Automotive Systems

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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.



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?

Figure 2-7.—Strokes and events in a two-stroke-cycle diesel engine cylinder.

Figure 2-8.—Comparison of two-stroke and four-stroke cycles.

Published by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy

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