Sequence of events
(1) Scavenging (intake)
(5) Expansion (power)
Description of Events
A fresh change of air is
forced into the cylinder
intake ports by the blower.
Exhaust gases escape
through the open exhaust
As the piston moves
upward, the intake ports are
covered and the exhaust
valves close. The air is
compressed in the cylinder;
the piston continues to
move towards TDC.
When the piston nears the
top of its stroke, fuel is
injected into the cylinder.
The fuel ignites due to the
heat of compression.
The rapid expansion of
burning gases forces the
As the piston nears BDC,
the exhaust valves open,
starting the release of
As shown earlier, a power stroke is produced every
crankshaft revolution within the two-stroke-cycle
engine, whereas the four-stroke-cycle engine requires
two revolutions for one power stroke. It might appear
then that the two-stroke-cycle engine can produce twice
as much power as the four-stroke-cycle engine of the
same size, operating at the same speed; however, this
power increase is limited to approximately 70 to 80
percent because some of the power is used to drive a
blower that forces the air charge into the cylinder under
pressure. Also, the burned gases are not completely
cleared from the cylinder, reducing combustion
efficiency. Additionally, because of the much shorter
period the intake port is open (compared to the period
the intake valve in a four stroke is open), a relatively
smaller amount of air is admitted. Hence, with less air,
less power per stroke is produced in a two-stroke-cycle
You need to know the differences between a two-
stroke and four-stroke engine. Study the following
One cycle equals one 1 . One cycle equals two
a n d t w o p i s t o n
tions and four piston
2. Requires a blower.
2. Blower is optional.
3. Requires intake and 3. Requires only intake
exhaust ports or
and exhaust valves.
intake ports and
Figure 2-8 shows a comparison of events that occur
during the same length of time for both two-stroke- and
four-stroke-cycle engines. Notice the shaded areas that
represent the overlapping of events.
For a vehicle to move, reciprocating motion must
be changed to what type of motion?
Q2. On what three things must an internal
combustion engine rely to operate ?
A one-cylinder engine consists of what number
Q4. A two-stroke engine has approximately what
percentage of power increase over a four-stroke
In a two-stroke diesel engine, what sequence of
events happens during the intake stroke?
CLASSIFICATION OF ENGINES
LEARNING OBJECTIVE: Recognize the
differences in the types, the cylinder
arrangements, and the valve arrangements of
internal combustion engines.
Engines for automotive and construction
equipment may be classified in a number of ways: type
of fuel used, type of cooling used, or valve and cylinder
arrangement. They all operate on the internal
combustion principle, and the application of basic
principles of construction to particular needs or systems
of manufacture has caused certain designs to be
recognized as conventional.
The most common method of classification is by
the type of fuel used; that is, whether the engine burns
gasoline or diesel fuel.
Mechanically and in overall appearance, gasoline
and diesel engines resemble one another; however, in