Piper Comanche PA-24 Service Manual page 576

Ground Boosted or
Ground Turbocharged
Deck Pressure
Manifold Pressure
Normalizing
Overboost
Overshoot
Bootstrapping
Critical Altitude
ADDED: 4/10/81
PIPER COMANCHE SERVICE MANUAL
These phrases indicate that the engine depends on a certain
amou~
turbocharging at sea level to produce the advertised horsepower. An e
that is so designed will usually include a lower compression ratio to av
detonation.
The pressure measured in the area downstream of the turbo compressor
discharge and upstream of the engine throttle valve. This should not be
confused with manifold pressure.
The pressure measured downstream of the engine throttle valve and is almost
directly proportioned to the engine power output.
If a turbocharger system is used only to regain power losses caused by
decreased air pressure of high altitude, it is considered that the engine has been
"normalized".
An overboost condition means that manifold pressure is exceeding the limits at
which the engine was tested and FAA certified and can be detrimental to the
life and performance of the engine. Overboost can be caused by
malfunctioning controllers or improperly operating wastegate in the
automatic system or by pilot error in a manual controlled system. Refer to
latest copy of Lycoming Service Bulletin No. 369.
Overshoot is a condition of the automatic controls not having the ability to
respond quickly enough to check the inertia of the turbocharger speed increase
with rapid engine throttle advance. Overshoot differs from overboost in that
the high manifold pressure lasts only for a few seconds. This conditio.n
usually be overcome by smooth throttle advance. A good metho
advancing the throttle is a follows. After allowing the engine oil to warm
approximately 140 0 F. advance the throllie to 28" to 30" manifold pressure.
hesitate I to 3 seconds and continue advancing to fullthrollie slow and easy.
This will eliminate any overshoot due to turbocharger inertia.
This is a term used in conjunction with turbo machinery. If you were
to
take all
the air coming from a turbocharger compressor and duct it directly back into
the turbine of the turbocharger, it would be called a bootstrap system and if no
losses were encountered, it would theoretically run continuously. It would also
be very unstable because iffor some reason the turbo speed would change,the
compressor would pump more air to drive the turbine faster, etc. A turbo-
charged engine above critical altitude (wastegate closed) is similar
to
the
example mentioned above, except now there is an engine placed between the
compressor discharge and turbine inlet. Slight system changes caused the
exhaust gas to change slightly, which causes the turbine speed to change
slightly, which causes the compressor air to the engine
to
change slightly, which
in turn again affects the exhaust gas, etc.
A turbocharged position engine's wastegate will be in a partially open position
at sea level. As the aircraft is flown to higher altitude (lower ambient pressures)
the wastegate closes gradually to maintain the preselected manifold pressure.
At the point where the wastegate reaches its full closed position, the preselected
manifold pressure will start to drop and this is considered critical altitude.
2G20