engineermike

Rick, the main reason these things use more fuel in winter is increased pumping losses due to a deeper vacuum in the plenum drawn to achieve the same power. It's about 5% loss for every 10 deg f.

Scarlet

I have an '11 Lariat 4x4 Supercrew, 5.5' bed, 6.2 with 3.73's. The first year I regularly recorded my gas mileage on every trip, almost fanatically. My "A" trip would be for each tankful run. The "B" trip was used for each individual trip. I kept a spread sheet that I would enter - Date, Grade of gas, Price per gallon, Miles on truck, "A" Trip mileage, Gallons used, ave. MPG, Fuel cost for that entry, and what type of trip it was (local, out to West Virginia, combined, etc.). I would calculate the numbers by hand and discovered that the trucks numbers were fairly close. The 2011 Brochure gives the numbers for a 6.2 as 13mpg city, 18mpg highway, 14mpg combined. It use to be that "It was Said" that average mileage for a vehicle is 15,000 miles/year. In 2011 I traveled 16,500. My combined gas mileage for 2011 was 16.3 MPG. Since then I was keeping a record as I went, But not as regular as I did in '11. I would take pictures of the Info. display on some trips to back up my claims. This year I am going to keep a constant record again, and at the end of the year I want to see what sort of gas mileage I am getting now.

RickNY

Here all my real world actual MPG measurements logged for every fill since I got the truck in May. I drive about 60% hwy @ 70 / 40% city. I also drive like a granny with the assistance of the driving coach on a Bully Dog GT for fuel economy.

No tunes or mods other than a bed cover:
http://www.fuelly.com/driver/rickny/f150

My avg is 14.8... Also, my dash display always reads at least 1 MPG higher than actual - I reset it after every fill.

Rick boomer

Either that is an esoteric engineering term or a vague industry term with little meaning. Can you be more specific? There are many physical phenomena that I could ascribe to "pumping losses. In the end we are talking about efficiency.




BTW: ethanol has a higher octane (activation energy, resistance to ignition, ignition temperature) but a vastly lower energy content per gallon. E85 has far less energy (mileage) per gallon than gasoline.

engineermike

It's not complicated, vague, or esoteric.

On the intake stroke of any gasoline engine, there is negative gauge pressure in the cylinder at idle and light loads. The negative pressure above the piston, along with near zero pressure below it, means that the crank is applying work to it and getting nothing in return. In other words, it takes fuel to pull a vacuum in the manifold, thereby reducing efficiency. Colder temps = deeper vacuum to get the same manifold density (power), which means less efficiency.

Vvt and dod both attempt to improve this situation.. Diesels do this perfectly (no throttle valve, so no vacuum) which is part of the reason they are more efficient.

Rick boomer

Just looked it up. It's much simpler than your explanation. It is just what it says literarily (which terms are often NOT). The losses due to pumping air in and out. If the air is denser the losses will be larger (more resistance). This is compensated in winter by the fact that the charge is larger, denser air, and therefor more efficient. It goes up at the square of the pumping speed.

engineermike

There are a few technical inaccuracies in that explanation, but I guess since its simpler, go with it.

CreepinDeth

Interesting, I had no idea how it worked......

Yesterday in 0 degree weather I only got 17-18mpg after I was doing 65mph and reset my MPG meter. Not quite 21mpg as advertised but this air density info plus the lower grade fuel in winter in the midwest would probably be attributing to that. I had no idea, and just assumed colder air would be better

Around town it has dropped to 12.5mpg.

engineermike

Except that's not at all how it works.

Edit: Unless you drive faster in cold temperatures. . .

Rick boomer

The density of air at 70 F is 0.0745 and at 0 F it is 0.0858. That means at 0 F the engine draws in 15% more air and 15% more fuel than at 70 F for the same throttle position. That represents a possible 13% reduction in fuel economy simply due to air density.




Response


That's not how it works.


Well, actually, that is how it works. Now, what did I say. I said that effectively the motor behaves somewhat (not exactly) like it is a slightly larger motor at low temperatures than it is a higher temperatures and is subject to some of the same kind of larger engine reductions in efficiencies.


In principle, at constant efficiency, the size of the motor would not matter. The engine would use just sufficient fuel to overcome the load on the truck due to air resistance and other frictions. Motor size does matter because efficiencies change with motors. Larger motors have larger inefficiencies. Larger minimum air and fuel charges. larger internal frictions. (for the same type of design)


Will you see the full 15% reduction in fuel economy? Of course not. You will back off the throttle. Some factors increase fuel efficiency. Some decrease it. A higher difference in external air temperature and maximum motor temperature will increase efficiency. There is more than one factor. many more.


I am not an automotive engineer studi9ed in the art of automotive efficiency. There are literally thousands of factors. I do not have all the data. I can only talk from basic physical principles.