Four-stroke engines power a wide range of applications, including cars and motorcycles. Also called Otto cycle or four-cycle engines, these form the principle on which most modern automobile engines function.

In this comprehensive guide, we’ll dive deep into the definition of a four-stroke engine, its functionalities, and the major difference between 2-stroke and 4-stroke engines.

It is a widely used type of internal combustion engine in today's market. It works through four piston strokes, namely power, compression, intake, and exhaust, that complete a full operating cycle. 

All these strokes work together in a defined sequence to convert fuel into mechanical energy. This helps power various applications like automobiles, and motorcycles, to name a few, that typically run on gasoline or diesel fuel.

• Piston: The role of the piston is to move up and down inside the cylinder, converting gas forces to the crankshaft’s mechanical rotation.
• Crankshaft: This part of a four-stroke engine converts the piston's up and down motion into smooth motion.
• Connecting Rod: The connecting rod acts as a lever arm, linking the piston to the crankshaft.
• Flywheel: A flywheel stores energy and helps maintain optimal engine momentum.
• Inlet and Exhaust Valves: These control fresh air flow in an air-fuel mixture out of the cylinder. 
• Spark Plug: This ignition device generates an electric spark to ignite the fuel-air mixture during the power stroke.

4-stroke engines have 4 valves through which the piston goes through to complete on full cycle. Let’s discuss each phase in detail.

1. Intake Stroke

• The intake stroke, which is the first step of the four-stroke engine cycle, takes place when the fuel-air mixture enters the combustion chamber inside the cylinder.
• The piston then moves from the top dead centre (TDC) to the bottom dead centre(BDC).
• The inlet valve remains open a few degrees after BDC, and the exhaust valve is shut. 
• Later, the inlet valve closes, sealing the air-fuel mixture in the cylinder.

2. Compression Stroke

• During the compression stroke, there is a movement of the piston from BDC to TDC in the cylinder.
• The outlet and inlet valves are closed to ensure the cylinder is sealed. This results in compression.
• The piston’s upward motion compresses the air-fuel mixture in the combustion chamber.

3. Power Stroke

• It happens before the compressed fuel-air mixture ignites. 
• The ignition and combustion forces the piston to move downwards from TDC to BDC, thus generating the power that allows the vehicle to move forward. 
• Both the inlet and outlet valves remain closed.

4. Exhaust Stroke

• Occurs after the BDC receives the piston in the power stroke.
• The combustion process here is complete, filling the cylinder with exhaust gases.
• The valves of the exhaust open, permitting the gases to exit.
• Additionally, the piston is pushed back to TDC via the inertia of the flywheel, which also marks the completion of one operating cycle.