Two Stroke Cycle Engine Operation

The 2 stroke is more complex in

operation than the 4 stroke, just because you have multiple things
happening at the same time. The Intake stroke is the same as the
power stroke and exhaust stroke. Confused yet? Let me explain.A stroke is one pass of the piston through the cylinder.  So one revolution of this crankshaft will produce two strokes of the piston in the cylinder.
parts of a two cycle engine
In a 2 stroke the gases flowing in and out of the cylinder are controlled by the piston. The piston acts as the
valve. When the piston is on the way towards the ignition source,
air/fuel is being pulled into the crank case or the bottom side of
the piston. This flow into the crankcase can either be
timed/controlled with a rotary valve or piston port, or it can be
controlled by a reed valve. On the top side of the piston air/fuel
is being compressed for an upcoming power stroke.


2 cycle intake and compression stroke
POW!!!!! The spark plug fires near
the top of the compression stroke and the piston starts its travel on
the power stroke.


2 cycle power stroke

The expanding gases produce power
until the exhaust port is revealed, by the piston and the expanding gases from the ignition leak out through the exhaust port. All this
time the air/fuel in the crankcase is gaining positive pressure
because the volume in the crankcase is shrinking due to the piston
moving closer to the crankshaft.


2 cycle power stroke
While the exhaust is being sucked out
of the ignition chamber air/fuel mixture in the crankcase continues
to get compressed. When the top of the piston uncovers the intake
ports, the pressurized mixture in the crankcase is forced into the
ignition chamber, charging the cylinder with fresh air/fuel to burn,
and purging the remaining exhaust gases. Some of the air fuel
mixture is pulled into the exhaust port as well, due to inertia of
the air/fuel.

2 cycle power stroke and exhuast stroke

By this time the Piston has hit BDC  (bottom dead center) and the piston is starting to go back to the ignition source. This  creates negative pressure in the crank case causing air fuel to be pulled in to the crank case..


2 cycle intake and compression stroke

The exhaust pipe or expansion chamber (if designed
correctly), will push some of this unspent fuel/air that traveled into the exhaust pipe back into the
ignition chamber (a pressure wave in the exhaust causes this). By
this time the piston is finishing covering the exhaust port and is on its way back to the ignition source, and
the process starts all over again.

Hope this explains the operation of a 2 cycle or 2 stroke engine.


Riding motorcycles and wrenching since my preteen years, I have moved from motocross to street bikes. Being a teen back in those days it was tough to get me off of the bike. Now days even though I am very busy being a dad I still have my weekends and go to the track to race and on occasion will do a track day or two.

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