Effect of temperature on mpg

[NateDN10]

Many of the threads I've seen on winter mpg focus on the effects of the winter fuel blend. However it's always been my belief that ambient temperature plays a bigger role, especially since most fuel is E10 anyway.

I've experienced this myself: I get 1 or 2 mpg higher on tanks where average ambient temperatures are in the 40F range than when they're around 20F.

Here's a study that shows (a) cold-start fuel enrichment contributes a 3% reduction in fuel efficiency and (b) in the range of 25C-60C, each 5C water temperature increase improves fuel efficiency by around 2-3%.

These numbers only deal with the first few minutes of driving, when the engine is warming up. However, I've observed lower fuel efficiency even over trips of several hours, when the impact of that first few minutes should be negligible. It seems to me that once the coolant and is up to operating temperature, efficiency losses to heat transfer should be pretty much the same for any ambient temperature; the thermostat ensures that heat transferred from the combustion chambers to the coolant is the same whether ambient temperature is -10C or 35C.

Does my reasoning make sense? Or would increased heat transfer to the oil and fuel, along with lower driveline efficiency make a significant difference? Or is this really just due to winter fuel blends?

Just thinking "out loud." Discuss!

 

[Danno]

Cold air is denser and harder for the engine to push the car through. More work = more fuel consumed.
As well, cold temps add to rolling resistance, tires, wheel bearings etc., which uses more fuel.

 

[Hokiefyd]

Our domestic cars would get better economy in the winter than in the summer. Living in North Carolina, our winters are relatively mild, and our summers are long and hot/humid. High loads on the air conditioning.

In contrast, both of our Hondas apparently have a valve in the fuel tank that dumps fuel right onto the pavement when the temperature is under 40*F or so...economy at colder temperatures is quite poor. Both seem to thrive in hotter weather. They run smoother, they run better in general, and they get better economy.

 

[PZR2874]

I get much better mpg's in the warm weather than in colder weather. Then again, I may be hitting the gas a bit more in the cooler weather.

 

[itguy08]

I definitely notice it in my car. Hover around 20MPG in the winter and 22-23 in the warmer weather. Not sure why or how as I keep the tires inflated, don't idle, and mainly drive the same route every day (30 some miles 1 way to work).

 

[KrisZ]

I track my gas mileage on a spreadsheet for each fill up. I record the date of each fill up as well. When you create a graph of the data, it is very clear that the fuel economy is worse during winter months and starts to climb as warmer months roll in.

 

[Coprolite]

Winter/Cold:
-More time spent below optimal engine temp (-)
-More time spent below optimal gear oil temp(e.g. differential) (-)
-More time spent on cold tires (-)
-Air conditioning usage (defrost mode) (-)
-Winter Blend Fuel (-)
-Cold air, more drag (-?)

Moderate temperatures (Spring/Fall)
-Warm up time moderate - all aspects (0)
-A/C not required (+)

Hot Temperatures
-Low RVP gas (+)
-Warm up minimized (+)
-A/C required (-)
-Heat Soak (?)

In North Texas, I witnessed the best fuel economy in the Spring/Fall with Honda Accord I4, Acura TSX I4, G37S. These cars had enough of my repetitive commute on tanks to be able to ignore long trips that would spike the efficiency or be just enough to throw off the calculations.

The local hills and relatively short commute amplified the FE penalties of engine warm up and passenger compartment cool down. On almost all my vehicles I average slightly over the City rating.

For most current vehicles
Car (city)(Highway)(My observed)(Total Miles)
2012 335i - 23/33/23.9/8046
2011 G37S Coupe 7AT- 19/27/20.64/15971
2012 F150 Ecoboost FX4 - 15/21/~15.1/1094 (Still breaking in)

With the TSX and Honda, I got at most 2 mpg better than City. I have misplaced my spreadsheet on those cars since it has been a while. The average speeds are usually around 31 on a commuting tank. The larger displacement engines suffer the most, with the G being skewed slightly higher with some road trip mileage. I never tried to categorize each tank. I found that the EPA ratings were pretty accurate for my driving style and accounted much of the impact on local terrain.

