SECTION IV OPERATION

TM 10-3930-623-12

SECTION IV

OPERATION

WARNING

Operation of this equipment presents a noise hazard to personnel in the area. The

noise level exceeds the allowable limits for unprotected personnel. Wear ear muffs

or ear plugs which were fitted by a trained professional. (Refer to TB MED 251)

4-6.

FOUR-STROKE CYCLE (see figure 4-1).

4-1.

PRINCIPLES CF OPERATION.

4-2. In the first part of this section, the principles of

4-7. INTAKE STROKE. The piston begins its intake

operation of the major assemblies of the fork lift truck will

stroke at the top of its travel in the cylinder with the

be explained.

camshaft opening the intake valve for that cylinder. The

intake stroke is a downstroke of the piston, which with

the intake valve open produces a suction from the

4-3.

ENGINE.

cylinder through the valve and intake manifold to the

4-4. The engine used in the truck (see figure 1-2) is a

carburetor. This suction draws a charge of air and

six cylinder, four stroke cycle gasoline engine of

vaporized gasoline into the cylinder. At the end of the

conventional L-head design, with battery and coil

intake stroke, the intake valve closes, Since the exhaust

ignition. One updraft carburetor supplies fuel-air mixture

valve is also closed at this time, the full charge (in

to all cylinders through a manifold. Exhaust from all

compressible gaseous form) is trapped in the cylinder

cylinders is delivered to a common exhaust system by

ready to be compressed.

an exhaust manifold. A camshaft in the engine block,

driven by the crankshaft through gears, turns at half

4-8. COMPRESSION STROKE. As the crankshaft

crankshaft speed to operate (and time) the valves and

continues to turn the piston returns upward to compress

ignition distributor, and drive the oil pump and fuel pump,

the fuel charge in the cylinder to about one-seventh its

The camshaft drive gear is positioned in relation to the

volume at the end of the intake stroke.

crankshaft so valves and ignition will operate at the

correct time in relation to piston travel which is controlled

NOTE

by the crankshaft. Hence, the expression "timing" of the

The

higher

the

degree

valves or ignition. Note that two revolutions of the

compression (compression ratio) the

crankshaft cause only one revolution of the camshaft,

more energy will be obtained from a

which results in two upstrokes and two downstrokes of

given quantity of fuel.

The

each piston to complete a cycle.

compression ratio of the engine is

limited by the grade of fuel for which

4-5. In the following explanation, a cycle of one

the engine is designed, and is as

cylinder will be discussed. What is said of one cylinder

high as practical for this application.

is true of each of the others; however, consider that each

piston (in firing order) lags behind the one ahead of it by

120 degrees of crankshaft rotation.

Figure 4-1. Four Stroke Engine Cycle

4-1