Wanted to discuss Exhaust Thermodynamics (wisemen welcomed)....

[suicidal2af]

You guys need to stop making comparisons between exhaust flow and water flow. Water is a virtualy incompressible fluid, meaning density does not change with volume. In this case mass flow is only dependant upon two factors, velocity and cross-sectional area where density remains constant.

Gas on the other hand is a compressible substance, meaning density will change with volume. Therefore mass flow is dependant upon density, velocity, and cross-sectional area. Maximizing mass flow in this case involves opitimizing the three different variables. Two large of a cross-sectional area will result in decreased density and velocity and may ultimately decrease mass flow.

Water may be incompressible, but gasses, much like water, are a fluid. Water can be under pressure also, and exhaust backpressure isn't compressing air -- it's putting it under pressure.

Therefore, water is a perfectly valid comparison.

 

[dynatos]

would the rule of high pressure always finding a median with low pressure be part of this situation too? If you are moving exhaust gas out that is hot, the air has a higher pressure after the turbine then the air that is cold(er) and dense at the tail pipe. Would this help the air move from the turbo to the tailpipe thus increasing velocity?

It seems this is just a big balancing act.. either too small and your velocity is great but mass air flow is low.. or too large and velocity sucks but mass air flow is good. If my above statement is true, i suppose that would mean the bigger the better -