3. The power stroke produces the energy to operate
the engine. With both valves still closed, the
spark plug arcs (sparks) and ignites the com-
pressed air-fuel mixture. The burning fuel
expands and develops pressure in the combustion
chamber and on the top of the piston. This pushes
the piston down with enough force to keep the
crankshaft spinning until the next power stroke.
4. The exhaust stroke removes the burned gases
from the combustion chamber. During this
stroke, the piston slides up while the exhaust
valve is open and the intake valve is closed. The
burned fuel mixture is pushed out of the engine
and into the exhaust system.
During engine operation, these four strokes are
repeated over and over. With the help of the heavy fly-
wheel, this action produces smooth, rotating power
output at the engine crankshaft.
Obviously, other devices are needed to lubricate the
engine parts, operate the spark plug, cool the engine, and
provide the correct fuel mixture. These devices will be
discussed shortly.
Chapter 1 The Automobile 7
Figure 1-9. A gasoline engine normally operates on a four-stroke cycle. Study the series of events.
Intake
valve
open
Exhaust
valve
closed
Piston and rod
moving down
Air-fuel
mixture
pulled into
cylinder
1—Intake stroke. Intake valve open. Exhaust valve closed.
Piston slides down, forming vacuum in cylinder. Atmospheric
pressure pushes air and fuel into combustion chamber.
Piston and rod
moving up
2—Compression stroke. Both valves are closed. Piston slides
up and pressurizes air-fuel mixture. This readies mixture for
combustion.
Air-fuel
mixture
compressed
Both
valves
closed
3—Power stroke. Spark plug sparks. Air-fuel mixture burns.
High pressure forces piston down with tremendous force.
Crankshaft rotates under power.
4—Exhaust stroke. Exhaust valve opens. Intake valve remains
closed. Piston slides up, pushing burned gases out of cylinder.
This prepares combustion chamber for another intake stroke.
Both
valves
closed
Spark plug
fires, mixture
burns and
expands
Piston forces
rod down
and turns
crankshaft
Piston slides up
and pushes
burned gases out
Intake
valve
closed
Exhaust
valve
open