In Western NY, it seemed I could crush the ratings and had very nice averages relative to EPA.

My averages would go up and down with the seasons, but I wouldn't call any delta statistically significant.

 

[Bamaro]

If you have TPMS on your car that shows actual tire pressure, you will quickly see another MPG issue with cold weather.

 

[Smokescreen]

Originally Posted By: Hokiefyd
....

In contrast, both of our Hondas apparently have a valve in the fuel tank that dumps fuel right onto the pavement when the temperature is under 40*F or so...economy at colder temperatures is quite poor. ..


What the...is this normal?

 

[UNC017]

There are lots of factors, all with pretty small effects, and so its really hard to say.

For one thing, there's more to "winter gas" than just being E-10 or not. Its always formulated with a much higher vapor pressure. This is allegedly to help the fuel vaporize and burn more efficiently, and in the carburetor days it may have been true. But now I think its about the refineries being able to dump lower quality product during winter rather than having to store or flare all those volatiles that contribute to the high vapor pressure but don't have much of a sale market elsewhere.

Also, thermal efficiency is about delta-T. Once the engine is warmed up, the cooling system has a thermostat to control the delta-T there, so you're right it shouldn't matter on a long trip. BUT, the engine also gulps air and spits out exhaust. The exhaust temp is pretty much fixed by the load on the engine, so when the intake temp is colder there's a bigger delta-T and more heat lost. Yes, the air is also denser so in theory the engine can gulp LESS of it, but I'd be surprised if the effects completely cancel out. Plus there's also more unregulated temp loss out the skin of the engine block and out the oil pan when the ambients are cold. Finally, the engine can produce more power when cold due to the colder/denser intake charge... and its hard to resist the temptation to USE that little extra power. Its not any more efficient when the intake air is cold, its just more powerful- more air molecules in means the PCM can feed more fuel molecules in and get more watts (horsepower) out of the crank.

 

[HerrStig]

Originally Posted By: Smokescreen
Originally Posted By: Hokiefyd
....

In contrast, both of our Hondas apparently have a valve in the fuel tank that dumps fuel right onto the pavement when the temperature is under 40*F or so...economy at colder temperatures is quite poor. ..


What the...is this normal?
Dime 'em out to the EPA!!!!

 

[UNC017]

Originally Posted By: Smokescreen
Originally Posted By: Hokiefyd
....

In contrast, both of our Hondas apparently have a valve in the fuel tank that dumps fuel right onto the pavement when the temperature is under 40*F or so...economy at colder temperatures is quite poor. ..


What the...is this normal?


Its the higher vapor pressure of the fuel. Around here, I see fuel dumping out of the vent on my older cars regularly on warm days during winter. But I think this is a little different- it seems like he's implying that it dumps fuel as the TEMP goes down. What I see is more dumping IN WINTER, but only on days when the temp is up, causing the vapor pressure to overwhelm the vapor controls, especially on older vehicles that vent at either atmospheric pressure or a couple of PSI.

 

[SwampSurvivor]

Commuting to work and back in the summer, the Focus gets 35-37MPG. In the winter, 30-32. Winter fuel, running colder longer and snow tires are the cause.

 

[Hokiefyd]

Originally Posted By: Smokescreen
Originally Posted By: Hokiefyd
....

In contrast, both of our Hondas apparently have a valve in the fuel tank that dumps fuel right onto the pavement when the temperature is under 40*F or so...economy at colder temperatures is quite poor. ..


What the...is this normal?


Okay...sorry...

It doesn't *literally* dump fuel onto the pavement...that was a tongue-in-cheek remark to say that it *seems* like there's an unregulated loss in fuel when the air is colder due to the apparently much richer fuel mixture. Honda has long prided itself regarding fuel economy and environmental sustainability, but I believe they have (or at least had) some room to improve regarding fuel mixtures on cold engines. Maybe that has been improved with their newer direct injected engines.

