500 General Compressed air is not only used on a diesel-locomotive for operating the air brakes and sanders but is also essential for the proper operation of many other items. The reverser switch, main power contactors, shutter operating cylinder, air throttle, horn, bell and windshield wipers are also air operated. Some of the items mentioned are in reality electro-pneumatically operated. This merely means that in such cases the flow of the air (as through a valve) is controlled by electrical circuits.
501 Air Compressor Each locomotive power plant is basically equipped with a 3 cylinder,two stage air Compressor. The air compressor is driven by an extension shaft through a flexible coupling, from the front end of engine. The compressor has its own oil pump and pressure lubricating oil system. With the engine stopped the oil level in the compressor crankcase can be checked on the bayonet type gauge. Lubricating oil pressure should be from 5 to 8 Pounds at idle speed (275 RPM).
The compressor consists of two low pressure cylinders and one high pressure cylinder. The pistons of all three cylinders are driven by a common crankshaft. The two low pressure cylinders are set at an angle to the vertical high pressure cylinder. Air from the low pressure cylinder goes to an intercooler, or radiator to be cooled before entering the high pressure cylinder. The intercooler is provided with a pressure gauge and relief val@e. The gauge normally reads approximately 32 pounds when the compressor is loaded. The intercooler relief valve is set for 50 pounds. Any marked deviation of intercooler pressure from 32 pounds should be reported at the maintenance terminal for attention.
Condensation and oil collects in the sump of the bottom header of the compressor intercooler and should be drained once at each crew change and at the regular maintenance period. A drain valve is provided in each sump for this purpose. Operate the intercooler safety valve by hand, when draining intercooler, to be certain that it functions properly.
Since the air compressor is directly connected to the engine and is in operation at all times when the engine is running, an unloader is provided in the hea of both high and low pressure cylinders which cuts out the compressing action when actuated by air pressure. The unloader accomplishes this by blocking open the suction or intake valves of the high and low pressure cylinders. When the air operating the unloader is cut off, the unloader releases the suction valves and the compressor resumes pumping. Main reservoir air pressure actuates the unloader valves. The air compressor governor is mechanically operated.
502 Draining Of Air System The air system should be drained periodically to prevent moisture from being carried into the air brake and electrical control air systems. The frequency of draining win depend on local conditions and can be determined by practice. It is recommended that draining be done at the time of each crew change, until a definite schedule can be determined by the individual railroad.
