I am not sure the stock 89-92 turbo is inadequate for the job. Mecanair,
I think, and others on here agree. I and others using it will find out.
Hugh MacInnes talks about this subject of POV's in his book "Turbochargers"
starting on page 71. As I read it the main drawback is; it does not
limit turbine RPM. Now since Greg is using a TO4 that may not be a
problem for him as the turbine housing can be made large enough
to limit turbine RPM. I on the other hand am worried about turbine RPM
because of the small size of the stock unit.
BTW the stock unit has a built in waste gate the passage of which are
rather
convoluted so I don't expect a lot of noise when the gate is open.
Also the bearings are water cooled.
Another trick that either Paul Yaw or Jeff Spitzer came up with, I think,
is to merely drill a small hole in the turbine housing to permanently dump
a percentage of exhaust gas directly into the tail pipe to limit RPM
and boost with a small turbine. This is a lighter weight solution than
using
a larger diameter turbine.
Paul Lamar
George Lendich wrote:
Paul,
The recent article in kitplanes (haven't got it to hand, going by memory)
suggests a simple solution is - a large Turbo housing with a much smaller
turbo wheel, which is only effective in the higher RPM range. It would
eliminate the need Wastegates and POV's (it would seem) - i'm not sure
how this would go for cost however - any comments !?
George (down under)
According to Hugh Macinnes that would be what is known as a free floating
turbo charger and as such it would only give about 7 percent more HP at
10,000 and 10% more at 12,000 feet assuming the same HP at sea level
as a normally aspirated engine.
On the other hand a turbo engine set up to be flat rated would
give the same HP at all altitudes up to about 14,000 feet (called
the critical altitude).
Putting it another way the NA engine would be about 25% down
on HP at 10,000 feet, the free floating turbo engine down about
18% and the flat rated turbo engine would not be down at all and would
have the same HP up to the critical altitude.
The large turbine housing is chosen to reduce the possibility of the
the turbo charger over speeding at high altitude. True... it does give
less boost at sea level if a waste gate IS NOT used.
The main purpose of a turbo charger in an airplane is to give lots of
HP at high altitude where the air density is lower and hence the
aerodynamic drag is lower so the airplane will fly faster with the
same HP it has at sea level.
The consensus is the waste gate is the way to go for an aircraft engine.
Unfortunately it adds cost and weight to those turbo's like the TO4
that do not have a built in waste gate. Greg Richter is out there on
the cutting edge relying on just an intake manifold pop off valve.
IMHO the 89-92 stock Mazda gen II turbo with built in waste gate
and water cooled bearing housing is a real bargain in a turbo aircraft engine.
We do need to explore its limits however and perhaps reduce its
RPM at high altitude by maybe drilling a small hole in the turbine housing
between the high pressure part and the tail pipe. This is a compromise of
course and no doubt this will reduce the critical altitude where the
engine is no longer flat rated.
Paul Lamar
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