12
Half the fuel energy comes out of the exhaust at 8,000 RPM as a blast
of exhaust gas. The speed of the exhaust gas is roughly 1000 MPH.
This impinges on the blades of the turbine spinning it up to 125,000 RPM.
The blades heat to 1800 F. This weakens them and they can fail from
centrifugal forces. The diameter of the turbo GT40 exhaust intake is roughly
3.5 inches. Since the rotary is positive displacement if you make the
turbine inlet smaller or the same as making the diameter of the
exhaust manifold smaller that will cause the speed the gases to increase
and the RPM of the turbine to increase.
A/r is the area of the turbine scroll divided by the radius of the blades.
For a given A a smaller radius blade would give a higher turbine RPM.
Centrifugal load is the speed of the blade squared so it is a good idea to keep
the radius large and the RPM down. Therefor an A/r approaching one is better
for our application.
Unlike in cars we don't give a damn about turbo charger acceleration.
In cars.
"Turbine performance is greatly affected by changing the A/R of the housing, as it is
used to adjust the flow capacity of the turbine. Using a smaller A/R will increase
the exhaust gas velocity into the turbine wheel. This provides increased turbine
power at lower engine speeds, resulting in a quicker boost rise. However, a small A/R
also causes the flow to enter the wheel more tangentially, which reduces the ultimate
flow capacity of the turbine wheel. This will tend to increase exhaust back pressure.
Likewise, this will reduce the engine’s ability to “breathe” effectively at a high
RPM, adversely affecting peak engine power."
"Conversely, using a larger A/R turbo housing will lower exhaust gas velocity, and
delay boost rise. The flow in a larger A/R turbo housing enters the wheel in a more
radial fashion. This will increase the wheel’s effective flow capacity, resulting in
lower back pressure and better power at higher engine speeds."
Paul Lamar
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