Two Stroke Cycle Engines
A two stroke cycle engine is a type of internal combustion engine which completes a power cycle with two strokes of the piston during only one crankshaft revolution.
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 of one working stroke in every revolution of the crankshaft has led to the development of two stroke cycle engine.
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 contribution in this direction and described a two stroke cycle known as Clerk Cycle.
Two stroke cycle engine employs for small powers required in auto cycles, scooters, motorcycles.
In two stroke engines, there is no suction and exhaust strokes. There are only two remaining strokes the compression stroke and power stroke. These are usually called the upward stroke and downward stroke. Also, instead of valves, there are inlet and exhaust ports in two stroke cycle engines.
Fresh charge enters the cylinder at the end of the working stroke through the inlet port. And then burnt exhaust gases are forced out through the exhaust port by a fresh charge.
Two strokes Cycle Spark Ignition (Petrol) Engine.
The principle of two stroke cycle spark ignition engine is shown in the figure. Its two strokes are as follows:
- Upward Stroke
- Downward Stroke
During upward stroke, the piston moves upward from the bottom dead centre to top dead centre. By compressing the charge air petrol mixture in the combustion chamber of the cylinder.
Due to 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 centre 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. and hence the exhaust starts through the exhaust port.
As soon as transfer port is open, 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 centre position.
The cylinder is completely filled with fresh charge, although it is somewhat with the exhaust gases. The cycle of events is then repeated, the piston making two strokes for each revolution of the crankshaft.
The figure shows a port diagram for a two stroke cycle petrol engine. which is self-explanatory.
A figure shows the shape of the p-v diagram for a two stroke cycle petrol engine. This diagram is only for the main cylinder or top side of the piston.
Two Stroke Cycle Compression Ignition (Diesel) Engine.
In this two stroke cycle engine only air is compressed inside the cylinder. and the fuel (diesel) is injected by an injector fitted in the head of the cylinder.
There is no spark plug in this engine. The remaining operations of the two stroke cycle compression ignition engine are exactly the same. as those of the spark ignition engine.
Port timing diagram for a two stroke cycle diesel engine.
The figure shows the cylinder pressures and temperatures acting on spark plug for two strokes and four stroke cycle engines.
Advantages of Two Stroke Cycle Engine:
Advantages of two stroke cycle engine over four-stroke cycle engine.
- The two stroke cycle engine gives one working stroke for each revolution of the crankshaft. The four-stroke cycle engine gives one working stroke for every two revolutions of the crankshaft. Hence , 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 two stroke cycle engine. Due to one working stroke for each revolution of the crankshaft, and so it need a lighter flywheel in it.
- For the same power, a two stroke cycle engine is more compact, light and requires less space than a four-stroke cycle engine. Thus it is more suited for auto-cycle, motorcycles and scooters.
- 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 cover and uncover by the movement of the piston itself.
- It has high mechanical efficiency due to the absence of cams, crankshaft and rockers, etc., of the valves.
- It gives less torsional oscillations.
- Two stroke engine requires fewer spare parts due to its simple design.
- It can be reversed if it is of valveless type.
- There is a saving in work required to overcome the friction of the inlet and exhaust port.
- In 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 crankshafts. Thus, the actual compression ration and hence the thermal efficiency of 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 greater wear and tear of moving parts.
Comparison between the four stroke engine and the two stroke engine.
- In four stroke engine, one working stroke for every two revolutions of the crankshaft. But in a two stroke engine, One working stroke for each revolution of the crankshaft.
- In a four stroke engine, the turning moment on the crankshaft is not even due to one working stroke for every two revolution of the crankshaft. hence it requires heavy flywheel and the engine runs unbalanced. But in the two stroke engine, the turning moment on the crankshaft is more even due to one working stroke for each revolution of the crankshaft. Hence it requires lighter flywheel and the engine runs balanced.
- The four stroke engine is heavy but the two stroke engine is light.
- Four stroke engine design is complicated but the design of the two stroke engine is simple.
- Cost of the four stroke engine is high whereas in the two stroke engine it is low.
- Less mechanical efficiency due to more friction on many parts of the four stroke engine. whereas in the two stroke engine the mechanical efficiency is more due to less friction on a few parts.
- In a four stroke engine, the output is more due to full fresh charge intake and full burnt gases exhaust. But in the two stroke engine, the output is low due to the mixing of fresh charge with the burnt gases.
- A four stroke engine runs at cold but the two stroke engine runs at hot.
- The four stroke engine is water cooled whereas the two stroke engine is air cooled.
- Less fuel consumption in the four stroke engine but in the two stroke engine, fuel consumption is more.
- The four stroke engine requires more space but the two stroke engine requires less space.
- Lubricating system is complicated in four stroke engine. whereas, in the two stroke engine, the lubricating system is simple.
- The four stroke engine produces less noise but in the two stroke engine, the noise is more.
- A four stroke engine is used in cars, buses, trucks. but the two stroke engine is used for mopeds, scooters, motorcycles.
- The four stroke engine consists of inlet and exhaust valves. whereas the two stroke engine is consists of inlet and exhaust ports.
- More thermal efficiency in the four stroke engine. but the two stroke has a low thermal efficiency.
- The four stroke engine consumes less lubricating oil. but the two stroke engine consumes more lubricating oil.
- Less wear and tear of moving parts in the four stroke engine. whereas in the two stroke engine, greater wear and tear of moving parts.
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