Piper Navajo PA-31 Service Manual page 478

Density Controller
NOTE
The density controller is designed to keep the air density constant at the
injector entrance. As ambient air temperature increases or density de-
creases due to change in altitude a higher manifold pressure is required to
maintain a constant density, also resulting in a higher injector inlet temp-
erature. 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.
PIPER NAVAJO SERVICE MANUAL
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 controller react to pressure and temp-
erature changes between the fuel injector inlet and the turbocharger com-
pressor. 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.
Differential Pressure
Controller
Ground Boosted or
Ground Turbocharged
Deck Pressure
Manifold Pressure
Normalizing
This controller uses a diaphragm rather than a bellows as is found in 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 differential 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.
These phrases indicate that the engine depends on a certain amount of
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.
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 de-
creased air pressure of high altitude, it is considered that the engine has
been "normalized".
Reissued: 10/12/79
214 POWER PLANT