[Klepper1]

Originally Posted by Leebro61

The pressure in the cylinder would have to be higher than 10psi, otherwise the flow dynamic pressure has been lost somewhere. If we are considering this an isentropic process, the total pressure is constant and no energy is lost (this is essentially Bernoulli's relation for flow along a streamline, the conservation of total pressure).

So you are saying that as the piston in the engine drops, air in the plenum at 10 psi flows into the engine, and when the flow stops into that cylinder, the prssure in the cylinder is higher than 10psi? How can that be? How can air flow from low to high pressure?

In my 5 years of undergrad engineering and 15 years of experience designing, testing, and flowing gas trough pipelines, I have never seen gas flow from a lower pressure to a higher pressure. Boy, if I could make that happen, I would save millions of dollars in compressors.

Great discussion though! Maybe I am just thinking too steady state. Maybe I will run it through my transient models at work tomrrow.

[Leebro61]

I'm not suggesting that air will NATURALLY flow from low to high pressure areas, but I'm saying that if the air has sufficient momentum, it will not stop instantly and reverse direction when it is flowing through an adverse pressure gradient.

Imagine this, you have a ball sitting on a 45* slope. You let go of the ball and it rolls down the slope. Now, you roll the ball UP the slope. It rolls up the hill for a ways (depending on how hard you threw it) before gravity catches up and it rolls back down. This is also a conservation of energy.

Another example...

My main design experience is thermodynamic design engineering. This involves turbine performance test analysis, Rankine cycle analysis, performance predictions, turbine design, etc.

Long story short, the diffuser at the exit of the turbine has a large effect on the efficiency of the machine. The diffuser allows the turbine to exhaust fluid BELOW atmospheric pressure (~anywhere from 2-3 in/hg), this increases the pressure ratio of the turbine which increases the turbine efficiency. The diffuser allows the pressure exiting the turbine at ~1psi to increase at a controllable rate (preventing the boundary layer from separating) to above atmospheric such that at the end of the diffuser, the flow is at a higher pressure than atmospheric and this pressure difference (diffuser exit - atmospheric) will drive the flow to the condensor.

If the exit pressure of the diffuser is ~10-15 times greater than the inlet pressure, how does flow exit the turbine and travel up the diffuser? Simple. The turbine actually acts as a nozzle, accelerating the flow as the static pressure decreases. This allows the flow to continue despite the increased upstream pressure

Edit- Should I expect some cash flow from your high concentration (I've just made you millions ) to my low concentration?

[Klepper1]

Originally Posted by Leebro61

Imagine this, you have a ball sitting on a 45* slope. You let go of the ball and it rolls down the slope. Now, you roll the ball UP the slope. It rolls up the hill for a ways (depending on how hard you threw it) before gravity catches up and it rolls back down. This is also a conservation of energy.

This example is the same as our turbo engine. Air at the suction side of the turbo is at a lower pressure. You add energy to air with the turbo (throwing the ball in the above example) and it comes out the other side of the turbo at a higher pressure (ball moves up your ramp) and is at a higher energy state. Now the air in the plenum is at 10 psi (and the ball is higher up the ramp) - But how does the air in the plenum at 10 psi get to a higher pressure in the cylinder without adding more energy to it? Or how does the ball get higher up the ramp without pushing it again?

...the flow is at a higher pressure than atmospheric and this pressure difference (diffuser exit - atmospheric) will drive the flow to the condensor.

Agreed the pressure difference is causing the flow. My mind says its the pressure difference that causes the flow from the plenum to the cylinder. The turbo imparts energy to the air to get it from the intake at atmospheric to the plenum at 10psi. Conservation of energy applies for sure. But conservation of energy must also apply from the plenum to the engine. And since no energy is added between the plenum and the cylinder, how can the gas be at a higher energy state (higher pressure) in the cylinder?

Should I expect some cash flow from your high concentration (I've just made you millions ) to my low concentration?

LOL - Not quite yet!

[scalbert]

Originally Posted by Klepper1

how can the gas be at a higher energy state (higher pressure) in the cylinder?

When the intake valve closes and the piston rises.