plug protrudes slightly into the
combustion chamber liner to
provide an effective spark.
constrainedgap plug which does
not closely follow the face of
the plug; instead it tends to
jump in an arc which carries it
beyond the face of the chamber
liner. Because the constrained
gap plug does not have to
protrude into the liner, the
electrode operates at a cooler
Annular Gap Igniter
temperature than that of the
The intense heat generated when combustion takes place means
that all internal combustion engines must be cooled by some means.
Aircooled reciprocating engines are cooled by air passing over fins
attached to the cylinders. Liquidcooled engines, as in an
automobile, use a liquid coolant that passes through jackets
surrounding the cylinders. In a reciprocating engine, combustion
takes place only during every fourth stroke of a fourcycle engine.
However, in a gas turbine engine, where the burning process is
continuous, nearly all the cooling air must pass through the inside
of the engine. If only enough air were admitted to the engine to
provide combustion, internal temperatures would increase to more than
4,000 F. Because of this, the amount of air admitted to the engine
is in excess of the amount required for combustion only; indeed,
about 75 percent of the air is used for cooling and 25 percent for
combustion. This large surplus of air (secondary air) cools the hot
expanding gases just before they enter the turbines. In some
engines, internal air is bled from the engine compressor section and
is vented through passages to the bearings and other parts of the
engine. This air is then vented into the exhaust stream.
Engine performance is monitored by instruments mounted on the
instrument panel in the cockpit.
a. Tachometer system. The tachometer gives the pilot a
continuous indication of engine rpm. A variety of systems or a