Greenlings: Why does mileage drop in winter?
This week's Greenlings topic came to us from another reader tip. Don asked why his fuel economy suffers so much in winter weather. In his own experience, mileage drops about 10 percent when the temperatures go from the 60-70 F range to near freezing. This is consistent with our own experience and in fact we've seen even bigger drops than that when testing hybrid vehicles in winter conditions. There are a number of reasons why winter driving reduces efficiency ranging from fuels to driver safety and comfort. If you happen to live somewhere like California or Florida where winter means night-time lows dipping into the sixties, you may not have experienced this phenomenon, but those of who live places with actual seasons have grown accustomed to this. Read to learn more about why your car uses more fuel when its cold.
Probably the best known reason for lower winter mileage is reformulated fuel. It's important to recall that there is actually no such chemical as gasoline. Gasoline is actually a blend of hydrocarbons along with other chemicals. Among the many standards established by ASTM (American Society for Testing and Materials) is pump gasoline. The standard specifies the properties that fuels must have but not the actual composition.
Over the years, refiners have modified the formula for gasoline in order to improve vehicle performance and drivability and reduce emissions. At normal ambient temperatures, gasoline will evaporate. The rate at which it evaporates is dependent on the composition. When petroleum is being refined a number of lighter and heavier components are produced and blended to create gasoline.
The lighter components evaporate more readily but have a lower freezing point. Similarly, the heavier bits are less prone to evaporation. The heavier components are used in so-called summer gas to make it easier to pass evaporative emissions standards and avoid issues like vapor lock. The lighter components make it easier to start the car in cold weather. A number of other components are added to winter gasoline to help lower the freezing point. However the combination of the lower density of the lighter components and the space taken up by additives means less overall energy per gallon when using winter formulas compared to summer formulas. As a result, the engine has to use more fuel to generate the same performance.
Cold temperatures also cause an increase in the viscosity of lubricants in the engine, transmission and axles all of which raise resistance to motion until they warm up.
Beyond fuel, there are other temperature related factors. While drivers looking for maximum efficiency are often willing to forego using air conditioning in the summer time, when the temperatures dip below freezing, just breathing inside the car causes the windows to fog up. That means it's absolutely necessary to run features like window defoggers which can put a significant drain on the car's electrical system.
Add in the fact that batteries tend to lose a lot of their power when they get cold, and it means the alternator has to work a lot harder on those cold mornings. If you have a car with other items like heated seats, it gets even worse. When we tested the Mercury Milan Hybrid in February, using the Smartgauge allowed us to see just how much energy the power accessories were consuming, particularly the defoggers. Factor in the effect on the battery of a hybrid and the situation gets even worse than for a conventional car. The hybrid is dependent on its battery and, when it is cold, the engine has to do even more work. Over a week of winter driving we averaged 28 mpg compared to 37 mpg for another Fusion hybrid that we tested during the summer over the same roads. We've seen similar results from other hybrids.
There are of course other factors outside the car that cause efficiency to drop. When roads are slick, tires slip more than they do on dry roads causing lost motion. Snow on the ground dramatically increases rolling resistance, making the car work harder to get motion. Even air resistance can increase somewhat as a result of increased density at low temperatures.
When plug-in vehicles arrive in the coming years, they will also take a performance hit in the dark season although automakers are working to mitigate this. Vehicles with a liquid cooled pack like the Chevy Volt may end up with an advantage because they can better regulate the temperature when plugged in. When ambient temperatures drop, the Volt battery will be pre-warmed while plugged in to ensure that it is at the optimum operating temperature when needed.
All of these factors and more mean that even the most thrifty drivers will see increased fuel consumption when the mercury drops. This is especially true for those that drive relatively short distances where the lubricants may not have an opportunity to full warm up or the driving distance beyond the warm up is short. Over longer trips, the warm-up period will be less of a disadvantage as it gets averaged out, but there will still be a penalty.
Reader Comments (Page 1 of 2)
gorr 7:59PM (11/12/2009)
I knew that batteries are not up to anything in winter it get even worse. Nobody will drive a battery car in canada and north of u.s.a and even in miami.
I said last week to start selling hydrostatics cars and trucks and trains and ships with powerful inboard rechargers.
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Luke 12:31PM (11/13/2009)
I drive a Prius in Illinois. You're mistaken, and I'll be driving a BEV as soon as I can afford one (or I afford the time and money to build one).
The mileage in the 2nd-gen Prius does drop quite a bit in the winter, but it's still better than any other car I've ever driven.
My intuition is that, since the Prius' computer won't shut off the engine until it's up to operating temperature, it runs the engine more during the first part of the trip than it does in the summer This ads up, since we make a lot of trips that are under 5 miles, this adds up. The effect is hidden on holiday road trips, though, since the car's warmed up after about 2.5 miles, and the road-trip is usually several hundred miles.
gorr 1:19PM (11/13/2009)
I know that you want to save gas, but why pay more for this. The idea is to get the thing for fewer money, not more money like you do by buying costly batteries sold by wall streets traders.
Just ask for green low cost non-poluting long-lasting technologies decreed by almost every folks here.
mpgomatic 9:01PM (11/12/2009)
Don't discount the warm-up idle factor ...
Start the car up, let it idle for five or ten minutes to warm up, and you're bound to see a significant dent in the MPG #s ...
