The main advantage of the side exhaust ports occurs down in the
idle and low power ranges of the engines. Mostly emissions.

Paul Lamar

I did a few more calculations on the stock Mazda turbo.
Looks like the turbine wheel is about 3 inches in diameter.
Lets use Francois's number for turbine RPM.
At 130,000 RPM or 2200 RPS the tips are going 21,000 inches per second.
1700 feet per second. Area of the scroll at a radius of 3.5 inches is
about 2.8 square inches. A/R = .8. Two, 2 inch exhaust pipes would have an

area

of 6.28 square inches total. The gas velocity in the scroll would then
be about 2.2 times the exit velocity from the 2 inch exhaust port.

According to Scientific Design of Exhaust and Intake Systems exhaust gas

velocity

is on the order of 200 to 300 feet per second from a typical four cycle

piston engine.

So exhaust gas velocity in the turbo scroll would then be 450 to 670 feet

per second.

Obviously the exhaust gas velocity in the rotary is two
or three times that number or one would get no torque out of the turbine.

Not in the ball park.

Another possibility is I am off on the diameter of the turbine wheel.
Lets see, a T3 wheel can be 2.319 or 2.559 inches in diameter. Using the

2.559

number that would bring the tip velocity down to 1470 feet per second.

Still not in the ball park.

Vance this is right down your alley. What say you? Where am I going wrong?
Is the exducer diameter the controlling factor?

How do I figure the exhaust gas velocity knowing the cubic feet of air
per minute going into the engine? (P1 x V1)/T1 = (P2 x V2)/T2 ?

Paul Lamar

Exhaust turbines are not free power. In order for the turbine to be
useful you need a high "spouting" velocity. The 1700  fps
sounds relatively reasonable since this would be well
below sonic for this temp and molecular weight. Gas turbine initial
nozzles are undoubtedly supersonic which requires a presssure
ratio above critical (abut 2.5) At sea level this would be a back
pressure about 20psig (not good for the engine). As you go up
in altitude this back pressure for sonic flow becomes more
rational, and turbine power is greater, and turbo compounding
begines to be worth the complexity and weight. This is just
one more reason why turbo compounding has no place in an
automobile application. Exhaust volume, as you seemed
to indicate, is a classic PV/WRT case using total temps (K 0r Rankine)
And absolute pressures. The voumetric flows you are using have to
reflect power levels and not just swept volumr. Vance

 
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