In a four stroke SI engine, there is one power stroke in two revolutions of crankshaft and two strokes, viz., suction and exhausts are non-productive. If these two non-productive strokes could be served by an alternative arrangement especially without movement of the piston then there will be one power stroke for each revolution of the crankshaft. In such an engine, the power output can be doubled, theoretically, for the same speed compared to four stroke engine. Based on this concept, D. Clark (1878) developed two-stroke engine. 
                                                             
In this engine, the filling process is accomplished by the charge compression in crankcase. The induction of the compressed charge pushes the burnt fuel products through the exhaust port. Therefore, no piston movement is required for suction and exhaust process. Two strokes are sufficient to complete the cycle, one for compressing the fresh charge and other for expansion or power stroke.
The charge is inducted into the crankcase through the spring loaded inlet valve when the pressure in the crankcase is reduced due to upward movement of the piston during compression stroke. After the compression and ignition, expansion takes place in the usual way. During expansion stroke the charge in crankcase is compressed. Near the end of expansion stroke piston uncovers the exhaust port and cylinder pressure drops to atmospheric pressure as combustion products leaves the cylinder. Further movement of piston uncovers the transfer port, permitting the slightly compressed charge in crankcase to enter the engine cylinder. 
The top of the piston has usually a projection to deflect the fresh charge towards the top of the cylinder before flowing to the exhaust port. This serves the double purpose—scavenging the combustion product in upper part of the cylinder and preventing the fresh charge from flowing directly to the exhaust port. The same objective can be achieved without piston deflector by proper shaping of the transfer port. During the upward motion of the piston from BDC, the transfer port is closed first and then the exhaust port is closed when compression of charge begins and cycle is repeated in the same way.
The port time diagram of a two stroke engine is as follows: