Non-Compression Of Air (Unloaded); Section 1: For Standard Compressor. See Figure; Section 2: For Closed Room Compressor. See Figure - BENDIX BA-921 COMPRESSOR-STD-CLOSED ROOM Service Data Sheet

Air Inlet Port
Cooling
Plate
Valve
Plate
Air in Pistons Shuttles Back and Forth from the
Piston to the Cylinder Head and Inlet Port During
Unloaded Mode
FIGURE 8 - OPERATION - UNLOADED (STANDARD)

NON-COMPRESSION OF AIR (UNLOADED)

Section 1: For Standard Compressor. See Figure 8.

When air pressure in the supply reservoir reaches the
cutout setting of the governor, the governor delivers
system air to the compressor unloader port. Air entering
the unloader port acts on the unloader piston causing the
piston to move upwards, away from its seat on the valve
plate assembly. When the unloader piston is unseated
an air passageway is opened between the cylinder bore
and the air inlet cavity in the cylinder head.
As the piston moves from bottom dead center (BDC) to
top dead center (TDC) air in the cylinder bore fl ows past
the unseated unloader piston, into the cylinder head inlet
cavity and out the inlet port. To prevent the air from fl owing
back into the engine air induction system, an inlet check
valve (ICV) is installed upstream of the air compressor inlet
port. The location of the device and the way it is plumbed
into the compressor air induction system is unique to the
specifi c engine and the type of air induction (naturally
aspirated or boosted air) the compressor uses. These air
induction systems will be explained in further detail in the
"Air Induction" section on page 4. On the piston down
stroke (TDC to BDC) air fl ows in the reverse direction, from
the inlet cavity past the unseated unloader piston and inlet
reed valve, and into the cylinder bore.
4
Air From
Governor
Unloader
Port
Unloader
Piston Up &
Unseated
Air in Pistons Shuttles Back and Forth from the
FIGURE 9 - OPERATION - UNLOADED (CLOSED ROOM)

Section 2: For Closed Room Compressor. See Figure 9.

When air pressure in the supply reservoir reaches the
cutout setting of the governor, the governor delivers
system air to the compressor unloader port. Air entering
the unloader port acts on the unloader piston causing
the piston to move away from its seat on the valve plate
assembly. When the unloader piston is unseated, an air
passageway is opened between the cylinder bore and a
secondary compartment or "closed room" in the interior of
the cylinder head.
As the piston moves from bottom dead center (BDC) to top
dead center (TDC) air in the cylinder bore fl ows past the
unseated unloader piston, into the "closed room". The size
of the closed room is suffi cient to accept the compressed
air provided by the compressor piston without creating
excessive air pressure in the "closed room". On the piston
down stroke (TDC to BDC) air fl ows in the reverse direction,
from the "closed room" past the unseated unloader piston
and inlet reed valve, and into the cylinder bore. Note: For
optimum performance, it is recommended that the air dryer
is equipped with "turbo cut-off".
Piston to the Closed Room
Air From
Governor
Unloader
Port
Closed
Room
Unloader
Piston Up &
Unseated