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Timing Gears, Camshaft, and Valve Mechanism | Up
Mechanical Science Volume 1 of 2 | Next
Diesel Engine Support Systems |
Diesel Engine Fundamentals
DOE-HDBK-1018/1-93
DIESEL ENGINES
pushrods and rocker arms transfer the reciprocating motion generated by the camshaft
lobes to the valves and injectors, opening and closing them as needed. The valves are
maintained closed by springs.
As the valve is opened by the camshaft, it compresses the valve spring. The energy
stored in the valve spring is then used to close the valve as the camshaft lobe rotates out
from under the follower. Because an engine experiences fairly large changes in
temperature (e.g., ambient to a normal running temperature of about 190°F), its
components must be designed to allow for thermal expansion. Therefore, the valves,
valve pushrods, and rocker arms must have some method of allowing for the expansion.
This is accomplished by the use of valve lash. Valve lash is the term given to the "slop"
or "give" in the valve train before the cam actually starts to open the valve.
The camshaft is driven by
Figure 10 Diesel Engine Valve Train
the engine's
crankshaft
through a series of gears
called idler gears and
timing gears. The gears
allow the rotation of the
camshaft to correspond or
be in time with, the
rotation of the crankshaft
and thereby allows the
valve
opening,
valve
closing, and injection of
fuel to be timed to occur at
precise intervals in the
piston's
travel.
To
increase the flexibility in
timing the valve opening,
valve closing, and injection
of fuel, and to increase
power or to reduce cost,
an engine may have one or
more camshafts. Typically,
in a medium to large V-type engine, each bank will have one or more camshafts per head.
In the larger engines, the intake valves, exhaust valves, and fuel injectors may share a
common camshaft or have independent camshafts.
Depending on the type and make of the engine, the location of the camshaft or shafts
varies. The camshaft(s) in an in-line engine is usually found either in the head of the
engine or in the top of the block running down one side of the cylinder bank. Figure 10
provides an example of an engine with the camshaft located on the side of the engine.
Figure 3 provides an example of an overhead cam arrangement as on a V-type engine.
On small or mid-sized V-type engines, the camshaft is usually located in the block at the
Rev. 0
ME-01
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