Question on siphoning for the physics majors.

[Courtney]

So reinstalled my special tank, and noticed the single line (single manual petcock modification) to the carbs takes an upward loop for about an 1" before heading down to the carb inlets. The loop is right at the same height or maybe 1/2" above the petcock outlet. I turned on the petcock and gas flowed easily into the carbs, but my mind says it should be a straight run downhill. Is this going to be okay the way it is? The loop allows the room for an inline filter, and the gas didn't mind climbing that 1/2" before going down??? Can you tell I didn't major in physics?

[TopCatGr58]

Hey Courtney,

I withdrew from physics in college..along with calculus...was just trying to take on too much too soon! Did finish Biology and Social Psych with A's, along with working 32 hours a week.

Remember, it's not just a siphon affect, but also a pressure/gravity feed from the ~4 gallons of pressure in the tank ABOVE the petcock pushing out the fuel as well, so it should be fine. The only time I would think it might cause a problem is when you are near empty, or trying to run on reserve, but again the fuel still flows faster than the carbs can empty, even from just 1 petcock.

T.C.

[MPittma100]

Hey Courtney,

I withdrew from physics in college..along with calculus...was just trying to take on too much too soon! Did finish Biology and Social Psych with A's, along with working 32 hours a week.

Remember, it's not just a siphon affect, but also a pressure/gravity feed from the ~4 gallons of pressure in the tank ABOVE the petcock pushing out the fuel as well, so it should be fine. The only time I would think it might cause a problem is when you are near empty, or trying to run on reserve, but again the fuel still flows faster than the carbs can empty, even from just 1 petcock.

T.C.

+1 here. T.C is correct. Gravity is your best friend in this case. Don't really need science. Just open the petcock like you did and the science will run all over the place until you turn it off.

MP

[Rasputin]

Ever siphoned gas from a car? It will go uphill, around corners and through restricted area with no problem. It will go from the bottom of your fuel tank in the car to a container of your choice as long as the final end of the flow is lower than the source fluid level.
Even some amounts of air in the lines would be ok. Those with clear fuel lines to the carbs can attest to the fact that the bike will run all week with an air bubble in the line (as long as the bubble is not too large).

[DGXSER]

It is a pressure thing. Not a vacuum siphon thing. Just as TC pointed out. without going back to Physics 402, for any material to flow through a device, or tube, it creates friction. The pressure pushing it through must overcome the friction required to move the fluid through the path. When fluid goes against gravity, it has to overcome gravitational pull at 32 feet per second per second. Well in this case it only needs to rise maybe an inch or so. So as long as the gas is more than 2 inches above the top of that rise in the piping, it will flow.

An interesting thing about closed loop systems, like say a heating water system in a house or building, where the same water circulates round and round, you do not need to account for the height the fluid changes, as for every inch it goes up, it also comes down equaling out the forces. However, in an open system, fluid goes through and out, like our fuel systems, you do need to account for the rises and do not get the benefit of the drops.

[Rasputin]

Stick a hose in a bucket of water and tell me that it does not self level as an open ended system. Major buildings have been levelled this way. Fluid like water or gas is self levelling regardless of drops. Drops may slow it down but they will eventually right themselves.

[WMarshy]

Its fluid mechanics at its simplest. Same idea why your towns water tower is elevated off the ground. A taller/higher column of fluid has more pressure at its base than a shorter column. As long as the little loop stays below the fuel level in your tank there will be sufficient pressure to force gas through it in a static world. Even if the level in the tank does drop below the little loop, gas will continue flowing until all the gas in the line and bowls are sucked dry. The air moving through the carbs causes a siphion effect on the fuel system but that is advanced theory with Dr. Bernoulli, not covered in todays lecture.

[Courtney]

OK, thanks guys, it's staying as it is then.

[clcorbin]

A man by the name of Bernoulli answered this question in 1738 with his famous equation:



The ENERGY ( sum of the pressure energy, kinetic energy due to flow and potential energy due to height differences) at any point in the system is a constant (with the necessary assumptions that are good enough for this case). So let's say the pressure at the end of the hose is the same at both ends (and it pretty much is as air pressure just doesn't change much in say 6"). Let's also say the height difference between the top of the gas in the tank and the bottom of the fuel line is 8". We also know that the gas isn't moving in the gas tank. From those assumptions, you can solve for the velocity of the gas in the hose at the exit.

It leaves out a few things (viscosity being the biggest, but gasoline doesn't have very high viscosity, so the impact, while it exists, isn't all that much), but overall it will give you a pretty good idea of how fast the fuel is flowing out of the hose. If you go with a tiny diameter line, the flow WILL be slower than you calculate (friction due to viscosity) or with a very long hose (again, viscous effects) you will also have slower flow than calculated. But for a quick look at what is going on, Bernoulli's equation is a very good place to start.

And notice that no where in there do we have to know ANYTHING about what is happening between the start point (gas tank fuel surface) and the exit (bottom of your fuel line). It just doesn't enter into the equation. So that little rise in your fuel line has nothing to do with it, practically. You could wrap the fuel line around the gas tank a couple of time and it would STILL flow (once you got it going as the line passes above the fuel level). Just a bit slower due to the higher friction in the longer line.

And I am working on my graduate degree in fluid dynamics...

[CaptonZap]

And I am working on my graduate degree in fluid dynamics...

Would your instructor give you extra credit if you worked out the ideal length of velocity stacks for 4000 RPM, standard day, half throttle?

CZ

[MPittma100]

So reinstalled my special tank, and noticed the single line (single manual petcock modification) to the carbs takes an upward loop for about an 1" before heading down to the carb inlets. The loop is right at the same height or maybe 1/2" above the petcock outlet. I turned on the petcock and gas flowed easily into the carbs, but my mind says it should be a straight run downhill. Is this going to be okay the way it is? The loop allows the room for an inline filter, and the gas didn't mind climbing that 1/2" before going down??? Can you tell I didn't major in physics?

Well, after all of these scientific responses, sounds like you may have gotten your question answered.

MP

[WMarshy]

... Don't really need science. Just open the petcock like you did and the science will run all over the place until you turn it off.

MP

Best response hands down. I laughed my @ss off when I read this.

[skids]

A man by the name of Bernoulli answered this question in 1738 with his famous equation:

And I am working on my graduate degree in fluid dynamics...

Well, I am wondering if surface tension could cause some interferrance. That is not a factor with Bernoullis. Personally I have never had issues with this at all, and I have seen a bubble through the clear gas line with the bike off, but it quickly disappeared when the gas was flowing.

[clcorbin]

Would your instructor give you extra credit if you worked out the ideal length of velocity stacks for 4000 RPM, standard day, half throttle?

My research is in shock interactions in multiphase mediums, so probably not... I have some friends in Formula SAE that have the software for calculating inlet (and exhaust) runner length, but I've never touched it.

CZ

Best response hands down. I laughed my @ss off when I read this.

Agreed!

Well, I am wondering if surface tension could cause some interferrance. That is not a factor with Bernoullis. Personally I have never had issues with this at all, and I have seen a bubble through the clear gas line with the bike off, but it quickly disappeared when the gas was flowing.

There are a lot of things that Bernoulli didn't take into account. But considering what he came up with in 1738, I'll cut the man some slack!

[WMarshy]

For your reading pleasure...

http://www.autozine.org/technical_sc...e_exhaust.html