16 Auto Engine Repair
The piston slides down with the intake valve
open and the exhaust valve closed. The
air-fuel charge is pulled into the cylinder.
Both valves are closed and the piston
slides up. This compresses the air-fuel
charge and prepares it for combustion.
The spark plug fires and the fuel begins to
burn. The heat of combustion causes
expansion of the gasses and creates
pressure. This pushes the piston down with
tremendous force to spin the crankshaft.
The piston slides up with the intake valve
closed and the exhaust valve open. This
pushes the exhaust gasses out of the
cylinder so a fresh charge can enter.
A—Intake Stroke B—Compression Stroke
C—Power Stroke D—Exhaust Stroke
Air-fuel mixture
pulled into
cylinder
Intake valve
open
Piston and
rod moving
down
Intake valve
closed
Pressure formed
on fuel charge
Exhaust
valve
closed
Piston and
rod moving up
Burned
gasses
pushed from
cylinder
Exhaust valve
open
Piston
and rod
moving up
Intake valved
closed
Spark
plug fires
Exhaust valve
closed
Air-fuel mixture
burns, expands,
and forms
pressure
Piston and
rod pushed
down to turn
crankshaft
Intake
valve
closed
Exhaust
valve
closed
Figure 1-16. Review the four-stroke cycle.You must be able to visualize these events to be a competent engine technician.