Since you asked, Devil's advocate here.  P-)
After thinking about the no waste gate layout Greg used I've

developed

an

appreciation for it.  This comes from some failure mode analysis

(strictly

armchair stuff).  OTOH Continental Lycoming, et al, have went
with

waste

gates most likely to reduce their liability, not increase it.
The

round

engine drivers of old had to watch the MP and know what kind of
fuel

they

took on last so they could set the throttle properly for take
off.

Flying

by MP was part of the job and from the perspective of this old

piston

(ahem) head, it keeps you in tune with your engine's
performance.

You

may

have already considered the following.

Waste gate = added complexity and failure modes:
Does it fail open?  That is if you are on takeoff at Leadville,
CO,

on a

hot August afternoon at full gross, will you get out of ground

effect

and

over the obstacles at the end of the runway?
Does it fail closed?  If you are not in the habit of watching
your

MP as

you firewall (nose cone wall??) the throttle while departing
Copper

Hill

International in the winter, will the engine survive?

Like I said, armchair analysis, I'm going NA 'cause a turbo
won't

fit in

a

RV-4.   P-)
Mike McGee
Control Systems Engineer

Mike McGee, RV-4 N996RV, O320-E2G, Hillsboro, OR
13B in gestation mode

Well, this is my take....
Most piston turbo'd engines have the oil pressure controlled
wastegates (usually controlled by a "differential controller" to
maintain a specific boost or sl pressure), spring loaded, AND a very

nice safety feature called a blowoff valve (to limit overboost).
Most

tc cars also have "blow off valves". Most aircraft failures are due
to

grit in the oil (sticks in the oil fed actuators, which is why you
run

the oil to the stuff after the filter!) or lack of maintenance
(mouse milk on Wastegate every so often!). I'm still waiting for
some electronic genius besides the people at GAMI (who have done
this electronic boost control for piston engine certification as
part of their electronic knock protected fi/ign product!) to come up

with an appropriate set up having the same features that eliminate
the oil stuff and use the same components electronically (again like

cars). This group might have some able person do make such a
product? Last I heard that might be in the making. BTW, Gami's set
up measures each pistons internal rising pressure, and does not
sense knock as noise! What a novel thought. [Marc Wiese]

I'm sure I'm over simplifying the requirements but couldn't the
wastegate be controlled  with a manifold pressure sensor in the intake

after the turbo and intercooler. This would give the pressure the
engine is seeing. This could control a solenoid that acts against a
spring loaded (to open)wastegate. I see the failure mode of the
wastegate going open but you would still have some power. [Stan
Blanton]

The stock turbo waste gate is controlled by a manifold pressure sensor.
The trouble is it is 5 psi above ambient pressure which is not what
you want at high altitude. You want more boost or constant intake
manifold pressure at altitude to maintain constant power.

I like Russell's idea of controling the pressure and boost with the
intake manifold temp. That is the key to eliminate detonation and
maximize power under all conditions. About 130 F max is about right.

Paul Lamar

To maintain an absolute manifold pressure could two sensors be used? One
would sense the atmospheric pressure and the other the manifold pressure
the engine is seeing. A comparison between the two with a lookup table
for extra boost needed? [Stan Blanton]

Another good reason to use a microcomputer to control the boost.

Paul Lamar
 
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