Compressor And The Air Brake System; Duty Cycle; Compressor Installation; Discharge Line - BENDIX DURAFLO 596 COMPRESSOR Manual

DISCHARGE
VALVE
DISCHARGE PORTS
CYLINDER HEAD SIDE OF VALVE BODY ASSY.
DISCHARGE SLIDING INLET
PORTS
COMPRESSOR LOADED
CYLINDER BLOCK SIDE OF VALVE BODY ASSY.
UNLOADER
PLUNGER
UNLOADER
PORT
UNLOADER AIR
PASSAGE
PIVOT
ARM
SLIDING
INLET VALVE
FIGURE 7 - UNLOADER MECHANISM
COMPRESSOR & THE AIR BRAKE SYSTEM
GENERAL
The compressor is part of the total air brake system, more
specifically, the charging portion of the air brake system. As
a component in the overall system its condition, duty cycle,
proper installation and operation will directly affect other
components in the system.
Powered by the vehicle engine, the air compressor builds
the air pressure for the air brake system. The air compres-
sor is typically cooled by the engine coolant system,
lubricated by the engine oil supply and has its inlet con-
nected to the engine induction system.
As the atmospheric air is compressed, all the water vapor
originally in the air is carried along into the air system, as
well as a small amount of the lubricating oil as vapor. If an
4
INLET PORTS
INLET
VALVES PORTS
PIVOT
ARMS
COMPRESSOR UNLOADED
GUIDE PIN
air dryer is not used to remove these contaminants prior to
entering the air system, the majority, but not all, will con-
dense in the reservoirs. The quantity of contaminants that
reach the air system depends on several factors including
installation, maintenance and contaminant handling devices
in the system. These contaminants must either be elimi-
nated prior to entering the air system or after they enter.

DUTY CYCLE

The duty cycle is the ratio of time the compressor spends
building air to the total engine running time. Air compres-
sors are designed to build air (run "loaded") up to 25% of the
time. Higher duty cycles cause conditions that affect air
brake charging system performance which may require ad-
ditional maintenance. Factors that add to the duty cycle
are: air suspension, additional air accessories, use of an
undersized compressor, frequent stops, excessive leakage
from fittings, connections, lines, chambers or valves, etc.
Refer to Table A in the Troubleshooting section for a guide to
various duty cycles and the consideration that must be given
to maintenance of other components.

COMPRESSOR INSTALLATION

While the original compressor installation is usually com-
pleted by the vehicle manufacturer, conditions of operation
and maintenance may require additional consideration. The
following presents base guidelines.

DISCHARGE LINE

The discharge line allows the air, water-vapor and oil-vapor
mixture to cool between the compressor and air dryer or
reservoir. The typical size of a vehicle's discharge line, (see
column 2 of Table A in the Troubleshooting section) assumes
a compressor with a normal (less than 25%) duty cycle,
operating in a temperate climate. See Bendix and/or other
air dryer manufacturer guidelines as needed.
The discharge line must maintain a constant slope down
from the compressor to the air dryer inlet fitting or reservoir
to avoid low points where ice may form and block the flow. If,
instead, ice blockages occur at the air dryer or reservoir
inlet, insulation may be added here, or if the inlet fitting is a
typical 90 degree fitting, it may be changed to a straight or
45 degree fitting. Shorter discharge line lengths or insula-
tion may be required in cold climates.
While not all compressors and charging systems are
equipped with a discharge line safety valve this component
is recommended. The discharge line safety valve is installed
in the cylinder head or close to the compressor discharge
port and protects against over pressurizing the compressor
in the event of a discharge line freezeup.