Short Answer:

A four-stroke engine completes one power cycle in four different strokes of the piston — suction, compression, power, and exhaust. In each stroke, the piston moves once up or down inside the cylinder. Two complete revolutions of the crankshaft are required to complete one full cycle.

During the suction stroke, the air-fuel mixture enters the cylinder. In the compression stroke, this mixture is compressed. The power stroke is when combustion occurs, producing energy. Finally, in the exhaust stroke, burnt gases are expelled. These four strokes together make the working of the four-stroke engine continuous and efficient.

Detailed Explanation:

Four Strokes of a Four-Stroke Engine

A four-stroke engine is an internal combustion engine that completes one cycle of operation in four distinct piston movements or strokes within the cylinder. These four strokes are suction, compression, power, and exhaust. Each stroke corresponds to a specific function in the conversion of chemical energy of the fuel into mechanical energy. One complete cycle of the engine requires two revolutions of the crankshaft and four movements of the piston—two upward and two downward.

Four-stroke engines are commonly used in cars, motorcycles, and industrial machinery because they offer better fuel efficiency, smoother operation, and lower emissions compared to two-stroke engines. The four strokes are explained below in simple terms.

1. Suction Stroke

The suction stroke, also known as the intake stroke, is the first stroke of a four-stroke engine. During this stroke, the piston moves from the top dead center (TDC) to the bottom dead center (BDC). As the piston moves downward, the inlet valve opens, and the exhaust valve remains closed. This movement creates a vacuum in the cylinder, causing the air-fuel mixture (in petrol engines) or only air (in diesel engines) to enter the cylinder from the carburetor or air filter.

The main purpose of this stroke is to fill the combustion chamber with the correct amount of mixture for efficient combustion. At the end of the suction stroke, the inlet valve closes, trapping the charge inside the cylinder for compression.

2. Compression Stroke

The compression stroke is the second stroke in which both the inlet and exhaust valves remain closed. The piston now moves upward from the bottom dead center to the top dead center, compressing the trapped air-fuel mixture or air.

As the piston moves upward, the pressure and temperature inside the cylinder increase significantly. The high compression makes the mixture more explosive, which ensures better combustion when the ignition occurs. In petrol engines, a spark plug ignites the mixture at the end of this stroke, while in diesel engines, the heat generated by compression ignites the fuel injected later.

This stroke prepares the mixture for an effective power stroke and increases the efficiency of the engine.

3. Power Stroke

The power stroke, also known as the expansion or working stroke, is the most important stroke in the cycle because it produces useful mechanical work. At the beginning of this stroke, the compressed mixture is ignited—by a spark plug in a petrol engine or by heat of compression in a diesel engine.

The combustion of fuel rapidly increases the pressure and temperature of the gases inside the cylinder. These expanding gases force the piston downward with great force. This movement is transferred through the connecting rod to the crankshaft, producing rotational motion that powers the vehicle or machine.

The power stroke is the only stroke that produces power; the other three strokes require energy from the flywheel to operate. At the end of this stroke, the exhaust valve opens to start the exhaust process.

4. Exhaust Stroke

The exhaust stroke is the final stroke of the four-stroke cycle. During this stroke, the piston moves upward from the bottom dead center to the top dead center. The exhaust valve opens, and the inlet valve remains closed.

As the piston moves upward, it pushes the burnt gases from the combustion process out of the cylinder through the exhaust valve into the atmosphere. This prepares the cylinder for a new charge of air-fuel mixture in the next cycle. Once the exhaust gases are expelled, the exhaust valve closes, and the engine is ready for the next suction stroke.

The flywheel stores energy during the power stroke and releases it during the other strokes to keep the engine running smoothly.

Summary of Four Strokes

1. Suction Stroke: Air-fuel mixture enters the cylinder.
2. Compression Stroke: Mixture is compressed by the upward motion of the piston.
3. Power Stroke: Mixture burns, producing high-pressure gases that move the piston downward.
4. Exhaust Stroke: Burnt gases are expelled from the cylinder.

Conclusion:

The four-stroke engine works efficiently by completing four piston strokes—suction, compression, power, and exhaust—in two revolutions of the crankshaft. Only one of these strokes (the power stroke) produces useful work, while the others support the process of combustion and gas exchange. This design ensures smooth operation, high efficiency, and durability, which is why four-stroke engines are widely used in automobiles and industrial machinery.