2 Stroke Engines

'Two Stroke' means that for every single revolution of the crankshaft fuel/air mixture is sucked in (inducted), compressed, fired and exhausted during which time the piston is performing 'two strokes' (one up and one down) inside the cylinder.

Basic 2 Stroke Actions
A - The engine is being turned over (e.g. for starting) and the piston is travelling up the cylinder.
This causes a lowering of pressure in the crankcase sucking in a mix of fuel/air from the carburettor
B - The piston has reached its uppermost position in the cylinder (or Top Dead Centre - 'TDC').
Sucking in fuel/air mix is complete with the crankshaft/cylinder below the piston full with fuel/air mixture.
C- Continued rotation of the crankshaft during starting carries the piston on from TDC back downward,
begining to compress the fuel/oil mixture inducted in B. Pressure closes the fuel/air intake valve.

D - Compression of fuel/air mix continues until piston has almost reached its lowest position (Bottom Dead Centre - 'BDC').
At this point the top of the piston uncovers an opening (transfer port) in the side of the cylinder to a connecting passage from crankcase.
The compressed mixture can now escape thru the passage (transfer passage) to the top of the cylinder above the piston.
E - The piston is travelling back up the cylinder again. Closing off the transfer port and
trapping the fuel/air mix in the top of the cylinder, and is compressed.
F - At some point just before TDC the mixture is ignited (in glow engines by glow plug element
or in diesel model engines by heat generated during compression)

G - Rapid expansion of gases during ignition of mixture forces piston down (this is when the engine starts)
H - As the piston travels down the exhaust port in the side of the cylinder is opened allowing burnt gases to escape
J - Shortly after the opening of the exhaust port the transfer port is uncovered by the piston allowing a fresh charge
of fuel into the top of the cylinder which also helps push out the previous burnt gasses thru the exhaust port in a 'scavenging action'.


The cycle repeats over & over again. A complete '2 stroke cycle' is covered by steps E to J.
Correct Timing is obviously important, the intake valve must open at the right moment for sufficient time to suck in (induct)
a full charge of fuel/air and close during compression. The transfer port must open at the correct time for compressed fuel/air to 'transfer'
to the top of the cylinder, and close for the upward compression stroke where the mixture must ignite at the right moment.
On downward stroke the exhaust must not open too early or pressure will be lost that ejects burnt gases, yet must open in time for long enough duration to eject gases similarly the transfer port must open early enought for proper scavenging and long enough to transfer a full fuel charge.

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