alt Hacker News> Gasoline engines are already 15% less efficient at 20F.
Is that actually true once the engine has reached operating temperature?
Replies
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.
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.
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.
Well you have to keep it at operating temperature
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
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.