Ignition And Starter System; Air Induction System - Cessna 172S Skyhawk Information Manual

SECTION 7
AIRPLANE & SYSTEMS DESCRIPTION
IGNITION AND STARTER SYSTEM
CESSNA
MODEL 172S
I Engine ignition is provided by two engine-driven magnetos, and
two spark plugs in each cylinder. The right magneto fires the lower
right and upper left spark plugs, and the left magneto fires the lower
left and upper right spark plugs. Normal operation is conducted with
Iboth magnetos due to the more complete burning of the fuel/air
mixture with dual ignition.
I Ignition and starter operation is controlled by a rotary-type switch
located on the left switch and control panel. The switch is labeled
clockwise, OFF, R, L, BOTH, and START. The engine should be
operated on both magnetos (BOTH position) except for magneto
checks. The Rand L positions are for checking purposes and
emergency use only. When the switch is rotated to the spring
loaded START position, (with the master switch in the ON position),
Ithe starter contactor is closed and the starter, now energized, will
crank the engine. When the switch is released, it will automatically
return to the BOTH position.
AIR INDUCTION SYSTEM
The engine air induction system receives ram air through an
intake on the lower front portion of the engine cowling. The intake is
covered by an air filter which removes dust and other foreign matter
from the induction air. Airflow passing through the filter enters an air
box. The air box has a spring-loaded alternate air door. If the air
induction filter should become blocked, suction created by the
engine will open the door and draw unfiltered air from inside the
lower cowl area. An open alternate air door will result in an
approximate 10% power loss at full throttle. After passing through
the air box, induction air enters a fuel/air control unit under the
engine, and is then ducted to the engine cylinders through intake
manifold tubes.
7-22
May 30/00