 

[UNC017]

Originally Posted By: Hokiefyd
Originally Posted By: Smokescreen
Originally Posted By: Hokiefyd
....

In contrast, both of our Hondas apparently have a valve in the fuel tank that dumps fuel right onto the pavement when the temperature is under 40*F or so...economy at colder temperatures is quite poor. ..


What the...is this normal?


Okay...sorry...

It doesn't *literally* dump fuel onto the pavement...that was a tongue-in-cheek remark to say that it *seems* like there's an unregulated loss in fuel when the air is colder due to the apparently much richer fuel mixture. Honda has long prided itself regarding fuel economy and environmental sustainability, but I believe they have (or at least had) some room to improve regarding fuel mixtures on cold engines. Maybe that has been improved with their newer direct injected engines.



OK, but I WAS serious about the older cars spitting fuel out of the vapor vents on warm winter days.



I think 'winter blend' fuels are one of the bad ideas motivated by good intentions that have come out of modern environmental consciousness. There should be college courses taught on unintended consequences. I consider myself a true environmentalist... and by that I mean that I want cleaner vehicles and lower impact on the planet, but I have ZERO tolerance for "feel good" legislation that doesn't consider all the consequences.

High vapor pressure fuel: originally reduced CO/CO2/unburned HC emissions by vaporizing better in carbureted engines. Now, high vapor pressure fuel mainly INCREASES ground-level ozone and smog by much higher evaporation from car fuel systems and from storage tanks.

Diesel particulate filters: Intended to improve air quality by removing fine carbon particles. And yes, its really much nicer driving behind a modern diesel than an old one... BUT, we're all up in arms over putting more carbon in the atmosphere. Particulate carbon washes out of the atmosphere in a matter of days and is bound back up into the soil. With DPF's, the trapped carbon soot is burned into CO2 during the filter's regeneration cycle, and so is converted INTO more greenhouse gas. Not only that, but the regeneration cycle requires burning more diesel to get the higher temps, so you're putting even MORE carbon into the air and lowering fuel efficiency of the vehicle.

Cash for 'clunkers:' Intended to stimulate replacement of vehicles with less polluting and more efficient ones, but fails to consider that saving the parts is in and of itself recycling and environmentally responsible, because it reduces all the pollution associated with producing new parts and vehicles.

 

[Hokiefyd]

Originally Posted By: 440Magnum
OK, but I WAS serious about the older cars spitting fuel out of the vapor vents on warm winter days.


My '84 Cutlass wouldn't get to the point of literally losing fuel, but you could REALLY smell it around the carbon canister area on days like that. I assume it was trying to push it out?

 

[UNC017]

Originally Posted By: Hokiefyd
Originally Posted By: 440Magnum
OK, but I WAS serious about the older cars spitting fuel out of the vapor vents on warm winter days.


My '84 Cutlass wouldn't get to the point of literally losing fuel, but you could REALLY smell it around the carbon canister area on days like that. I assume it was trying to push it out?


Typically pre-canister vehicles would spit fuel, vehicles with carbon canisters won't spit, but they'll saturate the canister to the point it can't retain anymore vapor. That's probably what you smelled.

 

[meep]

i store roughly 20 gallons year-around on some loose rotation. I don't like storing winter fuel. as soon as the temps get comfortable, the gas cans balloon out like little piggies.

 

[Kestas]

440Magnum, I don't think your beef is with winter fuel, but instead with summer fuel. Winter fuel is as low tech as you can get. With summer fuel, they remove the volatiles to prevent vapor lock, which makes it a tad more expensive. Plus, it's the summer fuel the EPA mostly messes with, for reasons of summer Ozone Action Days, which we have in SE Michigan, and others have around the country.

You make it sound like winter fuel is purposely blended that way. I understand your technical reasoning, but I don't believe the refiners go out of their way to make the winter fuel the way it is.

 

[NateDN10]

For reference, here's a chart of mpg on my Cobalt: