The piston ported two stroke engine

is beautifully simple in its operation. For those not familiar with the basics I hope the following goes part way to explain it and the main factors that affect its performance.

The part of these illustrations that may not be clear is that the transfer port is the opening into the cylinder from a channel behind the cylinder wall, the other end being the cut away section at the base of the cylinder.
 

To try and make this as clear as possible I'll set up the senario that our engine has had no fuel mixture pass through it for some time so that initially the cylinder above and below the piston contains fresh air. So with the piston at bottom dead centre we hit the starter button and the engine turns. This shows that primarily the action of engines is that of a pump!
Initial Upstroke  
Fig 1. On start of upstroke until piston skirt uncovers inlet port a partial vacuum is formed below the piston.

 

 Fig 2. As the piston skirt clears the inlet port the partial vacuum is equalized by air-petrol mixture draw in from carbs. Fig 3. the piston continues up continuing to draw mixture in through carbs until Top Dead Centre is reached.
Initial Downstroke  
Fig 4. On the down stroke until the piston skirt closes the inlet some mixture is pushed back through the inlet port. 

 

 

 Fig 5. Continuing down the mixture is compressed below the piston. (The crankcase is omitted for clarity but plays an important part in the performance of the two stroke engine. For example consider the effect on the compression ratio below the piston.) Fig 6. Towards the bottom of the stroke the transfer port is uncovered and the mixture pressure is equalized by entering the cylinder which is open to atmospheric pressure as the exhaust port is also uncovered .
 

 
Fig 6. The 'pumping' cycle  repeats but this time the cylinder has fuel mixture. For a small part of the cycle the mixture is not compressed and some is lost through the partially open exhaust port.  Fig 7. Once the piston has covered the exhaust port the mixture can be  compressed. (Note that the same upstroke is drawing in the next charge of mixture from the carbs.)  Fig 8. At just before top dead centre a spark is applied to ignite the mixture as it is finally compressed. This is timed so that maximum burn of the mixture occurs as the piston travels through TDC under the effect of maximum compression.
Fig 9. As can be seen the power element of the downstroke occurs, but unlike a fourstroke engine does not last for the full travel of the piston. In reality it is only about 30%, so effectively a GT750 (as it fires twice as often as a four stroke) it is a 500cc!!  Fig 10. the cycle continues with the exhaust port being uncovered allowing the cylinder to vent hot gasses and cool sufficiently to accept..  Fig 11. ..the transfer port is uncovered and the fresh charge of mixture pushes out the remaining exhaust gasses. The cycle then continues to repeat with fig 6, over and over again until you get where go are going and remove the spark!
As can be seen the action as a pump is basic. To tune the timing and efficiency the ports are designed in both relative sizes and opening and closing timing (position on the cylinder wall) for the designed rpm the 'pump' will operate at. It is evident that both the intake and exhaust are pulsed actions so the science of fluid dynamics plays a very large part in getting this to perform efficiently. This means in practice that there is a stage that once a certain rpm range is passed the design looses its ability to pump any where near an acceptable level. The ports are just not open long enough and the pulses become far to short to be effectively a 'flow'. It is at this peak efficiency as a pump that an engine will produce its maximum power.

 
As an engine the purpose is to pump fuel mixture, extract the heat in burning, and convert this to mechanical energy as efficiently as possible. Fig 6 and Fig 11 shows perhaps one of the biggest draw backs to this type of engine with respect to the amount of fuel used. For the fresh charge of mixture to enter the cylinder the cylinder must be open to the atmosphere via the exhaust so there is some loss in that fact alone.  The design also calls for this fresh charge to be the mechanism to flush as much of the exhaust out as possible so more mixture is to be wasted, by design! This is aided by the effect on the fresh charge from the exhaust pipes. The exhaust literally explodes out of the exhaust port, creating a flow of gasses away from the cylinder so that if not controlled the mixture can be partially 'sucked' out reducing the effective mixture to be burned on the following power stroke. Fortunately this is counteracted partially by the design of the exhaust system. In balancing the outlet from the exhaust pipes with the designed operating parameters of the engine an increasing back pressure can be 'built in' as rpm rises and more importantly for performance the internal design can be tuned to provide a shock wave of back pressure to force some of the escaping mixture back into the cylinder. However this effect in the main, can only be used to tune for peak performance at a narrow band of rpm.