Figure 3-3.Performance curves of a typical six-cylinder diesel engine.
Figure 3-4 is another example of a graph. It shows
that the amount of torque an engine produces
varies with its speed. The relationship between
torque and horsepower is shown in figure 3-5.
Horsepower is related to both torque and
speed. When both are increasing, as they do
between 1,200 and 1,600 rpm, then horsepower
goes up sharply. As torque reaches maximum and
begins to taper off, horsepower continues to rise
to maximum. The horsepower starts to decline
beyond rated speed where torque falls off sharply.
Engine timing is largely a matter of opening
and closing valves or ports and of adjusting igni-
tion or fuel injection so that these events occur at
the proper time in the cycle of engine operation.
Timing diagrams picture these events in relation to
each other and in relation to top dead center (TDC)
and bottom dead center (BDC). They are useful to
the CM for quick and easy reference. However,
before timing diagrams can be useful, the me-
chanic must recall a few facts about engine cycles.
The four-stroke-cycle engine makes two
complete crankshaft revolutions in one cycle
Figure 3-4.Graph showing relationship between torque and