Actually I think the main reason fuel economy declines in the winter is because the air density is higher and it takes more energy to push through the more dense atmosphere at the same speed.
Not to mention the fact that you run the accessories more in the winter. And I've noticed that the more electricity I use, the less my gas mileage (due to more drag from the alternator). The more I run the heater, the less the gas mileage (extra cooling of the engine? more pumping losses?). Etc.
Bottom line, more load on the engine by any source (and we tend to run more stuff in the winter) the lower the FE...
Winter does gives much more power but terrible FE except for one morning when I was almost snowed in, I just managed to rock my car out the parking space onto the road, I was in a hurry so I only cleared the wind screen and headlights but left all the cold snow on the rest of the car, my engine temperature soared very quickly but I thought its just a blip, but it stayed elevated, I got to work and noticed all the snow was still on my car, esp the grille, it blocked all the air flow into my radiator and engine bay, I didn't understand at the time that it was a grille block and warm intake until now.
Water is fuel, I just don't know how to make it work yet.
More dense atmosphere 10% more for about every 84 (approximately) degrees in temp. More work to push vehicle through more dense air.
Longer warmup time.
Colder fuel is harder to atomize.
Colder air also resists fuel atomization.
Emissions would also tend to be higher.
Heck at certain low temperatures it wont run at all.
Accessories are run more frequently.
Less flexibility in tires.
Lubricant viscosity is higher.
Increased air mass in air entering the engine means more fuel to match more air. This is why turbos run better in cold air. By "better" I mean faster 0-60 times, not "better" by using less fuel.
Less air density (hot air) means more incoming volume (though less dense and weighing less) of air due to larger throttle openings. This also means more compression pressure even though that same air has less mass. Larger throttle openings mean more compression which increases the ratio of compression pressure to combustion pressure, which means more efficiency.
It may be hard to understand this but let me try to explain.
The mass of the individual molecules in air and fuel is always the same. Mass airlfow sensors compensate for volume and density as well as temperature and humidity, by heating a wire and measuring the resistance across that wire due to the airflow cooling the wire.
At identical atmospheric pressures the mass is still different depending on the heat content of the air and fuel. Higher temperature means less mass per unit of volume, at the same atmospheric pressure.
Opening the throttle to compensate for the lower mass and slightly higher volume allows your effective compression to be higher, as well as reducing the suction and compression forces necessary to prepare the mixture for combustion.
Hotter air and fuel means less dense mixture and higher compression with less mass of material compressed.
We all know higher compression means more efficiency.
I am not trying to contradict valid points in each response. it's not soo much a matter of who is wrong or right. Its possible for every one to be right. The point is to better understand all the potential reasons for what is causing an effect that allows better understanding of the causes of inefficiency.
dont forget they change the fuel mix at the stations.
tommorow im gonna throw my stock intake box and paper filter back on my truck.its a snake maze that goes everywhere with the single opening going to the front of the truck in front of the radiator.(hopefully it will still fit)
If you are talking about getting air from in front of the radiator it would be cold air (LOL not when its 97 like it was here today). I doubt it would help your FE.
My idea wasn't about the temperature of the air (which I could still do between the opening and the engine) but about the pressure.
One of the ways I've had success is to try to minimize throttle-related pumping losses by opening the throttle as much as I can when possible. When I recently was reminded of the common ghetto ram air system*, using some sort of scoop/collector hidden behind a grille, it clicked with me -- if you're going 60mph anyway, couldn't some of that air pressure be collected and be used to push air through the other side of the throttle opening, changing the pressure delta and doing some of the pumping work?
The existing condition on both my vehicles is that air is not taken from anywhere that there's much pressure (Well, the VW might have a little pressure in front of it but the collector is pretty restrictive), nor is its temperature manipulated. I could certainly warm the air as it passes, and have the WAI while having the push too.
I look at it that better technology gives you more power AND economy. Compare the mileage of a typical four cylinder engine with a Honda VTEC engine. Honda designed engines that get better mileage and power. That's why I made the jump from Toyota to Honda.
For most of us, adding a little bit of power over stock gives us better mileage because you can give it less throttle for a shorter amount of time to get up hills or to get up to speed on the freeway.
In the case of my VX, I was able to increase the power about 40 hp and still maintain 39 mpg mixed driving (no hypermiling). The best purely freeway mileage I have achieved is 47 mpg. The car is capable of even better mileage with further tuning. I did this by improving the technology in the car. If I had money to get the Endyn roller wave pistons and head work, both power and economy would jump even further.
using some sort of scoop/collector hidden behind a grille
Just watch out for unintended consequences. Google "hydrolock corvette puddle" (without the quotes) and you'll read some interesting stories about people who forgot the Corvette is not an Amphicar (http://en.wikipedia.org/wiki/Amphicar).