Fuel Reserve - is it really a reserve?

[Daboo]

But, since the fuel in the lower tank cools the fuel pump, and heat shortens its life, you might consider filling up at two bars.

Short story that I hope you all can appreciate. One tragic, the other a bit humorous.

Some technicians at Grand Forks AFB were troubleshooting a fuel pump problem. The circuit breaker for the fuel pump (located in the wing and also submerged in fuel like the ST, kept popping. So they kept pushing it in. Eventually it got hot enough to blow the wing off. My squadron at Griffiss AFB in NY got involved by sending the only remaining G model wing to replace it from a electronics test airplane located on a remote hilltop in Upstate New York. We did ECM testing on airplanes on these remote hilltops and for the B-52, we had a 50 ton crane to pick it up and flip it upside down depending on the testing they were doing.

The more humorous story is how the early VW Beetles didn't have a fuel gauge. You were to flip the lever on the floor when you started getting low on gas to get the reserve fuel. Unfortunately, many owners forgot to flip the lever back after refueling and when their Beetle ran out of gas...there was no reserve left.

I find my XR to be quite incessant in getting more gas when it runs low. The dash will tell you how many miles you have left. I've never been brave enough to run it to the end yet. When it gets down to two bars on the gauge, an orange warning notice appears saying I'm running out of fuel...go find a gas station immediately. From that point on, I get to see both a readout on miles left and how many bars I have left. Maybe that's why you don't see many BMW's on the side of the road??? I usually have about 42 miles left when the warning comes on.

Chris

 

[Andrew Shadow]

When the last bar starts flashing on the ST1300 fuel gauge there are, excluding minor variances between ST1300's, 5 liters, 1.32 US gallons, 1.10 Imperial gallons, of fuel remaining in the lower tank.

 

[Larry Fine]

When I rode bikes with reserve petcocks, I developed the habit of leaving on reserve for a few miles, in case I got some water. I'd rather find out when I have a full tank than next time I set it to reserve.

 

[ST Gui]

The heat transfer from the pump switches from convection between the pump hardware and the liquid fuel to air.

I might be reading this wrong as it's been know to happen. But wouldn't that be more like:

The heat transfer from the pump switches from conduction between the pump hardware and the liquid fuel to convection between the pump hardware and air.

 

[Willsmotorcycle]

The fuel acts as a thermal ballast. It's heat bearing capacity is large compared to the air which displaces the volume of the lower tank as the fuel is consumed.

The heat transfer from the pump switches from convection between the pump hardware and the liquid fuel to air. This transfer is much more effective for liquid (fuel) than gas (air).

I'm following the science here, but not sure I follow the logic. If I have a pot of boiling water and a metal object suspended in the pot, the pot is 212* the water is 212* the object has to be 212*. The air outside the pot is less than 212*. I love stretching my brain.

 

[drbuzzard]

Fuel reserve?
Anybody remember ( I know you all do ) reaching frantically under your left leg for the reserve tank switch whilst riding amongst traffic. The good old days when a RD400 was considered a powerful bike.
Upt'North.

only to find that you were already on reserve because you forgot to swithch it back the last time you switched it on reserve

 

[gunsmoker]

Motors generate heat --including electric motors. So what if the electric motor in the fuel pump would normally get to a temperature much hotter than the ambient temperature of the substances around it, unless that substance is an excellent themo-conductor which draws that excess temperature away, into itself, which liquids tend to do well, but air does not do so well?

 

[jdmccright]

The more humorous story is how the early VW Beetles didn't have a fuel gauge. You were to flip the lever on the floor when you started getting low on gas to get the reserve fuel. Unfortunately, many owners forgot to flip the lever back after refueling and when their Beetle ran out of gas...there was no reserve left.

Not just Beetles. I've done that at least once on my old 400cc Yammy. At least it was easy to waddle to the gas station. If my ST ever quits, I'm leaving it. I love it but not worth a hernia over.

 

[ST Gui]

only to find that you were already on reserve because you forgot to swithch it back the last time you switched it on reserve

Did that more than once on my 305 Scrambler. That was back when a reserve was a Reserve!

 

[ST Gui]

I'm following the science here, but not sure I follow the logic. If I have a pot of boiling water and a metal object suspended in the pot, the pot is 212* the water is 212* the object has to be 212*. The air outside the pot is less than 212*. I love stretching my brain.

I'm betting you should wrap your brain around realistic figures over arbitrary ones. Where actual science is concerned I'd posit that under specific circumstances conduction is a more efficient method of heating or cooling than convection.

In your example the pot is generating the heat and transferring it to the metal object. It's the other way around in a gas tank.

A processor chip in a circuit heats up so a heat sink is added. To improve the conduction performance of the heat sink thermal paste is used. Then heat dissipation further relies on convection for cooling. Possibly a fan is used if the combination of conduction and convection is insufficient.

A fuel pump generates heat. I don't know that it has the capability of heating the fuel in the tank to its temperature or that it happens if it does. But fuel does have some efficiency in dissipating the pump's heat. That efficiency is lost when the pump is exposed to air as heat dissipation by conduction gives way to convection. So fuel level is reduced below the top of the pump I posit the efficiency of conduction is reduced.

I'll bandy the term thermal runaway. I'm not sure it applies but it's a cool term.

The air in the tank isn't likely to be the same temperature as ambient air temperature. Unless the air is significantly cooler around the pump when the fuel level drops the pumps temperature could easily rise faster than the stagnant air around it could cool by convection. I think in instances like this convection at least implies moving air currents. A convection oven heats more efficiently due to moving air just as a fan cools more efficiently due to moving air.

This is where actual temperatures and other variables need to be noted and compared.

Now NDT might laugh at the accuracy of my suppositions. But this is what I think — until something more edifying comes along as it often does.

 

[ST Gui]

Not just Beetles. I've done that at least once on my old 400cc Yammy.

Now that you mention it I image just about any bike back then without a fuel gauge probably had a manual reserve lever. Sure you could monitor your odo or trip odo if your ride had one and do the math. Without an trip odo I'd have done more pushing than riding.

If my ST ever quits, I'm leaving it. I love it but not worth a hernia over.

I pushed mine about 1.22 blocks on level ground and I hope to never do that again. To that end I top off at 3 bars unless I'm RTB.