TWO-STROKE-CYCLE ENGINE
In the
two-stroke-cycle engine (fig. 2-7), the same
four events (intake, compression, power, and exhaust) take place in only two strokes of
the piston and one complete revolution of the crankshaft. The two piston strokes are the
compression stroke (upward stroke of the piston) and power stroke (the downward stroke of
the piston). Remember that a diesel engine has six events that must happen to complete a
cycle of operation. To better understand the cycle of operation that happens inside the
cylinders of a two-stroke diesel engine, refer to the chart below while reviewing Figure 2-7.
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 engine.
You need to
know the differences between a two-stroke and four-stroke engine. Study the following
chart.
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.
Questions
Q1. 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 ?
Q3. A
one-cylinder engine consists of what number of parts?
Q4. A
two-stroke engine has approximately what percentage of power increase over a four-stroke
engine?
Q5. In a
two-stroke diesel engine, what sequence of events happens during the intake stroke? |
