TM 9-4310-394-13
Section III. PRINCIPLES OF OPERATION
1-12. THEORY OF OPERATIONS. The ACU is a self contained, transportable, diesel engine driven centrifugal air
compressor designed for compressing air and delivering it as needed. It consists of a two stage reciprocating air
compressor, a four stroke air cooled diesel engine, and a chassis for the mounting of the ACU components and to
provide a means of transporting the ACU. The ACU performs its operations in the following manner.
a. Air Compressor. The air compressor operates to compress air through the use of a two stage compressing
system. This two stage system consists of two complete piston assemblies which are mounted internally to the
compressor crankshaft.
As the compressor crankshaft rotates, the first stage (low pressure) piston moves downward to draw outside air. This air
is passed through a filter, through a set of valve plates, and then into the cylinder area of the first stage piston. As the
compressor crankshaft continues to rotate, the first stage piston moves upward to compress and push out this air through
the valve plates and into an intercooler tube which transports and cools the partially compressed air into the cylinder
head of the second stage piston.
This second stage (high pressure) piston is also attached to the compressor crankshaft. As the second stage piston
moves downward, it draws in the air partially compressed by the first stage piston. When the second stage piston moves
upward, it further compresses this air and then forces the air through an exit port of the cylinder head of the compressor.
The air is transferred from this exit port to a receiver tank by means of a discharge tube. The receiver tank stores the
fully compressed air until needed by the operator.
b. Diesel Engine. To cause the crankshaft inside the air compressor to rotate, the compressor flywheel mounted
on the air compressor crankshaft is driven by a V-belt attached to a pulley connected to the output shaft of a diesel
engine. This diesel engine is equipped with both an electrical starter and a manual recoil starter for starting the engine.
The operating speed of the engine is controlled by a throttle control mechanism which is factory set to provide a constant
operating speed for the air compressor. The electric starter can be powered by a 12 volt direct current battery (not
furnished with the ACU).
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