Four Stroke Engine: Parts, Working & Types of Strokes

A four-stroke engine is an internal combustion engine. It completes one operational cycle with four distinct piston strokes that are intake, compression, power, and exhaust. In a four-stroke engine, a whole operation involves two crankshaft rotations (7200).

Keyterms: Piston, flywheel, Four Stroke, Types of strokes

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What is Four Stroke Engine?

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A Four Stroke Engine is also known as a four-cycle engine. It is an internal combustion engine in which the piston rotates the crankshaft four times. The whole journey of the piston along the cylinder in each direction is referred to as a stroke. There are four-stroke used in a four-stroke engine that are intake, compression, power, and exhaust. A complete cycle in a four-stroke engine needs two crankshaft revolutions (7200).

Also read: Reversible and Irreversible Processes

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Parts of Four Stroke Engine

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The parts of a Four-Stroke Engine are: 

• Piston: The piston in a four-stroke engine transfers the expanding forces of gas to mechanical rotation of the crankshaft in an engine through a connecting rod. 
• Connecting Rod: The connecting rod acts as a lever arm. It sends motion from a piston to the crankshaft.
• Crankshaft: A crankshaft in a four-stroke engine turns reciprocating motion into rotating motion.
• Flywheel: The flywheel is a mechanical device; it stores energy produced by rotation.
• Valves for the inlet and outlet: It allows us to inject fuel into new air while simultaneously removing the spent air-fuel combination from the cylinder.
• Spark Plug: It's a method of sending an electric current across the combustion chamber, that ignites the air-fuel mixture and expands the gas rapidly.

Different strokes in Four Stroke Engine

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The four-stroke engine has four strokes which are explained below:

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Intake Stroke: Intake Stroke is also known as Suction Stroke. In intake stroke, the air-fuel combination is introduced to the combustion chamber. During this stroke, the piston moves from the Top Dead Center, which is the piston's furthest distance from the crankshaft to the Bottom Dead Center, the piston's closest distance from the crankshaft. The cylinder's low-pressure zone is caused by the piston's push toward the Bottom Dead Center. After Bottom Dead Center, the inlet valve remains open for a few degrees of crankshaft rotation. The intake valve closes after that, sealing the air-fuel mixture inside the cylinder. 

Intake Stroke

Also read: First Law of Thermodynamics

Key Points of Intake Stroke:

Compression Stroke: During the compression stroke, the trapped air-fuel combination within the cylinder is compressed. During the stroke, the piston moves from Bottom Dead Center to Top Dead Center, compressing the air-fuel combination. The pistons’ forward motion is aided by the flywheel's momentum. When the charge is ignited, the air-fuel mixture is compressed, allowing the release of more energy. The charge is the quantity of compressed air-fuel combination that has been trapped and is ready to be ignited within the combustion chamber. Both the inlet and exhaust valves must be closed to guarantee that the cylinder is closed and compressed.

Compression Stroke

Key Points of Compression Stroke:

Combustion Stroke: Combustion Stroke is also known as Power Stroke. The second rotation starts immediately when the crankshaft finishes a full rotation during the compression stroke. During the power stroke, the compressed air-fuel mixture is ignited with the help of a spark plug. Ignition or combustion is a quick, oxidizing chemical process in which a fuel chemically reacts with oxygen in the environment to generate energy in the form of heat. The hot expanding gases produced during the combustion stroke push the piston head away from the cylinder head.

Combustion Stroke

Key Points of Combustion Stroke:

Exhaust Stroke: The combustion process is complete when the piston reaches Bottom Dead Center during the power stroke, and the cylinder is filled with exhaust gases. In the exhaust stroke, the exhaust valves open, as a result the inertia of the flywheel and other moving parts pushes the piston back to Top Dead Center, compelling the exhaust gases through the open exhaust valve. The piston is at Top Dead Center at the end of the exhaust stroke, and one operating cycle has been finished.

Exhaust Stroke

Key Points of Exhaust Stroke:

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Difference Two Stroke vs Four Stroke Engine

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The major difference between the Two Stroke and Four Stroke Engine is explained in the table given below:

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Points to Remember

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• A Four-Stroke Engine is also known as a four-cycle engine. It is an internal combustion engine in which the piston rotates the crankshaft four times.
• The whole journey of the piston along the cylinder in each direction is referred to as a stroke. There are four-stroke used in a four-stroke engine that are intake, compression, power, and exhaust. 
• A complete cycle in a four-stroke engine needs two crankshaft revolutions (7200).
• The different parts of the Four-Stroke Engine are: Piston, Connecting Rod, Crankshaft, Flywheel, Valves for the inlet and outlet, and Spark Plug.
• Intake Stroke, also known as Suction Stroke. During the intake stroke, the air-fuel combination is introduced to the combustion chamber. During this stroke, the piston moves from the Top Dead Center, which is the piston's furthest distance from the crankshaft to the Bottom Dead Center, the piston's closest distance from the crankshaft.
• During the compression stroke, the trapped air-fuel combination within the cylinder is compressed. During the stroke, the piston moves from the Bottom Dead Center to Top Dead Center, compressing the air-fuel combination. The piston's forward motion is aided by the flywheel's momentum. When the charge is ignited, the air-fuel mixture is compressed, allowing the release of more energy.
• Combustion Stroke is also known as Power Stroke. The second rotation starts immediately when the crankshaft finishes a full rotation during the compression stroke. During the power stroke, the compressed air-fuel mixture is ignited with the help of a spark plug.
• The combustion process is complete when the piston reaches Bottom Dead Center during the power stroke, and the cylinder is filled with exhaust gases.
• The four-stroke engine is heavier than the two-stroke engine. A four-stroke engine weighs 50% more than a two-stroke engine.
• The four-stroke engine produces more torque at lower RPMs.
• The four-stroke engine has more parts and a much more complex structure, which makes the repair cost high.
• Four-stroke engines are majorly used in lawnmowers, car combustion engines, and go-karts.