In two-stroke cycle engines, the suction and exhaust strokes are eliminated. There are only two remaining strokes — the compression stroke and power stroke; and these are usually called the upward stroke and downward stroke respectively. Also, instead of valves, there are inlet and exhaust ports in two-stroke cycle engines. The burnt exhaust gases are forced out through the exhaust port by a fresh charge which enters the cylinder nearly at the end of the working stroke through the inlet port.
During the upward stroke, the piston moves upward from the bottom dead center to the top dead center, compressing the charge—air petrol mixture in the combustion chamber of the cylinder. Due to the upward movement of the piston, a partial vacuum is created in the crankcase and a new charge is drawn into the crankcase through the uncovered inlet port. The exhaust port and transfer port are covered when the piston is at the top-dead-center position. The compressed charge is ignited in the combustion chamber by a spark given by the spark plug.
As soon as the charge is ignited the hot gases compress the piston which moves downward, rotating the crankshaft thus doing the useful work. During this stroke, the inlet port is covered by the piston and the new charge is compressed in the crankcase. Further downward movement of the piston uncovers first the exhaust port and then the transfer port; hence the exhaust starts through the exhaust port. As soon as the transfer port opens, the charge through it is forced into the cylinder. The charge strikes the deflector on the piston crown, rises to the top of the cylinder, and pushes out most of the exhaust gases. The piston is now at the bottom dead center position. The cylinder is completely filled with the fresh charge, although it is somewhat diluted with the exhaust gases. The cycle of events is then repeated, the piston making two strokes for each revolution of the crankshaft.
Advantages and Disadvantages-
1. The two-stroke cycle engine gives one working stroke for each revolution of the crankshaft. The tour-stroke cycle engine gives one working stroke for every two revolutions of the crankshaft. Hence theoretically, the power developed by a two-stroke cycle engine is twice that developed by a four-stroke cycle engine for the same engine speed and cylinder volume.
2. The turning moment on the crankshaft is more even in a two-stroke cycle engine due to one working stroke for each revolution of the crankshaft, so a lighter flywheel is required in it.
3. For the same power, a two-stroke cycle engine is more compact, and light and requires less space than a four-stroke cycle engine. Therefore, it is more suited for auto cycles, motorcycles, and scooters.
4. A two-stroke cycle engine is simpler in construction and mechanism. There is no valve and valve mechanism in it. The ports are easy to design and they are covered and uncovered by the movement of the piston itself.
5. It has high mechanical efficiency due to the absence of cams, crankshafts, rockers, etc., of the valves.
6. It gives less torsional oscillations.
7. It requires fewer spare parts due to its simple design.
8. It can be easily reversed if it is of valveless type.
1. In a two-stroke cycle, Otto engine, the fuel consumption is high because the fresh charge is likely to be wasted by escaping through the exhaust port.
2. The actual compression starts when the ports are completely closed by the upward movement of the piston after a few degree revolutions of the crankshaft. Thus, the actual compression ratio and hence the thermal efficiency of the two-stroke cycle engine is less than that of the four-stroke cycle engine for the same dimensions.
3. The charge is diluted by burnt gases due to incomplete scavenging.
4 It gives greater noise.
5. It consumes more lubricating oil.
6. There is greater wear and tear of moving parts.