold weather, mileage will suffer.

 

You do need to keep the tire pressures up. Check them whenever the average outdoor temp changes more than about 10'C/20'F, or at least monthly.

 

It does use more fuel in cold air. The previous posters have answered your questions, but since you are still doubting the answers, I'll put it in my words:

1) Cold weather means more fuel to heat the engine and catalytic converter, regardless of whether you run the heater or not. Not running the heater helps, as would a block heater, but those things only lower the mileage hit, not eliminate it. The top priority for the Prius control system is to minimize emissions. To do this, it must keep the engine and emission system hot, and that's what it does.

2) Winter fuels are blended with lower energy per unit, which means lower miles per gallon regardless of what you drive or how you drive it.

3) Short trips are mileage killers. Cold startup involves warming the engine and emission system. You use this fuel whether you drive or sit still. Longer trips amortize this overhead over more miles, making for better mpg. In the winter, the cost of warming up is higher, so the mileage hit for a short trip is even greater.

4) Cold weather means increased friction. The rolling resistance of tires goes up with cold, and so does the drag from bearings and other lubricated devices.

5) Cold air is more dense than warm air. This contributes a small but finite amount of increased aerodynamic drag.

6) Cabin heat means fuel burned to stay warm, or electricity for the electric heaters.

7) Slush, rain, and snow increases rolling resistance.

There might be a few more lower order effects, but that should be enough to give you the general idea.

Tom

 

Not using the heater may be your problem.

Remember the traction battery was designed to operate most efficiently at a comfortable temperature. The ICE is working harder to make up for the loss of battery effeciency.

Keep the cabin at a comfortable temperature and your MPG will go up.

 

Check your defroster controls. In North America the defroster engages the AC and this is a real MPG killer.

Bob Wilson

 

Temp here has gone from the 20s to the 40s in the past week. My milage with no change in driving habits has jumped from 47 to 50 MPH.

Bring on Spring.

Soon, please.

 

The biggest thing that will kill your MPG is how bad do you want to stay warm while you drive. If you turn the climate control off and crack a window so that your windows don't fog, you will get near summer driving averages if your grill is blocked. But I'd rather have some heat.

 

It is not true that these only (or primarily) affect only the first 5 miles. Remember: heat transfer is proportional to the temperature differential, so the engine cools faster when it's hot. You still have to "spend gas" to keep the engine (and catalytic converter) hot, particularly if you're driving around town.

Several of Tom's (and other's) points extend beyond the first 5 miles:

• Winter gas is Winter gas and it's always gong to affect you; the mileage hit you take from this is very noticeable.
• Slush and rain affect you no matter what speed you're going (obviously not important on a dry day).
• The cold weather is still going to increase friction and rolling resistance (although this will get somewhat better after the first 5 miles)
• Of course, not using the heater will improve the mileage, but the engine still has to run more than it would on a warm day to keep the catalytic converter warm.

 

I believe everything work better on higher temperature.
Energy Citations Database (ECD) - Sponsored by OSTI
In the "3.2.3 Hybrid Drive System Losses Tests" section of above material, we can see less system losses on higher temp.

Anyway, attached graph is made by Japanese friend, and it shows higher temp is better for FE.
(X axis:temp in Celsius, Y axis:FE in km/L)

Ken@Japan

 

What he said.

For the past few months I've been averaging 36-37 mph (all short trips) with temps in the 20s and 30s fahrenheit. With my current tank I'm up to 40 mph and the only difference is slightly warmer temps -- lows still in the 30s but highs in the 40s-50s.

 

that is not quite correct. what Tom said is pretty much dead on...

you imply that after the "car" is warmed up, that will somehow lower the density of the cold air pushing on your Pri... not gonna happen...

that implies that somehow, the cold air rushing in to cool your Pri will not have an effect because your Pri, like other gas engines, puts out enough heat to keep the engine warm.... not gonna happen. (i did an experiment with CANVIEW, monitoring engine temps every 30 seconds... engine temps dropped as much as 20º F simply by waiting for a red light to change. when your engine does not run all the time, it struggles to maintain temps in cold air!)

so if your efficiency only drops 10% from summer to winter, you are pretty much doing it right. a smaller change in your efficiency means you are doing a stellar job!!

 

**sorry cannot edit my post...added to above**


best way to check it... monitor your local area for the change in formulation. around here, its occasionally announced in the paper... so watch for it.

usually the temps will be about the same... see the diff from one tank to another. it will usually be 2-3 mpg...without changing temps, driving conditions, etc...

 

I agree with your second two paragraphs. Cold air is cold air, and engine and catalytic converter (which is not helped by a blocked grill) need to stay warm. So in cold weather the engine will run more often and for longer times. As I pointed out in other threads, 30-50% change is not uncommon from summer to winter in the Chicago climate.

But that first part - density of the air? Is that really happening? Barometric pressure only varies a small amount, but I suppose a gradual cooling wouldn't affect the pressure significantly (is it still the same amount of air or does air at the top fill in from the sides as the cold air sinks). Can we measure density directly? At any rate, you should be able to see the same effect every time a high or low pressure system rolls into town if that theory is correct, summer or winter. Folks in Denver should average better mileage than those in St. Louis, if other factors are equal. Any readers here from La Paz?

 

barometric pressure is a general thing that takes pressure of a column of air... not density in general...

we all know that nearly everything (with exception of water) expands as it warms right....using the STP (standard temperature pressure) formula, it clearly shows that density of any gas is temperature dependent.