How a 4 stroke motorbike engine works

Suck, Squeeze, Bang, Blow!! (Sounds like an advert for a porn movie, but this is a simplified version of the 4 stroke theory!) 🙂

A Very Basic 4 stroke Engine

Please note-the diagram below represents a very simple version of a 4 stroke motorcycle engine, in reality, they are a little bit more complicated!!!

4 stroke engine

Engine Terminology…

Stroke: Either the up or down movement of the piston from the top to the bottom or bottom to top of the cylinder (So the piston going from the bottom of the cylinder to the top would be 1 stroke, from the top back to the bottom would be another stroke)

Induction: As the piston travels down the cylinder head, it ‘sucks’ the fuel/air mixture into the cylinder. This is known as ‘Induction’.

Compression: As the piston travels up to the top of the cylinder head, it ‘compresses’ the fuel/air mixture from the carburettor in the top of the cylinder head, making the fuel/air mix ready for ignighting by the spark plug. This is known as ‘Compression’.

Iginition: When the spark plug ignites the compressed fuel/air mixture, sometimes reffered to as the power stroke.

Exhaust: As the piston returns back to the top of the cylinder head after the fuel/air mix has been ignited, the piston pushes the burnt ‘exhaust’ gases out of the cylinder & through the exhaust system.

The 4 stroke Cycle

We have simplified this explanation as much as possible so some of the ‘correct’ terms have been replaced.
There are many more factors which enable an engine to run, such as fuel/air ratios, ignition timing & shaped piston heads (extensively used in 2 stroke engines) but the explanation below outlines the basic differences between 2 & 4 stroke engine operation.

4 Stroke Cycle

Stroke Piston Direction Inlet & Exhaust Valve Positions Actions Occurring During This Stroke Explanation
Stroke 1 Piston travels down the cylinder barrel Inlet valve open/Exhaust valve colsed Induction stroke As the Piston travels down the cylinder barrel, the inlet valve opens & fresh fuel/air mixture is sucked into the cylinder
Stroke 2 Piston travels up the cylinder barrel Inlet & exhaust valve closed Compression stroke As the piston travels back up the cylinder, the fresh fuel/air mix is compressed ready for ignition

Stroke 3

Piston travels down the cylinder barrel Inlet & exhaust valve closed Ignition (power) stroke The spark plug ignites the compressed fuel/air mix, the resulting explosion pushes the piston back to the bottom of the cylinder
Stroke 4 Piston travels up the cylinder barrel Inlet valve closed/Exhaust valve open Exhaust stroke As the piston travels back up the cylinder barrel, the spent exhaust gases are forced out of the exhaust valve

One thought on “How a 4 stroke motorbike engine works”

Leave a Reply

Your e-mail address will not be published. Required fields are marked *

Anti-Spam Quiz: