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# Pressure vs. volume?

Discussion in 'The Okie Corral' started by AutomotiveTech, Jan 14, 2010.

1. ### AutomotiveTech

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Since there are some people that are much smarter than me on here(not saying much, I know), can anyone explain to me why pressure is/is not directly proportional to volume. The reason I ask is, since I work on cars I frequently check fuel pressure. I hear people say that you can have good pressure, but the volume may not be enough. This makes no sense to me, if you have the correct pressure then how could you not have the volume? When I say volume, I am not refering to the actual flow from an orifice, but the amount the pump can provide. I can find references to Boyle and Charles on the internet, but these seem to deal with gases, not liquids.

2. ### fnfalmanChicks Dig It

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Volume is fixed. What you're talking about is volumetric flow rate which is not volume.

Gas laws are not the same as liquid laws because although they're both fluids, gas is compressible and liquid is incompressible. The laws are fairly similar until a certain pressure threshold is reached and then liquid becomes incompressible (that's how hydraulics work) but gas still be come quite compressible albeit at an extremely high pressure.

3. ### mikeflys1Pastafarian

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This is probably a terrible analogy but here goes...

Think of it comparing two rivers, both are the same depth and moving at the same speed but one is ten feet across and the other is one foot across. They would both have the same pressure but the wider one would have 10x the volume flowing through.

4. ### AutomotiveTech

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I do not see how pressure is related to volumetric flow rate. In a cars fuel system, how can the pressure be correct and there not be enough volume.

5. ### AutomotiveTech

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If they were both flowing through the same size orifice the 10x river would have greater pressure.

6. ### fnfalmanChicks Dig It

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What do you mean not enough volume? The volume is fixed. The volume of the gas tank is fixed. It ain't gonna get any larger or smaller on its own. The volume of the fuel line is fixed. The fuel lines are only so big in diameter and they only run for so long in linear feet. It ain't gonna shorten or lengthen on its own.

As far as how pressure is related to volumetric flow rates, it dictates the flow profile AND the flow rate across an orifice which has a fixed open surface area (aka hole).

I have no idea what you're talking about "pressure is good but no volume". No what "volume"?

I'm not an automotive engineer so I don't know what kind of pump they use for fuel delivery. Is the pump's performance fixed or is it variable?

7. ### fnfalmanChicks Dig It

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Not necessarily. More velocity, yes, but not necessarily more pressure.

8. ### FullClipNRA BenefactorCLM

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For a centrifugal pump, the pressure at the discharge will be highest with no flow. That pressure will decrease as the flow increases at a rate depending on the design of the pump. If the pump is worn this curve will change, but you may get the same shut-off head pressure, but less flow at normal operating conditions.

For a positive displacement pump, such as a piston, gear or screw type, the volume is fixed for every revolution or stroke of the pump. Normally, some type of relief valve limits the discharge pressure of a positive displacement pump so it doesn't self distruct on zero, or low flow conditions as it will keep trying to pump, and thus increasing pressure until something breaks or the driver can't supply the needed power.

9. ### AutomotiveTech

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The pumps volume is usually fixed, and the pressure is controled by a regulator valve on the fuel rail(on the engine). The test port is usually also located on the fuel rail. When diagnosing a fuel delivery problem, it is said that a restriced fuel filter(located between the pump and test port) can cause a loss of volume at times of higher demand. If this is true, then wouldn't the pressure also drop? Sorry, but it has been fifteen years since I had a physics class.

10. ### johnd

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Pressure is what makes it move
Volume is how much you get.
You have to have pressure to get any volume but then the limiting factor will be the delivery mechanism ( the pipe diameter)
Given a constant pressure the volume will be higher with a bigger pipe.

11. ### fnfalmanChicks Dig It

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Not necessarily just due to the same reason that Fullclip posted above. Pump performance curve dictates pressure reading and volume. And that pressure reading depends on the type of pump. If the pump is a centrifugal type then you may very well have good pressure reading but low or no flow.

I am not an automotive engineer so I have zero idea what style of pump the car manufacturers use for fuel pump. If we know that then we can even analyze the issue further.

12. ### tbhracingSenior Member

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^^^ beat me to the centrifugal pump answer.

In short- take a fire engine, depending on the pump, it could serve water in two modes- Pressure and Volume.

Pressure is the first stage with lower amount of PSI. If more pressure or PSI is needed, the pump can be moved in to Volume mode. (This is used if you plan to pump over 50% of the pump's rated capacity) So Volume is greater than Pressure.

Hope that makes sense.

13. ### AutomotiveTech

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Maybe I am not explaining this very well. Imagine a fuel pump, a line going to a filter, then a line going to an orifice with a test port in that line. I know a restriced filter will cause the line from the pump to the filter to have higher pressure, but after the filter the pressure and volume should drop, not just the pressure, right? I belive the fuel pumps are not centrifugal, but I could be wrong. We are talking around 60 psi, usually with a 5/16 line, if that tells you anything.

Last edited: Jan 14, 2010
14. ### bcmclane

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Think about if the fuel pump was connected to a pressure sensor. The pressure sensor will dial the pump up and down to maintain 60psi after the pump. With an open ended fuel line, -Y- volume of fuel will come out. If you squeeze the end of the line, the pressure sensor will keep pressure at 60psi, but volume will decrease due to a smaller opening. So in this situation pressure remains the same but volume changes.

It might be easier to think of this using lower pressures and a larger line. Say you are pushing 5psi of water through an 8" pipe. Then push 5psi through a 6" pipe. The volume of water moving through the 8" pipe would be greater, but pressure remains at 5psi.

15. ### FullClipNRA BenefactorCLM

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How can the 'volume' drop? If you have "X" amount of volume on one side of the filter, how can you have "Y" amount of volume on the other? The volume in the line is the same unless there is a leak along the pipe.

But are you measuring the pressure at a low demand conditon?

I'm used to dealing with much bigger pumps, but the idea is the same. Would help to know what kind of pump it is, and if the pump suction has a filter. A plugged filter or strainer on a pump suction will lower the capacity of the pump, starving the flow as the discharge flow demand increases.

16. ### tbhracingSenior Member

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Question- I think the example of "amps" vs "voltage" is almost the same deal, right?

17. ### AutomotiveTech

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I understand what you are saying. My problem is that if you have good pressure, how could testing volume by taking the fuel line loose condem the pump. I think that if you have the correct pressure, then there is sufficient volume to feed the engine the correct amount of fuel. If the engine was starving for fuel, then wouldn't the pressure also drop?

Last edited: Jan 14, 2010
18. ### UTKEngineer

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You keep saying "volume." I don't think that is what you mean. As fnfalman said, the volume of your system is fixed. Do you mean volumetric flow rate? (cc/min or something similar)?

Anyway, to answer you question, no. Your pump is putting out a given pressure. The difference in the pump outlet pressure and the pressure of you final destination (presumably the cylinder) minus the dissipation by friction determines the volumetric flow rate which will be constant through your system until it is mixed with air. Most liquids are nearly incompressible so changing the local pressure doesn't change their density.

19. ### MARKDANIEL

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What they might be refeering to is. The pressure may be correct durring a static test which would no fuel flow car not running or at idle. However when the fuel is flowing at a high rate the volume may not be enough which would actually cause the pressure to fall.

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