Piper SEMINOLE PA-44-180, Maintenance Manual

Page: 762 / 881

TURBOCHARGER NOMENCLATURE
TERM MEANING
Absolute Pressure Pressure measured above a vacuum rather than above ambient pressure.
Bootstrapping 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 if for 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 is similar to the example mentioned
above, except now there is an engine placed between the compressor discharge
and turbine inlet. Slight system changes cause 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.
Centrifugal Acting in a direction away from a center or axis, as in the centrifugal, radial
outflow of air in the turbocharger compressor.
Centripetal Acting in a direction toward a center or axis, as in the centripetal, radial inflow
of air in the turbocharger turbine.
Coking Common term for the process in which oil is carbonized by overheating.
Compressor That portion of the turbocharger which directs ambient air to the compressor
wheel which compresses the air and discharges it to the engine throttle for
mixing with fuel.
Critical Altitude Altitude at which the engine can no longer maintain rated horsepower.
Deck Pressure Pressure measured between the compressor discharge and the engine throttle
valve (not to be confused with “manifold pressure”.
Exhaust Bypass Valve Valve in the exhaust system which can be adjusted to bypass some of the
exhaust gas away from the turbocharger turbine thus reducing turbocharger
speed and the degree of compression of air.
Intake Manifold That part of the engine air intake system downstream of the throttle valve.
Manifold Pressure Pressure measured downstream of the engine throttle valve. This pressure is
almost directly proportional to engine power output.
Overboost Condition detrimental to engine life and performance in which manifold
pressure exceeds the limits at which the engine was tested and FAA certified.
PA - 4 4 - 1 8 0 / 1 8 0 T
AIRPLANE MAINTENANCE MANUAL
8 1 - 0 3 - 0 0
Page 81-06
Revised: May 15, 1989
3J8
PIPER AIRCRAFT