| Sequence of events | Description of Events |
| (1) Scavenging (intake) | A fresh change of air is forced into the cylinder intake ports by the blower. Exhaust gases escape through the open exhaust valves. |
| (2) Compression | 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. |
| (3) Injection/ignition and | When the piston nears the top of its stroke, fuel is injected into the cylinder. |
| (4) Combustion | The fuel ignites due to the heat of compression. |
| (5) Expansion (power) | The rapid expansion of burning gases forces the piston down. |
| (6) Exhaust | As the piston nears BDC, the exhaust valves open, starting the release of exhaust. |
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.
| TWO-STROKE | FOUR-STROKE |
| 1. One cycle equals one crankshaft revolution and two piston strokes. | 1 . One cycle equals two crankshaft revolutions and four piston strokes. |
| 2. Requires a blower. | 2. Blower is optional. |
| 3. Requires intake and exhaust ports or intake ports and exhaust valves. | 3. Requires only intake and exhaust valves. |
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?
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.
ENGINE COMPARISON
Mechanically and in overall appearance, gasoline and diesel engines resemble one another; however, in
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