A two stroke engine is one which completes its cycle of operation in one revolution of the crankshaft or in two strokes of the piston. In this engine the function of the intake and exhaust processes of the four stroke engine are taken care by the incoming fresh charge which is compressed either in the crank case or by means of a separate blower while the engine piston is near the bottom dead centre. The engine piston needs only to compress the fresh charge and expand the product of combustion.
In many two stroke engines the mechanical construction is greatly simplified by using the piston as a slide valve in conjunction with intake and exhaust ports cuts in the side of the cylinder. If the engine is running on Otto cycle, the charge consists of correct mixture of fuel and air where as for petrol or duel combustion cycles the charge will consists of pure air.
In 1878, Dugald -clerk, a British engineer introduced a cycle which could be completed in two strokes of the piston rather than as in the case with the four stroke cycle engines.The using this cycle were called two stroke cycle engines. In this engine suction and exhaust strokes are eliminated.
Here instead of valves, ports are used. The exhaust gases are driven out from engine cylinder by the fresh charge of fuel entering the cylinder nearly at the end of the working stroke.
The engine ignites fuel at every upward stroke, so there are two strokes for every ignition of fuel. They are called upward and downward stroke. As the piston moves in upward direction from bottom to top in the first stroke the air and fuel mixture gets compressed and ignited by spark plug as upward stroke comes to end. This results in an explosion of mixture which forces the piston to move downwards thereby producing power.
As there is a working stroke for each revolution, the power developed will be nearly twice that of a four stroke engine of the same dimensions and operating at the same speed. The work required to overcome the friction of the exhaust and suction strokes is saved.Two stroke engines are lighter than four stroke engines for the same power output and speed. For the same output, two stroke engines occupy lesser space.