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charlie 9:17PM (11/12/2009)
OK but suppose that your car is warmed up (or maybe it is stored in a heated garage) and that the roads are clear. Shouldn't the winter cold intake air allow you to get BETTER mileage than with hot summer air?
Maybe your engine has to do less work to compress the air, increasing efficiency? Something like that? It seems counterintuitive to me that hot summer intake would be better for efficiency than cold air.
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roflwaffle 9:53PM (11/12/2009)
I think it's the opposite for spark ignition engines to some extent. Pumping losses increase as the difference in pressure between the crankcase and the intake charge in whatever cylinder (at the bottom of the stroke) increases. The pressure in the crankcase is mostly independent of outside temperature, however the lower pressure on colder days means there's a sightly greater difference in the pressure between the crankcase and cylinder, although the difference is really quite small. This is also how EGR can improve mileage. Inert exhaust gases can be allowed into the engine so that there isn't as much of a difference in pressure while still allowing a 14.7:1 A/F ratio. Course, at very low loads this can lead to incomplete combustion, so it's only used when the air/fuel/exhaust gases can mix well.
http://www.mechadyne-int.com/vva-reference/part-load-pumping-losses-si-engine
Charlie 11:28PM (11/12/2009)
I'm thinking that cold air is higher pressure. Other than that I agree with what you were saying, it's just that I'm thinking that cold air is denser and has more oxygen by volume.
MikeW 11:33PM (11/12/2009)
No, more power with colder air, but less mileage.
There was one of those hyper-milers who intentionally built a hot-air intake system. For lower air density, for larger throttle angle, for less pumping losses.
Charlie 12:20AM (11/13/2009)
I don't know if u have more oxygen and can make more energy per stroke u should be able to run at lower rpm- less friction...
Mattias 1:46AM (11/13/2009)
This is true for naturally aspirated diesels. My old Peugeots (I got four with the XUD9) always had more power and besser mileage in winter.
roflwaffle 1:49AM (11/13/2009)
The thing w/ SI engines is that they're relatively overpowered, so even if air with greater density can increase maximum power output, a SI engine will end up having to close the throttle more to compensate at typical power levels, and it's the difference in pressure between the cylinder/crankcase that's the problem. If we had an engine that only needed to produce a certain amount of power, then we probably could optimize efficiency and speed depending on air density, but we can't do that for most cars.
Taylor. Yes, Taylor. 4:27AM (11/13/2009)
Actually, I always used to think this, till just now. Reading the comments, I think i can put it together a bit better:
Most every car operates with a MAF, or Mass Air Flow sensor, on the intake. It calculates the mass of the air coming in and then uses that to determine how much fuel to shoot into the engine, so that you get the correct air fuel ratio, which is a really important to your engine operating properly. When air gets colder, it gets more dense, and since the engine pistons are a fixed pump, one piston full of colder air contains more mass of air, which requires more fuel. So yeah, your engine puts in more fuel when the air is colder, lowering MPGs. :)
In fact now I can't believe I never realized that before!
-Taylor
maximelap2 1:11PM (11/13/2009)
I also think that colder air would mean better mpgs, however only by a small percentage.
Your MAF sensor is not temperature dependent and therefore no additional fuel is consumed, unless you have an air intake temperature sensor. If you have a higher oxygen concentration per volume, you will have a better performance, (ei. more power per piston cycle) which should result in a slight decrease in RPMs.
This is the principle behind performance cold air intakes. Here's a reference or for a more detailed explanation:
http://www.articles43.com/article/How-Cold-Air-Intakes-Work-Towards-Performance-and-Efficiency-in-your-Vehicle-22925.html
Josh 10:11PM (11/12/2009)
@mpgomatig: I thought warm up idle was something from the 80's. Cars nowadays don't need to warm up!
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Dave 7:34AM (11/13/2009)
If your car is covered with ice and snow, you need to warm it up so you can clear it.
Also, it is more comfortable to get into a warm car.
I don't know where you're from, but around here (Rhode Island) a lot of people have remote starters so they can start their cars and warm them up without even going outside to turn the key.
maximelap2 12:56PM (11/13/2009)
It's good practice to warm up the engine before putting a load on it since material are temperature sensitive. eg. Metals expand when temperature increase, oil is more viscous when cold, etc.
nrb 10:31PM (11/12/2009)
"mileage drops about 10 percent when the temperatures go from the the 60-70 F range to near freezing"
Near freezing? You southerners need to stop writing cold weather articles.
Freezing is swimsuit weather. :)
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Dave 7:35AM (11/13/2009)
No kidding.
I've had my top down at less than 32F.
john1701a 10:38PM (11/12/2009)
The impact of winter on Prius is reduced with each generation.
Here in Minnesota, I clearly saw that with my upgrade from a Classic (2001) to an Iconic (2004) model. Now with my 2010, I'm already seeing a pleasing improvement.
It's interesting to note that even though overall battery capacity is impacted by the cold, operation isn't. In fact, it's easy to keep the pack from getting too hot... and the system does indeed take advantage of that opportunity.
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nrb 10:11AM (11/13/2009)
" I clearly saw that with my upgrade from a Classic (2001) to an Iconic (2004) model. Now with my 2010, I'm already seeing a pleasing improvement. "
Could some of this have to do with going from an aging battery pack to a brand new one?