within the cylinder. The relationship between volume,
pressure, and temperature within a cylinder of the
engine is explained in the chart below and shown in
figure 2-2. Note the changes within the cylinder while
the temperature outside remains a constant 70°F.
A and B
T h e p i s t o n m o v e s u p w a r d ,
compressing the air in the cylinder.
B and C
As volume decreases, pressure
increases, and temperature rises.
These changing conditions continue,
as the piston moves upward.
As the piston nears TDC, volume is
still decreasing. Because of
compression within the cylinder,
both pressure and temperature of
the air are now greater than at the
This up-and-down motion is known as
RECIPROCATING MOTION. This motion
(straight-line motion) must be changed into ROTARY
Figure 2-2.Volume, pressure, and temperature relationships.
MOTION (turning motion) to turn the wheels of a
vehicle. A crankshaft and a connecting rod change their
reciprocating motion to rotary motion.
All internal combustion engines, whether gasoline
or diesel, are basically the same.
We can best
demonstrate this by saying they all rely on three
thingsFUEL, AIR, and IGNITION.
FUEL contains potential energy for operating the
engine; AIR contains the oxygen necessary for
combustion; and IGNITION starts combustion. Each
one is fundamental, and an engine cannot operate
without them. Any discussion of engines must be based
on these three factors and the steps and mechanisms
involved in delivering them to the combustion chamber
at the proper time.
DEVELOPMENT OF POWER
The power of an internal combustion engine comes
from burning a mixture of fuel and air in a small,
enclosed space. When this mixture bums, it expands
greatly, and the push or pressure created is used to move
the piston, thereby rotating the crankshaft. This motion
is eventually sent to the wheels that move the vehicle.
Since similar action occurs in each cylinder of an
engine, lets use one cylinder to describe the steps in the
development of power. The one-cylinder engine
consists of four basic parts, as shown in figure 2-3.
First, we must have a CYLINDER that is closed at
one end; this cylinder is similar to a tall metal can that is
stationary within the engine block.
Inside this cylinder is the PISTONa movable
plug. It fits snugly into the cylinder but can still slide up
and down easily. This piston movement is caused by
fuel burning in the cylinder and results in production of
You have already learned that the up-and-down
movement of the piston is called reciprocating motion.
This motion must be changed into rotary motion, so the
wheels or tracks of a vehicle can rotate. This change is
accomplished by a throw on the CRANKSHAFT and
the CONNECTING ROD which connects the piston
and crankshaft throw.
The throw is an offset section of the crankshaft that
scribes a circle, as the shaft rotates. The top end of the
connecting rod is connected to the piston and must
therefore go up and down. The lower end of the
connecting rod is attached to the Crankshaft. The lower
end of the connecting rod also, moves up and down but,