Piper ARROW III, Service Manual

Page: 303 / 594

Density Controller The density controller is designed to allow the engine to develop full
rated power no matter what ambient temperature and pressure
conditions are. This controller regulates wastegate bleed oil only at
full throttle position to maintain a constant air density at the injector
inlet. The pressure and temperature sensing bellows of the con troller
react to pressure and temperature changes between the fuel injector
inlet and the turbocharger compressor. The bellows, filled with dry
nitrogen gas, maintains a constant density by allowing the pressure to
increase as temperature increases. Movement of the bellows re-
positions the bleed valve, causing a change in quantity of bleed oil,
which changes oil pressure to the wastegate piston.
NOTE
The density controller is designed to keep the air density constant at the
injector entrance. As ambient air temperature increases or density decreases
due to change in altitude a higher manifold pressure is required to maintain
a constant density, also resulting in a higher injector inlet temperature. This
is why wide open throttle manifold pressure increases with either altitude or
outside air temperature. In a full throttle climb, a gain of 3 to 4 inches of
manifold pressure between sea level and critical altitude will be seen.
Differential Pressure This controller uses a diaphragm rather than a bellows as is found in
Controller the density controller. It is used in conjunction with the density
controller. Its function is to override the density controller so that the
compressor discharge pressure is not held at an unnecessarily high
level when lower manifold pressure is being used. The diff e r e n t i a l
controller will usually maintain a compressor discharge pressure
(deck pressure) approximately 6.5” Hg above the selected manifold
pressure. In this system, the density controller is only effective at
wide open engine throttle conditions.
Ground Boosted or These phrases indicate that the engine depends on a certain amount of
Ground Turbocharged turbocharging at sea level to produce the advertised horsepower. An
engine that is so designed will usually include a lower compression
ratio to avoid detonation.
Deck Pressure The pressure measured in the area downstream of the turb compressor
discharge and upstream of the engine throttle valve. This should not
be confused with manifold pressure.
Manifold Pressure The pressure measured downstream of the engine throttle valve and is
almost directly proportioned to the engine power output.
Normalizing 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”.
Added: 5/1/80
2B1
CHEROKEE ARROW III SERVICE MANUAL
POWER PLANT - CONTINENTAL