In four-stroke cycle engines, there is only 1 Power stroke in every two revolutions of the crankshaft or in 1 cycle of 4 strokes of the piston. The desire of one working stroke in every revolution of the crankshaft has led to the development of 2 Stroke Engine.
In 1838, an Englishman called ‘Barnett’ described the mechanism of supplying a charge to the engine cylinder by means of separate pumps, In 1878, Dugald Clerk also gives a lot of contribution in this direction: and described a 2-stroke cycle known as Clerk cycle. Two-stroke cycle engines are widely used for small powers required in autocycles, scooters, and motorcycles.
In two-stroke engines, the suction and exhaust strokes do not happen. There are only 2 remaining strokes – the compression stroke and the power stroke, and these are usually called the upward 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 the fresh charge which enters the engine cylinder nearly at the end of the power stroke through the inlet port.
2 Stroke Cycle, Spark Ignition (Petrol) Engine
The principle of the two-stroke cycle spark ignition engine is shown in Fig.3.14. Its two strokes are described as follows:
During the upward stroke, the piston moves upward from bottom dead center to top dead center. Compressing the charge- (air-fuel mixture) in the combustion chamber of the cylinder. Because of the upward movement of the piston, a partial vacuum is created in the crankcase and a fresh charge is enters into the crankcase through the 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 of engine compress the piston. It causes the piston to move downward, thus rotating the crankshaft and 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 piston uncovers first the exhaust port and then the transfer port, and hence the exhaust starts through the exhaust port. When the transfer port opens, the charge through it is entered into the cylinder. The charge strikes the deflector on the piston crown, and rises to the top of the cylinder thus push 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 has then repeated the piston making two strokes for each revolution of the crankshaft.
How does a 2 Stroke Engine work?
2 Stroke Cycle, Compression Ignition (DIESEL) Engine
In the 2-stroke compression ignition engine, only air is compressed inside the engine cylinder and the fuel(diesel) is injected by an injector that fitted in the head of the cylinder. There is no spark plug in this engine. The remaining process of the 2-stroke compression ignition engine is exactly the same as those of the spark-ignition engine.
Advantages Of 2 Stroke Cycle Engine Over 4 Stroke Cycle Engine
- The 2-stroke engine gives 1 working stroke for each revolution of the crankshaft. The 4-stroke engine gives 1 working stroke for every two revolutions of the crankshaft. Hence theoretically, the power developed by two-stroke cycle engine is twice that developed by four-stroke cycle engine for the same engine speed and cylinder volume.
- The turning moment on the crankshaft is more even in the two-stroke cycle engine due to one working stroke for each revolution of the crankshaft, and so a lighter flywheel is required in it.
- For the same power, a 2-stroke engine is more compact, lightweight and requires less space than a 4-stroke engine. Therefore, it is more suited for autocycles, motorcycles, and scooters.
- A two-stroke cycle engine is simpler in mechanism and construction. There is no valve and valve the mechanism in it. The ports are easy to design and they are covered and uncovered by the movement of the piston itself.
- It has high mechanical efficiency because of the absence of cams arrangements crankshaft and rockers, etc of the valves.
- It gives less torsional oscillations.
- It requires less spare parts due to its simple design.
- It can be easily reversed if it is of valves less type.
- There is a saving of energy required to overcome the frictional losses of the inlet and exhaust port.
- In the 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.
- 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.
- The charge is diluted by the burnt gases due to incomplete scavenging.
- It gives greater noise.
- It consumes more lubricating oil.
- There are more wear and tear due to moving parts.