air pressure in an exhaust manifold behind a turbocharger.

[fireslave]

i need a rough idea of what the air pressure would be behind a turbo making about 7psi of boost.

turbo would be t25-ish in size and would be serving about 1.7 liters of engine

i only need a very rough idea... if you say, for example, its 10-15psi above atmospheric than that's close enough for me...

any ideas?

 

[BrianPetersen]

Are you talking about the pressure in the exhaust manifold *ahead* of the exhaust turbine (between the engine and the turbine)?

In other words, the exhaust manifold pressure?

It will be extremely variable during the cycle due to the pulsations from each exhaust stroke, but a first-approximation "guesstimate" that the exhaust manifold back pressure is between 1.0 and 1.5 times the intake manifold boost pressure (relative to barometric pressure). A more accurate estimation would require knowledge of the performance map of both the compressor and the exhaust turbine, and to some extent the characteristics of the engine and the manifold.

Very efficient turbochargers can achieve exhaust manifold pressure lower than intake manifold pressure (does not violate thermodynamics, because the volume flow rate is far higher due to the higher temperature) but it is unlikely and normally doesn't happen throughout the engine's operating range.

 

[fireslave]

yes, that is exactly what I am talking about. i had heard before that some turbochargers can operate with lower pressure in the exhaust manifold than in the intake, but that it would only be in a certain band of engine performance where such optimal things happened.


but lets say, for example, the turbo is sized to make 7psi of boost. are there any events where the exhaust manifold would spike to 20psi for example? or would it stay within a small range for all its operation? would it only happen when the turbo is spooling from a very low speed?


also, when you say 1 to 1.5x do you mean 10psi of boost = 10-15 psi above ambient in the exhaust manifold, or do you mean 24.7 psi absolute in the intake manifold and 24.7 psi to 37 psi absolute in the exhaust manifold... because that ends up being like 22 above ambient...

i would guess relative because that's how turbo speak goes, but mathematical formulas don't always work that way...

 

[BrianPetersen]

I am talking relative to ambient because that's what the compressor has to work against and that's what the exhaust turbine has available to it.

Spikes well above this will occur every time an exhaust valve opens. It will decay to well below this just before the next exhaust valve opens depending on the volume of the manifold and the number of cylinders feeding the turbo. The average pressure (above ambient) will vary widely depending on engine operation but can be expected to remain in the 1.0 - 1.5 x intake manifold pressure(above ambient).

If you have an unduly restricted exhaust downstream of the turbo or an unduly restricted air intake, or mount the turbo far from the engine, all bets are off.

 

[Lou Scannon]

I would expect that with any modern efficient turbo that is decently matched to the engine there will always be part of the operating range where the intake to exhaust pressure differential is favorable. I say that because every engine that I have instrumented or modeled has demonstrated this.