Key points of compression stroke in aviation piston engines

The key points of the compression stroke of an aviation piston engine are as follows:

1. Valve condition

Intake valve closure: At the beginning of the compression stroke, the intake valve closes to prevent fresh air (or mixture) from entering the cylinder.

Exhaust valve closed: At the same time, the exhaust valve remains closed to ensure that the gas inside the cylinder is sealed and compressed.

2. Piston movement

Piston upward movement: The piston starts to move upward from the bottom dead center (the lowest point of the piston movement in the cylinder) until the top dead center (the highest point of the piston movement in the cylinder).

Cylinder volume reduction: As the piston moves upward, the volume inside the cylinder gradually decreases, causing the gas inside the cylinder to be compressed.

3. Gas compression

The mixture is compressed: During the compression stroke, the mixture (a mixture of gasoline and air) drawn in during the intake stroke is strongly compressed by the piston.

Pressure and temperature rise: As the gas is compressed, its pressure and temperature gradually increase. When the piston reaches the top dead center, the pressure of the mixed gas can reach about 10 atmospheres, and the temperature increases to approximately 400℃.

4. Compression ratio

Definition: The compression ratio refers to the ratio of the total volume of the cylinder to the volume of the combustion chamber. It is one of the important indicators for measuring the performance of piston engines.

The compression ratio of an aviation piston engine is typically between 5 and 8. The greater the compression ratio, the more severely the gas is compressed, and the greater the power generated by the engine.

Impact: Increasing the compression ratio can enhance the engine's power output and fuel utilization rate, but it will also increase the engine's operating noise and vibration.

5. The purpose of compressing the stroke

Improving combustion efficiency: By compressing the mixture, increasing its pressure and temperature to make it more flammable, a greater explosive force is generated during the combustion stroke, thereby enhancing the combustion efficiency of the engine.

Preparing for the combustion stroke: The compression stroke is the preparatory work for the combustion stroke, which ensures that the mixture can burn rapidly and completely during combustion.

6. The end of the compression stroke

Spark plug ready for ignition: Near the end of the compression stroke, when the piston is close to the top dead center, the spark plug on the cylinder head is ready for ignition.

Entering the combustion stroke: Once the spark plug generates an electric spark and ignites the mixture, the compression stroke ends and the combustion stroke begins.

7. The role of the auxiliary system

Intake system: Although the intake stroke has ended, the design and layout of the intake system will still affect the smooth progress of the compression stroke. For instance, the smoothness of the intake passage and the efficiency of the intake filter will all affect the quality of the mixture entering the cylinder.

Ignition system: The performance of the ignition system directly affects the end of the compression stroke and the beginning of the combustion stroke. A reliable ignition system can ensure that the spark plugs generate sufficient electric sparks at the right time to ignite the mixture.

In conclusion, the compression stroke of an aviation piston engine is a crucial part of the engine's working cycle. It compresses the mixture through the upward movement of the piston, increasing its pressure and temperature, and preparing for the combustion stroke. The size and uniformity of the compression ratio directly affect the effect of ignition and explosion as well as the completeness of combustion, and it is one of the important indicators for evaluating the performance of piston engines.