[seiferteric]

> Gasoline engines are already 15% less efficient at 20F.

Is that actually true once the engine has reached operating temperature?

[epistasis]

Short trips are worse:

> Fuel economy tests show that, in city driving, a conventional gasoline car's gas mileage is roughly 15% lower at 20°F than it would be at 77°F. It can drop as much as 24% for short (3- to 4-mile) trips.

[helterskelter]

The temperature difference should in principle increase thermodynamic efficiency. You get loss of MPG from other factors though mentioned in the link, like increased friction of moving parts, idling to warm up (0MPG), defrosters/seat heaters, lower tire pressure, denser air to drive through, winter fuel mixes which may not have as much energy, etc.

[mylifeandtimes]

Once had a Porsche 914. Air cooled engine. Drove it across Montana and the Dakotas one winter. One very cold winter.

Not sure the engine ever reached "operating temperature" on that drive.

[EvanAnderson]

Sticking a piece of cardboard over a portion of the radiator was a common sight during the winter when I was growing up in rural Ohio. I didn't think our winters were that cold, but maybe late 70s to early 80s vehicles were more susceptible to running cold.

I had a car that developed a stuck-open thermostat and did the cardboard trick to get by until I could replace the faulty part.

[rootusrootus]

I've had that happen, too, on a [more] regular car. I drove a Mustang 5.0 from Oklahoma to Oregon, and as I went through eastern Colorado the coolant temperature steadily dropped until it was resting at the bottom of the gauge. I don't recall whether the gas mileage suffered noticeably or not during that phase of the drive.

[colechristensen]

There are a bunch of things going on, and some people's measure of efficiency needs work.

1) winter blend fuels have less energy per volume, that doesn't make your engine any less efficient by energy but it does by volume of gas

2) lots of temporary cold effects: fuel vaporization, thick lubricants, etc. these things become less of a problem as the engine warms up but some energy is still lost on long drives

3) air resistance: all aerodynamic forces are linearly proportional with air density. At a constant pressure there's about a 15% difference in air density between the hottest and coldest places you can drive (and thus 15% less drag on a hot summer day than a cold winter day). aerodynamic forces are proportional to the square of your velocity and they become the largest resistive force around 50mph -- so at highway speeds you're losing efficiency because you have to push more air out of the way

4) energy used to maintain temperature: this is hard to calculate but some engine power is lost because the energy is used heating up the engine block and lost to the environment

5) the Thermodynamics 101 engine efficiency goes UP with increased temperature, but it's got a lot of real world effects to compete with, no spherical cows and all

[anjel]

Partial pressure variant fx on combustion outputs

[jurgenburgen]

Assuming you can get the car to start (mine needs an engine warmer at that temperature), it takes at least 15 minutes of driving to reach that temperature. Unless you’re going on a longer trip the engine most likely wont be warm by the time you reach your destination.

[yetihehe]

I had to drive in -30C once, the engine could not get up to final temperature after 2 hours of highway driving because I had to run cabin heater at full blast on windshield and side windows so they didn't cover with fog inside. But that was in very old low power car.

[gambiting]

Garages exist though.

[number6]

Well you have to keep it at operating temperature