Two-Stroke Petrol Engine | Operation, Advantages, Disadvantages.


Two-Stroke Engine- 

In four-stroke cycle engines, there is one working stroke in two revolutions of the crankshaft or in a cycle of four strokes of the piston. The desire for one working stroke in every revolution of the crankshaft has led to the development of two-stroke cycle engines. 

In 1838, Barnett, an Englishman, described the mechanism for supplying a charge to the cylinder by means of separate pumps,  In 1878, Dugald Clerk also made a lot of contributions in this direction, and described a two-stroke stroke cycle known as the Clerk cycle. Two-stroke cycle engines are widely used for small powers required in auto cycles, scooters, and motorcycles.

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.


The two-stroke engine has invented a long time ago. It has great power because of the engine crankshaft revolves two times and makes power. Whereas the four-stroke engine completes the four revolutions of the crankshaft to produce one power stroke. In two-stroke engines, we need to mix lubricating oil with the fuel to lubricate all the engine parts. The crankshaft has a scoop-type design which through the lubricant to the cylinder body for better operation. Mainly the power stroke happens in two different strokes. Its two strokes are described as follows: 

Upward Stoke- 

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.

Downward Stroke- 

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- 

In nowadays two-stroke engines are not on market. But the engines have great power along with great noise. Here are some of the advantages and disadvantages discussed- 


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. 

9. There is a saving in work required to overcome the friction of the inlet and exhaust port.


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.

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