How Does A Four Stroke Engine Works?

4 Stroke Engine:-

Four-stroke engine

The name itself gives us an idea – it is an Internal Combustion Engine where the piston completes 4 strokes while turning the crankshaft twice. A stroke refers to the piston traveling full in either of the direction. A cycle gets completed when all the 4 strokes get completed. The four-stroke engine was first demonstrated by Nikolaus Otto in 1876, hence it is also known as the Otto cycle.

Let us come to the parts which a 4 stroke engine has,

Piston – In an engine, the piston is used to transfer the expanding force of gases to mechanical rotation of crankshaft via a connecting rod. The piston is able to do this because it is secured tightly within the cylinder using piston rings to minimize the clearance between cylinder and piston!

Crankshaft – A crankshaft is a part which is able to convert the reciprocating motion to rotational motion.

Connecting rod – A connecting rod transfers motion from a piston to crankshaft which acts as a lever arm.

Flywheel – Flywheel is a rotating mechanical device that is used to store energy.

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

Spark Plug – A spark plug delivers electric current to the combustion chamber which ignites the air-fuel mixture leading to the abrupt expansion of gas.

The four strokes of a 4 stroke engine go by the name –

Four Stroke engine working process

1. Suction/Intake Stroke:-

In this stroke, the piston moves from TDC to BDC [(Top Dead Centre – the farthest position of piston to the crankshaft) to  (Bottom Dead Centre – the nearest position of piston to the crankshaft)].

The piston moves downward sucking the air-fuel mixture from the intake valve.

Key points:-

Intake valve – OPEN

Exhaust valve – CLOSED

Crankshaft rotation – 180°

2. Compression Stroke:-

Here, the piston moves from BDC to TDC compressing the air-fuel mixture. The momentum of flywheel helps the piston to move upwards.

Key points:-

Intake valve – CLOSED

Exhaust valve – CLOSED

Crankshaft rotation – 180° (total = 360°)

3. Power Stroke:-

The second rotation of crankshaft has begun as it completes one full rotation during the compression stroke. Power stroke begins with the expansion of the air-fuel mixture ignited with the help of a spark plug. Here, the piston moves from TDC to BDC. This stroke produces mechanical work to rotate the crankshaft.

Key points:-

Intake valve – CLOSED

Exhaust valve – CLOSED

Crankshaft rotation – 180° (total = 540°)

4. Exhaust Stroke:-

Again the momentum of the flywheel moves the piston up from BDC to TDC thereby driving the exhaust gases outside through the exhaust valve.

Key points:-

Intake valve – CLOSED

Exhaust valve – OPEN

Crankshaft rotation – 180°(total = 720°)

Here completes two complete rotation(720°) of the crankshaft along with one cycle ( One cycle because a thermodynamic cycle is a series of thermodynamic processes that returns a system to its initial state. Here, a series of thermodynamic processes happening is during the strokes. 4 strokes = 4 processes !)