INTRODUCTION TO THE HMMWV - OD1615 - LESSON 1/TASK 2
In the paragraphs that follow, the operation of the
engine will be followed through each of the four strokes that comprise one
(1) Intake. When the intake stroke begins, the piston is at top dead
center (TDC). As the piston moves downward, the intake valve opens and the
downward movement of the piston creates a vacuum that sucks air into the
When the piston reaches bottom dead center (BDC), the intake
valve closes and the intake stroke is complete.
piston travels upward toward TDC, the air that was drawn into the cylinder
is compressed to a ratio of nearly 22 to 1, almost three times the ratio of
This ultra-high compression causes the air to
superheat, making it hot enough to ignite the diesel fuel. The compression
stroke ends when the piston reaches TDC.
When the piston reaches TDC, the fuel injector sprays a precisely measured
combustion chamber ignites the fuel as it is injected.
This is where the
term compression ignition comes from.
The very high compression ratio is
part of the reason a diesel provides high efficiency and impressive fuel
(3) Power Stroke. The power stroke begins with the initial burning
forces the injector fuel into the cylinder area, where combustion is
completed. The exploding gases push the piston downward, providing power to
the engine's crankshaft and in turn to the vehicle's drive train.
valve opens, and the piston again moves up in the cylinder, this time
forcing out exhaust gases.
As the piston reaches TDC again, the exhaust
valve closes, the intake valve opens, and the entire process starts all