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 TM 55-1905-223-24-5
1-11. Safety, Care, and Handling. Safety precautions must be observed at all times while performing
maintenance. General WARNINGS and first-aid data appear in front of this manual. Review all safety
information before starting any task. Carefully read through an entire maintenance procedure before
performing any maintenance function. Make sure the task can be done safely. All WARNINGS, CAUTIONS,
and NOTES are of great importance to your safety and the safety of the equipment.
SECTION III. PRINCIPLES OF OPERATION
Overview. The principles of operation section will give you an overall
description of how the bowthruster engine operates. Keep in mind that proper
engine operation depends on two things: 1) Compression for ignition, and 2)
that fuel be measured and injected into cylinders in the proper quantity at the
proper time.
1-12. Diesel Engine Operation. Diesel engine operation is different from spark ignited engine operation.
Compression ratios are higher and the charge taken into combustion chambers during the intake stroke
consists of air only, with no fuel mixture. Injectors receive low-pressure fuel from the fuel pump and deliver it
into individual combustion chambers at the proper time, in equal quantity, and in an atomized condition for
burning. Ignition of fuel is caused by heat of compressed air in the combustion chamber.
a. Diesel Engine Cycles. It is easier to understand the function of engine parts if you know what
happens in the combustion chamber during each of the four piston strokes of the cycle. The four strokes
happen in this order: intake stroke, compression stroke, power stroke, and exhaust stroke (FIGURE 1-5). In
order for the four strokes to function properly, valves and injectors must act in direct relation to each of the four
strokes of the piston. The intake valves, exhaust valves, and injectors are camshaft actuated, linked by tappets
or cam followers, push rods, rocker levers, and valve crossheads. The camshaft is driven by the crankshaft
gear and that means rotation of the crankshaft directs the action of the camshaft, which then controls the
opening and closing sequence of the valves and the injection timing (fuel delivery).
(1) Intake Stroke. During intake stroke, the piston travels downward, the intake valve is open, and
exhaust valves are closed. The downward travel of the pistons allows air from the intake manifold to enter the
cylinders. The intake manifold is pressurized with air from the turbocharger.
(2) Compression Stroke. At the end of the intake stroke, the intake valves close and the pistons starts
upward on the compression stroke. The exhaust valves remain closed. At end of compression stroke, air in
combustion chambers has been forced by the pistons to occupy a smaller space than it occupied at beginning
of stroke, causing the air temperature to rise to a point high enough for ignition of fuel. During last part of
compression stroke and early part of power stroke, a small metered charge of fuel is injected into the
combustion chamber. Almost immediately after fuel charge is injected into combustion chamber, fuel is ignited
by the existing hot compressed air.
1-8
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