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!!!
Engine
Terminology
A Longer list of commonly used Engine Terminology is available HERE
(link will open in a new window)
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 |
|
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 |
