MPG drop in winter - thoughts?

[james]

Just about everyone knows that we see mpg drop in cold weather. Mostly it seems to be put down to a combination of cold engine/intake air and winter fuel mixes. However, I was thinking about it today coming back from skiing. This is a road I drive frequently all seasons, and I know just about what speeds I'll reach without using the throttle.

What I noticed today, though, is that even coasting in neutral I don't reach anywhere near the same speed as in summer (in gear with a bit of regen), therefore there must be some other factor at work. Seems like it could be tires taking more energy to flex (they're at 50 psi), cold lubricants in bearings or transmission, or maybe brakes tightening up because of cold. Any thoughts?

I think I'm getting lower mpg this year than previously. It seems almost as though the car had been dipped in molasses I did get new tires on all four wheels last month, but they're the same Potenzas that were original equipment.

[b1shmu63]

New tires will get slightly worse mileage. (They will also have better grip on the road.) Their outside diameter is slightly larger too so the mileage will register lower too.

Many other factors influence mileage in the cold as well, including the battery chemistry, wind resistance, tire slippage, thicker transmission oil, thicker CV joint grease. Add a blower running to heat the cabin, cold engine oil on startup, poor fuel vaporization, winter blend gasoline. You get the idea.

Imperical results seem to say that a warm air intake, a radiator block and a block heater help somewhat. These ideas have been used on cars and trucks for years in one form or another.

[Armin]

colder and stiffer bearing grease?

[Mike Dabrowski 2000]

CV joints, wheel bearings are right in the cold air flow, so the grease stays relatively cold and stiff compared to summer. The air density is much higher as well. A friend that flys ultralite planes, said that winter takeoffs take 50 feet, hot summer takeoffs can take 300 feet. As Kip said, it is the combination of all of these factors, not just one.

[james]

I do have the radiator block, hot air mod, MIMA, etc. However, even when I remove all the factors related to the engine by coasting in neutral, there seems to be much more drag in winter. The point is whether this is something that could be fixed. If for instance the cause was lubricants becoming stiff in the cold, then it might be possible to change them.

[IamIan]

Originally Posted by james

I do have the radiator block, hot air mod, MIMA, etc. However, even when I remove all the factors related to the engine by coasting in neutral, there seems to be much more drag in winter. The point is whether this is something that could be fixed. If for instance the cause was lubricants becoming stiff in the cold, then it might be possible to change them.

short answer = no


Long Answer .... see below :


Aerodynamic Drag Force :

F = -1/2 * p * v * v * A * Cd

Power of Drag = Force Of Drag Times Velocity

P = F * v

where :
P = Power of Drag ( Watts per unit time )
F is the force of drag ( Newton )
p is the density of the Air ( kg/m³ )
v is the velocity of the car ( meters per unit time )
A is the frontal Area ( Insight = 1.9 Square Meters )
Cd is the drag coefficient ( Insight = 0.25 )

The air density will also change with humidity… but on average :

At -10 degrees C the Density of the air is about 1.341 kg/m³
At +10 degrees C the Density of the air is about 1.247 kg/m³
At +30 degrees C the Density of the air is about 1.164 kg/m³

Between -10 C and +30 C the aerodynamic drag at the same speed for the same object will increase the by about ~13.2% ... so 13.2% more power from the engine would be needed to go the same speed in the same car at the colder temperatures... just from Aerodynamic losses..

---------------------- Plus ---------------------

Rolling Resistance Force :

F = Cr * m * g

P = F * V

F = Force of rolling resistance ( Newton )
P = Power of rolling resistance ( Watts per unit time )
Cr = Coefficient of rolling resistance ( See Chart Bellow )
m * g = Total Weight ( Insight + Driver + Fuel + Etc )
V = velocity ( meters per unit time )

Cr___________Description
0.006-0.01_____low rolling resistance car tire on a smooth road
0.010-0.015_____Ordinary car tires on concrete

P = Cr * m * g * V (rolling resistance)

It has been said :

Originally Posted by chrs

the published Bayer Rubber curve that I cited earlier (and that I'll gladly send to anyone who PMs me with an email address). That shows tan delta going up by a factor of 4.7 from 30 C to -10 C

This will vary greatly on the exact chemistry of the tire and allot of other variables like the road surface chemistry … but any increase in Cr will directly increase the rolling resistance by the same amount… so a 50% increase in Cr due to lower temperatures will cause a 50% increase in Rolling Resistance force.

------------------------- PLUS ---------------------

Engine Lubricants are less effective the colder they are

--------------------- PLUS -------------------------

The insight will charge the NiMH battery pack more in the colder weather as part of the battery care process to protect the batteries from some of the effects of colder weather.... which means more drag on the engine from the IMA... and less of that drag back from the batteries...

--------------- PLUS --------------------

Longer Engine Warm up times.

------------ PLUS -------------------

Harder to get into lean burn mode

----------- Etc ----- Etc ----------

You can try to make improvements and I encourage you to ....

but you will always get better FE / MPG in the summer than the winter.

[ikhoudvanu199]

thanks Ian for that very technical explination, even tho i didnt understand all of it, it was nice to see some more reasons as to why

and i have also been noticing that accelerating on the same roads i do daily, under what seemed to be the same pressure on the accelerator...took longer and further to get to the same speeds