MPG in the winter

We all know that gas MPH drops in cold weather. But why? Is it due to the colder air being fed to the engine (my guess)?

(this is about after the engine reaches operating temperature, not during the warm up period)

But engines have lots of waste heat, so why would the engine designers not use some of that to warm incoming air before combustion? seems like a simple thing to do, specially when the manufacturers are struggling to meet federal standards and seem to jump through hoops to gain a few MPG.

Or perhaps the conditions for the federally mandated MPG standards so not include a test with low ambient temperature.


It happens primarily because the engine simply takes longer to get to full operating temperature, a lot longer in NH this past week, and until it does it runs rich. Secondarily, the engine has to move transmission and differential parts through much thicker fluids. It takes more energy to mix cement than to mix chocolate milk.

So, you might ask, why does it run rich? Excellent question.
The process of combustion occurs when sufficient heat energy exists in a system of hydrocarbon molecules I contact with oxygen to cause the hydrogen and the carbon atoms to separate and bond to the oxygen atoms. The more complete and faster the combustion process happens in the power stroke is the more heat energy that gets converted to torque.

Key on that “sufficient heat energy” statement, particularly the word “sufficient”. The heat energy comes from the total of the spark and from the combustion process itself. When the gasses in the cylinder are cold, insufficient heat energy is release in the combusting gasses to burn the mix completely enough to keep the engine running. To compensate, the mix is made with more fuel (richer) per volume. If there’s enough, there’ll be sufficient heat to keep the process going even though much of it doesn’t combust. The waste is simply considered a necessary evil.

Diesels are an excellent example of this. Diesels don’t use spark plugs, they get their heat from the high compression of the gasses (all matter emits heat when compressed, diesels have much higher compression ratios than gas engines, typically I the range of 14:1). However, when very cold the compression alone doesn’t create to begin the combustion process. So diesels use “glow plugs” to add some extra heat when cold. A glow plug is a form of spark plug, it creates heat to get the process going.

Post back and let me know if this helps.

Regulars, let me know if I missed something. These types of posts can always use an objective proof-reading.

the same mountainbike
Diesels don’t use spark plugs, they get their heat from the high compression of the gasses (all matter emits heat when compressed, diesels have much higher compression ratios than gas engines, typically I the range of 14:1).


However, when very cold the compression alone doesn’t create to begin the combustion process. So diesels use “glow plugs” to add some extra heat when cold.

But only on start-up and maybe a coupla minutes after.

Good additions, auto-owner. Thanks.

Regardless of what the factors are, and there are many non motor factors including colder and thicker lubricating fluids, less air pressure in the tires, winter tires if you use them, more idling time and greater air resistance form cooler more dense air. Because of this, it’s best to just not look at the MPG gauge and fill the tank up with your eyes closed. Only take your mileage in warm weather. It’s way too discouraging otherwise to even think about, as many factors you can do so little about.

Heating up the incoming combustion air can theoretically save only 1 or 2% out of (typically) a 10% loss in winter mpg. I remember the heater snorkels from the exhaust manifold, and there was still the loss in winter mpg. Dagosa’s other factors are the key.

Dag’s factors are definitely significant contributors. It’d be interesting to see an analysis of how much each factor contributes. It’d probably vary widely based on the vehicle.

Earlier this week (or was it the end of last week?) it never got above 17F where I live. It took forever for the engine to warm up. It took even longer for my bones to warm up. Today the weather has begun to improve. It got up to 31F this afternoon. It hasn’t reached freezing all week!

I think the reason the mixture has to be extra rich is so enough fuel will evaporate to form combustible vapors when the engine is cold. Only the vapors will burn, not the liquid. In a cold engine, not all the fuel vaporizes.
Because gasoline is a blend of hydrocarbons each with its own boiling point, at low temperatures, the heavy hydrocarbons may not evaporate so you need extra fuel in order for the air/fuel vapor mixture to not be too lean to burn.
Once the engine is hot, all the hydrocarbons evaporate and the mixture does not have to be extra rich.

Many of us notice that a not completely warmed up engine will idle ok with the choke off but if you open the throttle, the engine stumbles and misses. It’s natural to think that this is because the idle mixture is rich. Actually, the high idle vacuum in the intake manifold lowers the boiling point of the fuel and so the fuel provides combustible vapors when the vacuum is high but not when the vacuum is low.

In short, it’s a temporary lean condition.

Don Prudhomme was asked in an interview once why they do “burnouts”. He said that while most people think it’s to soften the tires, it’s actually to heat up the motors. That comment always stuck with me. Hot burns the fuel better.

A number of reasons cold be behind it. The intake air temperature sensor is also reading colder air entering the engine and that can translate to a bit more fuel. The resistance drops as the temp goes up and it doesn’t take much to create a little extra resistance in the circuit which then makes the ECM think the engine is a bit colder than it actually is. Same goes for the engine coolant temp sensor.

Colder air is also denser which can lead to more drag. Maybe they should area rule auto bodies. That could lead to some unusual designs… :slight_smile:

I haven’t actually checked the MPG on my Lincoln in quite a while other than with the on-board display. I did so on the last fill-up with the gallons used divided by the miles driven and was a bit surprised to find that it varied from the display but in a good way. The car is going to be filled up tomorrow for a second check just to make sure of the first one and it’s been brutally cold here; finally getting over freezing the last few days. Sixties on the weekend!

