What is a  Four-stroke IC engine?

A four-stroke engine is an internal combustion engine that utilises four distinct piston strokes (intake, compression, power, and exhaust) to complete one operating cycle. A complete operation in a four-stroke engine requires two revolutions (7200) of the crankshaft .

Parts of a Four Stroke Engine

Piston

In an engine, a piston transfers the expanding forces of gas to the mechanical rotation of the crankshaft through a connecting rod.

Crankshaft

A crankshaft is a part that converts the reciprocating motion to rotational motion.

Connecting Rod

It transfers motion from a piston to a crankshaft, acting as a lever arm

Flywheel

The flywheel is a rotating mechanical device that is used to store energy.

Inlet and Outlet Valves

It allows us to enter fresh air with fuel & to exit the spent air-fuel mixture from the cylinder.

Spark Plug

It is a device that delivers electric current to the combustion chamber, which ignites the air-fuel mixture leading to the abrupt gas expansion.

Four Stroke Engine Cycle

The four strokes of the engine go by the following names:

Suction/Intake Stroke

Intake stroke occurs when the air-fuel mixture is introduced to the combustion chamber. In this stroke, the piston moves from TDC (Top Dead Center – the farthest position of the piston to the crankshaft) to BDC (Bottom Dead Center – the nearest position of the piston to the crankshaft.) The movement of the piston towards the BDC creates a low-pressure area in the cylinder. The inlet valve remains to open a few degrees of crankshaft rotation after BDC. The intake valve then closes, and the air-fuel mixture is sealed in the cylinder

• Key points

• Inlet Valve – Open

• Outlet Valve ­­ – Closed

• Crankshaft Rotation – 1800

Compression Stroke

In compression stroke, the trapped air-fuel mixture is compressed inside the cylinder. During the stroke, the piston moves from BDC to TDC, compressing the air-fuel mixture. The momentum of the flywheel helps the piston move forward. Compressing the air-fuel mixture allows more energy to be released when the charge is ignited. The charge is the volume of compressed air-fuel mixture trapped inside the combustion chamber ready for ignition. The inlet and outlet valves must be closed to ensure that the cylinder is sealed, resulting in compression.

• Key points

• Inlet Valve – Closed

• Outlet Valve ­­ – Closed

• Crankshaft Rotation – 1800 (Total 3600)

Power/Combustion Stroke

The second rotation of the crankshaft begins when it completes a full rotation during the compression stroke. The power stroke occurs when the compressed air-fuel mixture is ignited with the help of a spark plug. Ignition or Combustion is the rapid, oxidizing chemical reaction in which a fuel chemically combines with oxygen in the atmosphere and releases energy in the form of heat. The hot expanding gases force the piston head away from the cylinder head.

• Key points

• Inlet Valve – Closed

• Outlet Valve ­­ – Closed

• Crankshaft Rotation – 1800 (Total 5400)

Exhaust Stroke

As the piston reaches BDC during the power stroke, combustion is complete, and the cylinder is filled with exhaust gases. The exhaust valves open during this stroke, and the inertia of the flywheel and other moving parts push the piston back to TDC, forcing the exhaust gases through the open exhaust valve. At the end of the exhaust stroke, the piston is at TDC, and one operating cycle has been completed.

• Key points

• Inlet Valve – Closed

• Outlet Valve ­­ – Open

• Crankshaft Rotation – 1800 (Total 7200)