Diesel Engine Fundamentals
engine. When the piston travels below the level of the ports, the ports are "opened" and
fresh air or exhaust gasses are able to enter or leave, depending on the type of port.
The ports are then "closed" when the
Figure 8 Diesel Engine Valve
piston travels back above the level of
the ports. Valves (refer to figure 8)
are mechanically opened and closed to
admit or exhaust the gasses as needed.
The valves are located in the head
casting of the engine. The point at
which the valve seals against the head
is called the valve seat.
medium-sized diesels have either
intake ports or exhaust valves or both
intake and exhaust valves.
Timing Gears, Camshaft, and
In order for a diesel engine to
operate, all of its components must
perform their functions at very precise intervals in relation to the motion of the piston.
To accomplish this, a component called a camshaft is used. Figure 9 illustrates a
camshaft and camshaft drive gear. Figure 2 and Figure 3 illustrate the location of a
camshaft in a large overhead cam diesel engine.
Figure 9 Diesel Engine Camshaft and Drive Gear
A camshaft is a long
bar with egg-shaped
eccentric lobes, one
lobe for each valve and
fuel injector (discussed
later). Each lobe has a
follower as shown on
camshaft is rotated, the
follower is forced up
and down as it follows
the profile of the cam
lobe. The followers are
engine's valves and fuel
i n j e c t o r s
t h r o u g h
linkages called pushrods
and rocker arms. The