What Is Two-stroke Engines
TWO-STROKE ENGINES The two strokes of the cycle are completed once during each revolution of the crankshaft. Diagram below represents a two-stroke SI engine with crankcase compression. As the piston moves upwards in the cylinder and covers both the transfer port (T) and exhaust port (E), there is compression of the charge into the cylinder. At the same time in the crankcase the charge expands and the pressure is reduced below the atmospheric pressure. So, fresh charge enters into the crankcase through the spring-loaded automatic valve (S). Ignition occurs before the TDC. The gas pressure rises rapidly and pushes the piston downwards to produce power.
As the piston descends through about 80 per cent of the working stroke, the exhaust port (E) is uncovered by the piston and exhaust begins. The transfer port (T) is uncovered a little later in the stroke due to its position in relation to the exhaust port (E). The charge in the crankcase, which has been compressed by the descending piston, enters the cylinder through the transfer port (T).
The piston is shaped to deflect the fresh charge upwards in the cylinder and prevent its escape through the exhaust port. As the piston rises, the transfer port is closed slightly before the exhaust port and after the exhaust port is closed, compression of the charge in the cylinder begins. The cycle is then repeated. The pressure diagram and the timing diagram for a two-stroke SI engine are shown in Figures Diagram (a) and diagram (b) respectively.
The description of the two-stroke cycle as given in this section also app with the exception that only air is compressed and the spark plug 18 replaced A higher compression ratio is also used in the case of CI engines
Diagram (a) P_ V Diagram Of A Two Stoke Engine
Diagram (b) Timing diagram of a two Stoke engine