sjwoodruff, I'm with you on the tire pressure and idling, but I don't understand the winter blend fuel and atomization points. I thought the primary reason mileage dropped in cold temperatures was that the denser air requires more fuel to create an efficient burn. Since more oxygen fits into the same cylinder charge, you have to let more fuel through the injectors or it will burn too lean. I've never heard atomization brought into the discussion before. That seems like a function of surface tension, but I don't see how that would be affected by cold temperatures. I'm not doubting you, because I'm certainly not an expert. I just don't understand the mechanism you're describing. Along the same lines, what is the winter fuel blend in your area? Here in the Chicago area, winter blend fuel may contain less alcohol than the summer blend.
Good question. I'll try an add info w/o making the explanation too long and drawn out...
Point 1) Cooler, denser air will require more fuel to avoid burning lean. However, more air + more fuel = more power. Think of turbocharging or supercharging. An engine will create more power at cooler ambient temps - that's why many real world performance tests are done at dawn when the air tends to be heavier. This will decrease fuel economy slighty, but only if you're driving the car harder than you normally would, follow? Similar driving habits should yield nearly imperceptable decreases in fuel economy if all else is equal. But we know this isn't the case due to our winter fuel mix, bringing us to...
Point 2) <Courtesy of Chevron> Composition Changes: Gasoline is composed of hundreds of hydrocarbons, each with its own energy content. The energy content of the gasoline as a whole is the weighted sum of the energy contents of the component hydrocarbons. The relationship between gasoline fuel economy and energy content is advantageous because it can be used to predict how composition changes will affect fuel economy. Replacing some hydrocarbons with others may change the energy content of a gasoline. A more significant change in energy content occurs when a gasoline is oxygenated
– when some hydrocarbons are replaced by oxygenate(s). Oxygenates
are combustible liquids that contain oxygen. At present, the most common oxygenates are ethanol and methyl tertiary
-butyl ether (MTBE). Ethyl tertiary
-butyl ether (ETBE) and tertiary
-amyl methyl ether (TAME) are used to lesser extents. Because oxygenates can be viewed as being "partially burned" hydrocarbons, they have lower energy contents than hydrocarbons, and, consequently, oxygenated gasolines have lower energy contents and lower fuel economies than conventional gasolines.
Point 3) Atomization: To assure that fuels have the proper volatility characteristics, refineries adjust gasoline seasonally. Due to additives, during the cold, winter months, fuel will be more volatile. This helps good “cold” start and warm-up performance. Conversely, during the hot summer months, the fuel mix tends to be less volatile. This will help minimize the incidence of vapor lock and hot driveability problems.
On it's own, fuel is very volatile (it likes to evaporate). The warmer the temperature, the more it will evaporate (atomize). IC engines don't operate well under a wide range of variables. The fuel blends tend to seasonally adjust the volatility of the fuel in order for more efficient engine operation. Though this is the case, the pro-volatility agents used in the winter result in a lower energy burn in the combustion chamber. Less power from combustion = more fuel used to create power = lower mpg.
Point 4) Other items to consider: Engines take longer to warm up. During this time, they are running heavy at high RPMs. Even after you get going, all the car's systems have to work harder to maintain a normal operating temperature. The air is denser when it’s cold and it’s likely to be windy in the winter resulting in more wind friction. The roads can be covered in snow and ice. Tires are stiffer. Until they warm up on the highway, they roll less easily. And the tire pressure is likely to fall below the desired range. This all adds up to more resistance. In the winter, you use your headlights, defrosters, and heaters more resulting in more strain on the alternator and parasitic losses to the engine.
Two-thirds of the friction losses in an engine are estimated to occur during hydrodynamic lubrication and one-third during boundary lubrication or mixed hydrodynamic/boundary lubrication. The newer energy-conserving motor oils are designed to reduce friction losses from both types of lubrication by tailoring the viscosity characteristics of the base oil and the chemistries of the friction-modifying additives. Colder oil doesn't lubricate as well though, so there will be more friction losses there. The same holds true for transmission fluid.
OK, longer than I hoped for, and I now officially feel like a complete nerd