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overheating, roughness, or loss of power.
Detonation is characterized by high cylinder head temperatures
and is most likely to occur when operating at high power
settings. Some common operational causes of detonation
include:
• Using a lower fuel grade than that specifi ed by the
aircraft manufacturer or operating the engine after
it has been sitting for an extended period; after 3
weeks or as indicated by the POH, drain old fuel and
replenish with fresh fuel.
• Operating the engine at high power settings with an
excessively lean mixture.
• Extended ground operations.
Detonation may be avoided by following these basic
guidelines during the various phases of ground and fl ight
operations:
• Make sure the proper grade of fuel is being used. Drain
and refuel if the fuel is old.
• Develop a habit of monitoring the engine instruments
to verify proper operation according to procedures
established by the manufacturer.
Preignition occurs when the fuel/air mixture ignites prior
to the engine’s normal ignition event. Premature burning
is usually caused by a residual hot spot in the combustion
chamber, often created by a small carbon deposit on a spark
plug, a cracked spark plug insulator, or other damage in the
cylinder that causes a part to heat suffi ciently to ignite the
fuel/air charge. Preignition causes the engine to lose power
and produces high operating temperature. As with detonation,
preignition may also cause severe engine damage because
the expanding gases exert excessive pressure on the piston
while still on its compression stroke.
Detonation and preignition often occur simultaneously and
one may cause the other. Since either condition causes high
engine temperature accompanied by a decrease in engine
performance, it is often diffi cult to distinguish between the
two. Using the recommended grade of fuel and operating the
engine within its proper temperature and RPM ranges reduce
the chance of detonation or preignition.
Fuel Systems
The fuel system is designed to provide an uninterrupted fl ow
of clean fuel from the fuel tank to the engine. See Chapter
3, Components and Systems, for more information on fuel
tanks. See earlier section in this chapter for specifi cs on fuel
injection systems. The fuel must be available to the engine
under all conditions of engine power, altitude, attitude, and
during all approved fl ight maneuvers. [Figure 4-17]
Fuel Pumps
WSC aircraft with carburetors have engine-driven fuel pump
systems. A diaphragm pump is the primary pump in the fuel
system for two-stroke engines. Air pulses in the crankcase
actuate a diaphragm and provide fuel under pressure to the
carburetor. Four-stroke engines have a mechanical pump
driven directly off the engine.
Sometimes an electric auxiliary pump is provided for use in
engine starting and in the event the engine pump fails. The
auxiliary pump, also known as a boost pump, provides added
reliability to the fuel system. The electric auxiliary pump is
controlled by a switch in the fl ight deck.
4-14
Carburetor Fuel Bowls
FUEL FLOW
Fuel Filter
Fuel Pump
Figure 4-18. Fuel system showing fuel filter to fuel pump to
carburetor float bowls.
Fuel Plunger Primer
The optional fuel plunger primer is used to draw fuel from
the tanks to supply it directly into the engine prior to starting.
This is particularly helpful during cold weather when engines
are hard to start because there is not enough heat available to
vaporize the fuel in the carburetor. For some aircraft, it is the
only way to deliver fuel to the engine when fi rst starting. After
the engine starts and is running, the fuel pump pushes fuel
to the carburetors and begins normal fuel delivery. To avoid
overpriming, read the priming instructions in the POH.
Choke
A choke or fuel enriching system is an alternate method to
provide additional fuel to the engine for initial cold starting.
Actuating the choke control allows more fuel to fl ow into
the carburetor.
Fuel Bulb Primer
The fuel bulb primer is manually actuated by squeezing the
bulb to draw fuel from the fuel tanks. This charges the fuel
lines and carburetor fl oat bowls before starting the engine
the fi rst time on a given day. After the engine starts, the fuel
pump is able to deliver the fuel to the fuel bowls. An electric
auxiliary fuel pump can also be used to charge the fuel lines
and carburetor fuel bowls before starting. This auxiliary fuel
pump is also used as a backup pump of the engine driven
fuel pump fails.
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Weight-Shift Control Aircraft Flying Handbook(46)