The car usually gets 27-27.5 on the highway but the last check showed 29.4 with a bit of city driving factored in. Not shabby for 2 tons of RWD V-8 car.
Who needs Ecoboost… :wink:

“area rule”? You lost me on that one. Is that another term for cross-sectional area?

That’s where the center is slimmer than the ends. This was really noticeable on a lot of jet engine aircraft such as the B-58 Hustler, F-102, etc which gave them a coke bottle shape.
Would it make any difference at sustained highway speeds on a car? No idea, but maybe it’s worth a shot… :slight_smile:

If the new E-class (electric, not Mercedes) race bodies and F1 designs are any indication, it’d definitely help. Salt flats cars going for Land World Speed Records are all coke bottle shaped too.

Thanks for your comments. It’s a bit more complicated than I first thought, but then most things are.

Area rule only applies when you approach the speed of sound.


I think there are 3 factors:

  1. Heating things up takes energy.

  2. Cold doesn’t allow the gas to vaporize as easily

  3. The formulation for gas changes in the winter.

Here’s a link that lists 9!

There’s more to cold engine inefficiency than needing a rich mixture. Even engines that burn natural gas or propane need to warm up before they are efficient, they just don’t need an extra rich mixture while cold because the fuel they burn is already a vapor.
Every BTU of heat that flows from the combustion chamber to the engine block for the cooling system to dispose of is yet another BTU of heat that the engine will never be able to convert into 778 ft-lb of energy and heat flows from hot to cold faster than it flows from hot to warm.

Long stroke “undersquare” engines have made a big comeback in automobiles mostly because that design minimizes combustion chamber surface area and along with it, thermal losses, not to mention that such an engine has an easier time meeting emission standards.

Many years ago in upstate NY they had a 15 day cold spell of never getting above 0. Average high temps was -10 during that stretch. My little Chevy luv truck usually averaged about 22mph for my weekly commute during the winter months. Summer mpg was about 25mpg. During that cold stretch it dropped to below 10mpg. When the cold stretch was over…it went back up to 22. I did adjust my air pressure during that cold spell.

Let me summarize. I’m only considering MPG when the engine (and car) is fully warmed up, such as on a long trip.

  1. Temperature of the air into the combustion chamber. This could be fixed by preheating the air with some of the waste heat, but is not done, perhaps because this is a small part of the losses.

  2. More air drag due to the fact that air is denser at cold temperature. I can see this being a significant factor at high speeds, as air resistance is a large part of the losses.

  3. More friction in the transmission, differentials, etc due to the oil being of lower viscosity at lower temperature. I don’t see this as a major factor as I would think it would quickly warm up to normal operating temp.

  4. A lot of discussion about rich mixture. I don’t see how this is an issue once the engine is warmed up.

  5. Tires, if the pressure has not been adjusted for the low temp. If it has been adjusted, then this is not a factor.


  1. not a significant factor. The denser air simply allows the capacity for more power. The same amount of energy is required to move a given total load (resistance from all sources) a given speed.

  2. a factor, but I don’t believe it’s significant until altitude reaches thousands of feet above sea level. Pilots aren’t even required to go on oxygen below 12,000 feet.

  3. a biggie. Definitely. My years in ND confirmed that beyond all question.

  4. it isn’t… but your question said nothing about the engine being warmed up. I call a foul!

  5. it was a definite issue with bias ply tires. I drive many a thump-thump-thump mile in ND in the early '70s. If a tire doesn’t roll smoothly, resistance will be greater. I don’t believe it’s an issue with radial tires, at least not that I’m aware of. I might defer that question to Capriracer.

By the way, my car is warming up in the driveway as I write this. Today’s high in my neighborhood is predicted to be 17F. It’s 17F right now, so I guess that’s as hot as it’ll get today. I’m warming it up only because I’m only going to the store one mile away. I’d rather get it to temp and also keep the battery fully charged.

^Yeah, but you’re describing an academic issue re: “systems at equilibrium” that (IMO) really doesn’t exist in the real world.

Think: my experience is that it takes about 5 miles of driving to get the coolant to temp (with my new car, you can get “real-time” coolant readouts). Motor oil is NOT thermostatically regulated, and (from what I’ve read) takes longer to get to temp, and may NEVER get to temp in sufficiently cold air! (So, let’s be safe any say “10 miles.”)

Differential and MT are wholly unregulated, and I’d expect them to rise merely to a temperature N degrees warmer than ambient, where an equilibrium is established: within reason, they’ll vary directly with outside temperature. (AT generates enough heat to warm itself over time.)

So…you have a car that takes from 5 miles, to infinite miles, to fully warm! For a given application (such as “grocery getter”), the “warmed to equilibrium” answer is moot, as it never completely happens.

And there’s a few others: the denser air means you have a more powerful engine…which means more pumping losses behind a cracked throttle. Poor atomization is an issue (more so for carb than EFI, but still relevant).

But mostly, winter mileage is lousy because of the transient “warm-up” inefficiencies. A pre-warmed car that is never shut off should be a lot closer to summertime performance.

Re: “area rule,” supposedly the styling cues for the Corvette Stingray were cribbed from the area-ruled fuselages of fighter jets of the day…much like tail fins were cribbed. Area rule only matters if you have wings around the “waist,” and then only as you approach the speed of sound: any faster (or slower) and it actually hurts aerodynamics slightly.

So, in the end, just fashion…but really awesome looking!