I have been asked several times why people get worse gas mileage in the cold months. Well let's narrow down the reasons why A Three words: "time to temperature." In cold weather, our vehicles take a much longer period of time to reach full operating temperature. And they take this extra time each and every time we start them up, even if they have not fully cooled down. Modern engine-management systems are very efficient at optimizing the fuel/air ratio entering the engine. The oxygen sensor monitors the percentage of oxygen in the exhaust, compares this with the percentage of oxygen in the atmosphere, and generates a low-voltage signal that communicates this ratio to the computer. The computer then adjusts the pulse-width of the fuel injectors -- the precise period of time each injector is open on each injection cycle --to fine-tune the amount of fuel reaching each cylinder. The system makes this very fine adjustment dozens of times each second, working very hard to reach the optimum air/fuel ratio for any given situation, and at steady-state cruise speed seeks to approach the perfect ratio of 14.7 to 1, called the stoichometric ratio. The system is running in a "closed loop" when it is relying upon the oxygen-sensor signal to fine-tune engine operation. But the engine-management system can seek this optimum air/fuel ratio only when the engine is up to full temperature. In fact, the computer does not look for a signal from the oxygen sensor until it approaches full temperature. During the warm-up cycle -- which takes considerably longer in cold weather -- the computer operates on a warm-up program based on coolant temperature, mass airflow or MAP sensor input of air volume and temperature, throttle position and engine rpm. This is called "open loop" operation, meaning the system is not operating off the feedback from the oxygen sensor. In open-loop operation during the warm-up period, the engine requires -- and is provided with -- a richer air/fuel ratio to ensure good combustion. It needs this extra fuel for the simple reason that a percentage of the atomized, then vaporized, fuel delivered to the engine condenses into liquid gasoline on cold internal engine components such as the intake manifold, intake valves, pistons and cylinder. And remember, it's gasoline vapor, not liquid, that burns. Until those parts warm up, the engine needs more fuel to operate with reasonable drivability. Thus, the colder the weather, the longer it takes for your engine to reach full operating temperature and closed-loop operation, and thus the more fuel the engine uses to deliver the same driving cycle. That's the primary reason your engine consumes more fuel in winter driving. **Another factor in winter mileage is air density, defined as the number of air molecules per cubic foot of air entering the engine. Remember that 14.7-1 air/fuel ratio? That means 14.7 parts of air are mixed with one part of fuel for the perfect air/fuel ratio -- that's a lot of air! In winter, colder air means denser air -- more molecules per cubic foot. At any specific throttle setting or opening, the same number of cubic feet of air, but containing more air molecules, will enter the engine. The computer will provide more fuel to create the proper air/fuel ratio at that moment. This is a bit more subtle, since "more air/more fuel" produces more power, so you may be able to operate at a slightly lower throttle setting -- sort of rebalancing the equation.
These kind of insights you provide are very valuable. They should almost be in the Tech section since they are basically that. It would boost that section and help promote us as a Tech then Party forum, and not a Party then Tech forum. Nice write up though for real!
I'm in full agreement.....I want a tech/truck oriented website and not a party site with a little bit of trucks. There are other sites for that type of thing. At the same time having fun is my other goal I will leave this in the General area for now (alot of tech topics get swept under the table fast) but I will take your advice and move it to the tech section later.
not to mention I at least let my truck sit for at least 10 minutes at idle to warm the engine up to where the engine oil temp is at least 70 before I start driving it around.
^x2. Idling kills my fuel mileage during the winter. We also have additives added to the fuel to help compensate for the cold and that doesn't help mpg either.
One More Thing: A very informative write-up. There is one other rather significant factor in cold-weather fuel mileage that I didn't see in the write up: Torque converter clutch engagement is locked out until the transmission fluid reaches a certain minimum temperature, at least with the 4R70W-type transmissions. I notice this feature in my Lincoln TC with its digital dash and "instant econ" display. Cruising at 50mph shortly after a cold-start on a long, level stretch of road the econ will read 17-18mpg. As the tranny fluid reaches operating temperature the TC clutch engages and the econ shoots up to 26-30mpg. The fuel mileage improvement comes from recovery of the loss through the unlocked torque converter and pumping losses through the intake as the engine rpm is brought down and the throttle opened to compensate.
