Locomotives need an air compressor to operate
the air brake systems, horn, bell, and in older locomotives, the "control"
air system. Control air is used to operate the reverser, cam switch (in
dynamic brake equipped locomotives) and power contactors. The steam
boiler uses main reservoir air and has its own pressure regulator system.
The compressor has 3 cylinders, two low pressure
and one high pressure. Air is taken in by a low pressure cylinder's piston's
downward stroke. When the piston returns upwards, it compresses the air,
circulates it through the intercooler and into the high pressure cylinder
which compresses it further and finally, exhausts it into the main reservoirs.
The intercooler is nothing more than a radiator
connected between the low and high pressure cylinders. In an air cooled
compressor, the intercooler would have fins for air circulation, exactly
like an automobile radiator. In this water cooled version, the intercooler
has a water jacket and does not rely on air cooling. S.P. was a big believer
in water cooled compressors as they are much less prone to failure due
to their cooler running nature. They are, however, subject to freezing
ans one does not run antifreeze in the cooling system of a EMD engine.
This the the high pressure cylinder and the exhaust
line to the main reservoirs. The small line connected to the center of
the head is the unloader line. The unloader system connects to all 3 heads
and forces the intake valves open at the proper pressure in order to stop
the compressor from pumping. The brass thing on top of the intercooler
is a 35 pound safety valve used to protect the intercooler from excessive
pressure. The air system has a safety valve set at 150 pounds, mounted
near the main reservoirs.
This view from the fireman's side shows a low
pressure cylinder and its connection to the intercooler. You can also see
the cooling water line running between the high pressure head and this
low pressure side. The compressor air intake and air filter are to the
right, out of view.