Tecnico Bachiller

BENDIX® BA-921® COMPRESSOR
DESCRIPTION
The function of the air compressor is to provide and maintain air under pressure to operate devices in air brake systems. The Bendix® BA-921® compressor is a single-cylinder reciprocating compressor with a rated displacement of 15.8 cubic feet per minute at 1250 RPM. The compressor consists of a water-cooled cylinder head assembly and an integralair-cooled crankcase assembly. The cylinder head assembly is made up of the cylinder head, cooling plate and valve plate assembly and uses two sealing gaskets. Depending on the application, the cylinder head and cooling plate may be aluminum or cast iron. The cylinder head contains air and water ports as well as an unloader assembly. A cooling plate is located between the cylinder head and valve plateassemblies and assists in cooling. The valve plate assembly consists of brazed steel plates which have valve openings and passages for air and engine coolant to flow into and out of the cylinder head. The compressor's discharge valves are part of the valve plate assembly. The inlet reed valve/gasket is installed between the valve plate assembly and the top of the crankcase.
Splash Shield UnloaderCover Unloader Piston Discharge Reed Valves (2) Piston Air Intake Cooling Plate Crankshaft Coolant Ports (3 total) Crankcase Cylinder Head Valve Plate Assembly Cooling Plate Safety Valve

FIGURE 2 - BENDIX® BA-921® COMPRESSOR

The cast iron crankcase houses the piston assembly, connecting rod, crankshaft and related bearings. All Bendix® BA-921® compressors are equipped with a safety valve toprotect the compressor head in the event of, for example, a discharge line blockage downstream of the compressor. Excessive air pressure will cause the safety valve to unseat, release air pressure and give an audible alert to the operator. The safety valve is installed in the cylinder head safety valve port, directly connected to the cylinder head discharge port. The crankcase cover located at thebottom of the crankcase is stamped with information identifying the compressor model, customer piece number, Bendix piece number and serial number. See Figures 2 and 3.
Compressor Model, Customer Piece Number, Bendix Piece Number and Serial Number shown here FIGURE 3 - CRANKCASE BASE COVER

Piston Rod Crankcase Cover FIGURE 1 - BENDIX® BA-921® COMPRESSOR (CUT-AWAY)

SD-01-690

Caterpillar C7,C9 Engines

Caterpillar HD Acert Engines

Detroit Diesel S60

FIGURE 4 - TYPICAL COMPRESSOR DRIVE FLANGES

piston to move upwards, away from its seat on the valve plate assembly. When the unloader piston is unseated an
Air Inlet Port Discharge Valve Closed

OPERATION
The compressor is driven by the vehicle's engine and functions continuously while the engine is in operation. Actualcompression of air is controlled by the compressor unloading mechanism operating in conjunction with a governor.
Air Discharge Port Unloader Port

AIR INTAKE (LOADED)
Just as the piston begins the down stroke, (a position known as top dead center, or TDC), the vacuum created in the cylinder bore above the piston causes the inlet reed valve to flex open. Atmospheric air (in naturally aspiratedapplications) or pressurized air (in turbocharged applications) flows through the open inlet valve and fills the cylinder bore above the piston. See Figure 5.

Cooling Plate Valve Plate

Unloader Piston Down & Seated Inlet Valve Open

AIR COMPRESSION (LOADED)
When the piston reaches the bottom of the stroke, (a position known as bottom dead center, or BDC), the inlet reed valve closes. Air abovethe piston is trapped by the closed inlet reed valve and is compressed as the piston moves upwards. When air in the cylinder bore reaches a pressure greater than that of the system pressure, the discharge reed valves open and allow air to flow into the discharge line and air brake system. At the same time air flows into the hollow center of the unloader piston through an opening in the end of the...