How a 2 stroke motorbike engine works

All thrash and blue smoke baby :-)

A Very Basic 2 Stroke Engine

Please note – the diagram below represents a very simple version of a 2 stroke motorcycle engine, in reality, they are a little bit more complicated!

2 stroke motorcycle 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 carburetor in the top of the cylinder head, making the fuel/air mix ready for igniting by the spark plug. This is known as ‘Compression’.

Ignition: When the spark plug ignites the compressed fuel/air mixture, sometimes referred 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 and through the exhaust system.

Transfer Port: The port (or passageway) in a 2 stroke engine that transfers the fuel/air mixture from the bottom of the engine to the top of the cylinder

The 2 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 and shaped piston heads (extensively used in 2 stroke engines) but the explanation below outlines the basic differences between 2 and 4 stroke engine operation.

2 Stroke Cycle

Piston Direction
Actions Occurring during This Stroke
Stroke 1
Piston travels up the cylinder barrel
Induction and Compression
As the Piston travels up the barrel, fresh fuel/air mix is sucked into the crankcase (bottom of the engine) and the fuel/air mix in the cylinder (top of the engine) is compressed ready for ignition
Stroke 2
Piston travels down the cylinder barrel
Ignition and Exhaust
The spark plug ignites the fuel/air mix in the cylinder, the resulting explosion pushes the piston back down to the bottom of the cylinder, as the piston travels down, the transfer port openings are exposed and the fresh fuel/air mix is sucked from the crankcase into the cylinder. As the fresh fuel/air mix is drawn into the cylinder, it forces the spent exhaust gases out through the exhaust port.

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