4 Stroke Engine Basic

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!!!

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.

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