Gas: Full up or half a tank??? - Page 5

the other pc · 01-05-08, 12:55

Quote:

Originally Posted by Setec Astronomy

You're kidding, right? Oh...you're an electrical guy, IIRC. ....

Hey, wait a minute! I’m an electrical guy, and they made us take physical chemistry, thermodynamics and fluid mechanics.

I know how to drink a milkshake through a straw and everything! Well, not at Wendy’s. At Wendy’s I have to use a spoon.

PC.

Corey Bit Spank · 01-05-08, 01:29

Quote:

Originally Posted by KnuckleBuckett

Pump readout feedback is calculated using the feedback port (kicks it off when full). The higher the speed the more that air in venturi form (zipping past the feedback port) increases readout error. In effect you are paying for air. Further, though to lesser effect, the higher the fill speed the more air is incorporated into the fuel (temporarily) thus again you are paying for air. An exagerated example of this would be whipped cream.

But what if the error was in my favor?

mhadden · 01-05-08, 01:37

Personally, I will drive until the light comes on, or just before it. I try not to run the vehicle to much further after the light came on since most fuel pumps nowadays use the fuel to lubricate and cool the pump. Either way, I don't think it really matters, but I'm a believer in running your vehicle too low too many times can reduce the life of the pump.

Setec Astronomy · 01-05-08, 02:02

Quote:

Originally Posted by the other pc

Hey, wait a minute! I’m an electrical guy, and they made us take physical chemistry, thermodynamics and fluid mechanics.

I know how to drink a milkshake through a straw and everything! Well, not at Wendy’s. At Wendy’s I have to use a spoon.

I can't eat a whole Frosty anymore, it makes my teeth hurt. Eh, I learned about straws from reading the back of cereal boxes, anyway. Do you know how many straws you would have to lay end to end to reach the moon? Me neither.

Aurora40 · 01-05-08, 02:20

Quote:

Originally Posted by Setec Astronomy

If you want to change the phase of the fluid as you suggest, you'll need to increase the temperature.

Well, I do know that pressure will dictate a phase change just the same as temperature will. I believe gas caps only vent one way, and that is simply to prevent the pressure from getting too high in the tank (this is why many states test the cap when doing an emissions test, to make sure it isn't venting at too low a pressure).

I absolutely don't understand why it would starve your motor. The pump is in the liquid, it isn't at the motor using suction. There could be total vacuum in the space above the liquid gasoline, and the fuel pump would still work just fine.

But yes, lol, I'm an "electrical" guy I suppose.

KnuckleBuckett · 01-05-08, 02:30

The error is not in your favor.

I really hope you were kidding.

Setec Astronomy · 01-05-08, 03:03

Yeah, you're going to lower the boiling point of the fluid, yes you will get some vapor to replace liquid as you draw down the tank. From my 1976 Buick Factory Service Manual, in the description of the evaporative emission control system:

3. To prevent the tank from mechanical damage in the even of excessive internal or external pressures resulting from the operation of this closed system, a pressure-vacuum relief valve, located in the gas cap, will control the tank internal pressure.

If the state only checks the cap for pressure relief, it's because they don't care if your car stalls, only if it barfs out vapor or gas.

Here's one online reference that discusses why you need to relieve the vacuum on the tank for structrual reasons Tank protection division - storage tank venting for conservation, safety, and environmental protection

Aurora40 · 01-06-08, 08:00

I could see how a 1976 Buick might stall as it probably had a mechanical fuel pump mounted on the motor. But do you agree an electric pump submerged in the fluid would not suffer that problem?

I wouldn't think vacuum would hurt a modern gas tank, but I don't really know that. Maybe the gas cap does also allow air in. If you drove the car regularly and only filled up to 1/2 a tank, I doubt condensation or water in the fuel would be a problem because of this though. It just seems like the benefit of doing this is pretty dubious and it would be quite a hassle to have a driving range of like 150 miles.

jtford95 · 01-06-08, 09:02

If I'm not mistaken, you get better gas mileage with a full tank of gas.

Setec Astronomy · 01-06-08, 09:07

I only used my '76 manual because back in those days the manuals actually explained how the systems worked. And no I don't agree that an electric centrifugal pump is different than an engine-mounted diapraghm pump, you still won't pump anything out of the tank if you don't vent the ullage. Even those college kids who punch a hole in their pressurized beer can have to open the tab before they try to drown themselves. And don't look at me on the half-tank stuff, as previously mentioned I run mine down to nothing then fill it all the way back up. Anyway, that's enough on this subject.

Setec Astronomy · 01-06-08, 09:08

Quote:

Originally Posted by jtford95

If I'm not mistaken, you get better gas mileage with a full tank of gas.

What's the concept on that one? Or is that only if you have one of those magnetic vortex fuel straighteners that aligns the fuel molecules so they spray thru the injector in an orderly fashion?

kelmar · 01-06-08, 02:00

When I get gas I fill it up. I don't want to go back to the gas station any more then I need to.