Paul,
I can't remember what the A/R ratio stands for, I think it is the
ratio of the inlet tube area, to the turbo area.
Can you clarify that point for those of us that don't quite understand.
George (down under)
"* Turbine A/R - Turbine performance [HP] is greatly affected by
changing the A/R of the housing. Using a smaller A/R will
increase the exhaust gas velocity into the turbine wheel,
causing the wheel to spin faster at lower engine RPMs giving
a quicker boost rise. This will also tend to increase exhaust
back pressure and reduce the max power at high RPM. Conversely,
using a larger A/R will lower exhaust gas velocity, and delay
boost rise, but the lower back pressure will give better high RPM power."
Somebody on the Internet :)
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Basically what you have for a given size turbo charger is a fixed
R or radius. Changing A (area) changes the velocity of the exhaust gases
impinging on the turbine blades. Of course this puts back pressure on the
engine and the harder it must work to keep up the flow.
Heating the air increases its pressure and therefore its velocity with
a given pipe size. That is why it is important to retain the heat in
the exhaust gas before feeding it to the turbine. Exhaust gas can approach
super sonic velocities depending the size of the exhaust pipe. The combination
of heat and high exhaust gas velocities add up to tremendous potential
energy in the exhaust. Close to 250 HP in a 200 mechanical net HP rotary engine.
In the case of the rotary we have too much heat so we must be careful
not to run the turbine too fast as the blades are weakened by the high
heat and might fly off. Centrifugal force on the blade = M x V^2/R... M is
the mass of the blade.
The force imparted to the turbine blades per square foot by the exhaust
gas is V^2 x .0026. The .0026 constant is a function of the gas density.
V of course is the velocity in MPH. V^2 means V squared.
Think of the scroll as a fire hose nozzle. The smaller the area of the hole
in the nozzle the faster the water comes out. If that nozzle is pointed
at a water wheel the faster the water hitting the wheel the faster the wheel
will rotate up to a point. The faster the wheel rotates at a given torque
the higher the HP. All turbines have peak torque when they are not rotating.
Without rotation there is no HP is generated. That is when the velocity of
the gas striking the blade is a maximum. As the turbine speeds up the relative
velocity of the gas striking the turbine blade is reduced until the blade
is moving at the speed of the gas. In that case there is no torque so
the HP becomes zero again. See the attached chart.
Paul Lamar ...No rotor no motor.
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"VNT technology ? variable nozzle technology ? will be as beneficial
to gasoline as diesel. The hot side of the turbocharger, the turbine,
is where we try to obstruct [extract] energy out of the hot exhaust flow. At
low engine speeds, where there is relatively little flow, little
temperature, we locally accelerate the flow and there is better kinetic
transfer of kinetic energy from the gas to the wheel. I like to compare
it to a garden hose: if you have the hose further away, you squeeze the
hose. VNT does miracles; it brings available torque down to almost idle
speed. In the old days, you had to rev up your engine ? gas or diesel ?
up to 2,000 rpm. Today you see full boost at about 1,500 rpm."
By Garrett?s Martin Verschoor
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Good article on understanding turbo charts.
Paul Lamar ...No rotor no motor.
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