Understanding the Four-Stroke Engine Cycle

A four-stroke engine is a type of internal combustion engine that completes a working cycle in four distinct piston strokes. These strokes are Intake, Compression, Power, and Exhaust. Understanding how these strokes relate to the crankshaft's rotation is key to understanding the engine's operation.

The Four Strokes Explained

Let's break down each of the four strokes and see how much the crankshaft rotates during each one:

1. Intake Stroke: The piston moves downwards, creating space for the fuel-air mixture to enter the cylinder through the open intake valve. During this downward movement, the crankshaft completes 180 degrees (or half a revolution).
2. Compression Stroke: The intake valve closes, and the piston moves upwards. Both valves are now closed, trapping the fuel-air mixture. The piston compresses the mixture against the cylinder head. This upward movement causes the crankshaft to complete another 180 degrees, totaling 360 degrees (one full revolution) from the start of the intake stroke.
3. Power Stroke: A spark ignites the compressed fuel-air mixture, causing a rapid expansion of gases. This high-pressure gas pushes the piston downwards forcefully. This is the stroke where the engine produces power, driving the crankshaft. The crankshaft completes another 180 degrees during this downward movement, totaling 540 degrees (one and a half revolutions) from the start.
4. Exhaust Stroke: The exhaust valve opens, and the piston moves upwards. This movement pushes the burnt gases out of the cylinder through the open exhaust valve. The crankshaft completes the final 180 degrees during this upward movement, bringing the total rotation to 720 degrees (two full revolutions).

Crankshaft Revolutions per Cycle

As we can see from the stroke descriptions, the crankshaft rotates 180 degrees during each of the four strokes. Therefore, the total rotation for one complete working cycle is the sum of the rotations for each stroke:

Total rotation = Rotation per stroke $\times$ Number of strokes

Total rotation = $180^\circ \times 4 = 720^\circ$

Since one full revolution is 360 degrees, 720 degrees of rotation equals two full revolutions ($720^\circ / 360^\circ = 2$).

Thus, a complete working cycle in a four-stroke engine requires two revolutions of the crankshaft.

In summary, the sequence of Intake, Compression, Power, and Exhaust strokes covers a total of 720 degrees of crankshaft rotation, which is equivalent to two full revolutions of the crankshaft.

Revision Table: Four-Stroke Engine Cycle

Let's quickly review the key points about the four-stroke engine cycle and crankshaft rotation:

• A four-stroke engine completes its cycle in four strokes: Intake, Compression, Power, and Exhaust.
• Each stroke corresponds to 180 degrees of crankshaft rotation.
• Total crankshaft rotation for one cycle = $4 \times 180^\circ = 720^\circ$.
• $720^\circ$ is equal to two complete revolutions ($2 \times 360^\circ$).
• Therefore, the working cycle is completed in two revolutions of the crankshaft.

Additional Information: Engine Types and Cycles

While the four-stroke engine is common, other types of engines exist, such as two-stroke engines. It's useful to compare their cycles:

• Four-Stroke Engine: Completes the working cycle (Intake, Compression, Power, Exhaust) in four piston strokes and two crankshaft revolutions. Each stroke is distinct.
• Two-Stroke Engine: Completes the working cycle in two piston strokes and one crankshaft revolution. The intake/compression and power/exhaust phases are combined within the two strokes.

Understanding the relationship between piston strokes, valve operation, and crankshaft rotation is fundamental to comprehending how internal combustion engines function and generate power efficiently over crankshaft revolutions.