The 2 Stroke / Cycle Engine

 The following is an explanation of the basic operation of the two-stroke engine. 

1. Starting with the piston at top dead center (TDC 0 degrees) ignition has occurred and the gasses in the combustion chamber are expanding and pushing down the piston. This pressurizes the crankcase causing the reed valve to close.

At about 90 degrees after TDC the exhaust port opens ending the power stroke. A pressure wave of hot expanding gasses flows down the exhaust pipe. The blow-down phase has started and will end when the transfer ports open. The pressure in the cylinder must blow-down to below the pressure in the crankcase in order for the unburned mixture gasses to flow out the transfer ports during the scavenging phase.

Now the transfer ports are uncovered at about 120 degrees after TDC. The scavenging phase has begun. Meaning that the unburned mixture gasses are flowing out of the transfers and merging together to form a loop. The gasses travel up the backside of the cylinder and loops around in the cylinder head to scavenge out the burnt mixture gasses from the previous power stroke.

It is critical that the burnt gasses are scavenged from the combustion chamber, to make room for as much unburned gasses as possible. That is the key to making more power in a two-stroke engine. The more unburned gasses you can squeeze into the combustion chamber, the more the engine will produce. Now the loop of unburned mixture gasses have traveled into the exhaust pipe's header section. Most of the gasses aren't lost because a compression pressure wave has reflected from the baffle cone of the exhaust pipe, to pack the unburned gasses back into the cylinder before the piston closes off the exhaust port. 

3.

Now the crankshaft has rotated past bottom dead center (BDC 180 degrees) and the piston is on the upstroke. The compression wave reflected from the exhaust pipe is packing the unburned gasses back

in through the exhaust port as the piston closes off the port the start the compression phase. In the crankcase the pressure is below atmospheric producing a vacuum and a fresh charge of unburned mixture gasses is flowing through the reed valve into the crankcase.

4. The unburned mixture gasses are compressed and just before the piston reaches TDC, the ignition system discharges a spark causing the gasses to ignite and start the process all over again.

Some notes about the above diagrams, the intake type depicted is what is commonly called a "case reed" type, ( a one way reed style valve allowing air/fuel into the crankcase from the carb). Another common type is a piston port style, whereas the reciprocating motion of the piston opens and closes a port in the side of the cyl allowing air/fuel into the crankcase when the piston moves toward the top of its stroke (TDC) and closes the port as the piston moves toward the bottom of its stroke (BDC). A third type is a rotary valve style using a rotating disc timed and driven off the crankshaft, opening and closing ports in the crankcase. Rotax used this type of induction system for many yrs on there performance engines.