Fuel Expansion in Aux. Cells

[Ignacio]

Got from another list from the IBA technical authority, Tom Austin. Good, nerdy, arcane stuff.

Re: Expansion and Catch Tank Size Calculation
All gasolines are not the same so I use a volumetric expansion coefficient of 0.07% per degree F increase in temperature, which is 0.13% per degree C.

Here is my summary of overflow tank/vapor space design calcs:

When you fill the tank, the temperature of the gasoline is usually a little lower than the daily mean temperature of the location where you purchased it. 60°F is a typical temperature for fuel dispensed from underground tanks in areas with maximum daily temperatures in the 70-80°F range. Given daily variations in temperature, it is easy to find 60°F fuel in an area that will achieve a daily peak temperature of nearly 100°F. If the bike sits in the sun in a black tank, fuel dispensed at 60°F could easily rise to over 100°F. Designing for a 40°F rise is the minimum, in my opinion.

The volume change going from 60 to 100°F is 2.8% (40*0.07). Each gallon is 231 cubic inches. 2.8% of 5 gallons is 32.3 cubic inches, which is 17.9 ounces. (Vehicle manufacturers typically provide more than double this much expansion space to deal with more extreme conditions and to have a safety margin. That's why all motorcycle tanks are designed with dead space than can't be filled.) It is not that difficult to experience a 60°F rise in gasoline temperature in an aux tank. Designing for a 40° rise does not give you a fail safe system.

Based on the above calculations, an overflow tank that is 2.625" dia. (the diameter of a soda can) would have to be 7.8 inches tall to handle the effect of a 40°F rise in a 5 gallon tank. Note that the catch tank sold by Tour Tank is only 1.625" dia. By 4" tall. That's only 16.6 cubic inches, about half what you need for a 40°F rise in a 5 gallon tank.

 

[Groo]

Is this something that most Aux. tanks account for, or you just underfill to make sure you don't have gas sloshing out if it heats up? How does a design like Dean Tanji's account for expansion, or does it?

 

[Ignacio]

I don't know about the Tanji specifically, but think you can fill that sucker to the brim. Unless he's engineered a bubble somewhere that gets released after you fill that neck thing up....

I happen to have a BLM with a flush mount cap. The cap is recessed enough that I think I have some head space left in the cell and can get 4.6 gallons in it's 4.8 rated capactiy.

That said I've seen and smelled a few aux tanks at rallies that were dripping on the ground through their vent hoses. Seemed like people filled first thing in the morning trying to squeeze in that last drop of fuel and by the afternoon there were puddles here and there in the afternoon sun.

Personally, I also have a small catch basin (a largish Wix fuel filter) and hope that combined with a bit of head space won't leak to the ground through expansion or extended corners to the left.

 

[chickey191]

Is this something that most Aux. tanks account for, or you just underfill to make sure you don't have gas sloshing out if it heats up? How does a design like Dean Tanji's account for expansion, or does it?

whether by design or happenstance - most fuel cells that I've seen have a certain degree or amount of expansion space built in - usually in the filler opening. Either a baffle or plate or the design itself prevent the cell from being fully filled to the brim. Whether this is designed in (with appropriate space per capacity) or whether it just sort of naturally happens (larger footprint of cell provides larger expansion space). This thread started because of a cylindrical designed tank that doesnt lend itself to offering a lot of expansion room - and that company providing an expansion tank (which doesn't look like it would be big enough based on Tom's calculations).

The question came about because there was/is some confustion about requiring a catch tank at all. IBA rules only state that overflow must be provided for and routed away from moving parts. MERA rules (for UT 1088) require that a catch tank be provided, though they don't stipulate size). Adding to this was the desire to provide an expansion tank to recycle overflow rather than just catch it -- and to further complicate things whether or not the catch tank would be considered part of the total 11.5 capacity limitation.

 

[Ignacio]

The question came about because there was/is some confustion about requiring a catch tank at all. IBA rules only state that overflow must be provided for and routed away from moving parts. MERA rules (for UT 1088) require that a catch tank be provided, though they don't stipulate size). Adding to this was the desire to provide an expansion tank to recycle overflow rather than just catch it -- and to further complicate things whether or not the catch tank would be considered part of the total 11.5 capacity limitation.

See how my efforts to tease away a nerdy part of trivia without fully explaining the backstory and paralell discussion thread on a listserv worked out..... :blink:

What Chuck said.

 

[Woodstock]

Isn't this why _all_ fuel cells have an overflow which excess fuel can be expelled out the tank onto the road, and why it's critical that the overflow does not drain either in front of a tire or onto a hot surface (like the exchaust).

Mind you, the overflow of the normal FJR tank comes out "in front" of the rear tire, so maybe that part isn't as critical??