In this article, we discuss two stroke spark ignition engines and the difference between two stroke spark ignition engines and four stroke spark ignition engines.
Two Stroke Spark Ignition Engines or Two Stroke Petrol Engines
In two stroke spark ignition engines, there is one power stroke in one cycle i.e. in two revolutions of the crankshaft.
In two stroke spark ignition engines the suction and exhaust strokes are eliminated. Also, there are no inlet and exhaust valves, instead of that these engines have inlet and exhaust ports.
A piston of these engines is given a specific crown shape which helps to prevent the loss of fresh incoming charge which can be escaped along with the gases through an exhaust port.
A two stroke spark ignition engine consists of the following strokes:
1. First stroke
2. Second stroke
1. First stroke
Initially, the piston is at BDC. The arrangement of the ports is such that the piston performs two operations simultaneously.
When the piston starts rising from BDC it closes the transfer port and exhaust port and the already existing charge is compressed.
At that time, a vacuum is created in the crankcase which is gas-tight.
As soon as the inlet port is uncovered the fresh air is sucked into the crankcase and the charging is continued until the crankcase is filled.
At the end of this first stroke, the piston reaches TDC.
2. Second stroke
In this stroke, the piston moves from TDC to BDC.
Before the completion of the compression stroke, the compressed charge is ignited using a spark plug and the gas pressure is exerted on the crown of the piston.
This pressure forces the piston in a downward direction which produces some useful work.
The downward movement of the piston closes the inlet port and compresses the charge already sucked into the crankcase. This fresh charge is transferred to the top side of the piston through the transfer port.
At that time, the exhaust port opens and burnt gases are escaped.
Difference between Two Stroke Spark Ignition Engines and Four Stroke Spark Ignition Engines
Two Stroke Spark Ignition Engines
1. The cycle is completed in one revolution of the crankshaft.
2. Power stroke is obtained in every revolution of crankshaft.
3. Due to one power stroke in one revolution, power produced for same size of engine is more.
4. In these engines ports are present.
5. Due to simplicity and light weight, its initial cost is low.
6. The piston is having crown shape or dome shape.
7. Low thermal efficiency.
8. It consumes more amount of lubricating oil.
9. During the operation it produces more noise.
10. These types of engines are used in scooters, mopeds, etc.
Four Stroke Spark Ignition Engines
The cycle is completed in two revolutions of the crankshaft.
The power stroke is obtained in every two revolutions of the crankshaft.
Due to one power stroke for two revolutions, the power produced for the same size engine is small.
Instead of ports, these engines contain valves.
Due to complications and heavy weight, its initial cost is high.
The crown of the piston is flat.
High thermal efficiency.
It consumes less amount of lubricating oil.
It produces less noise during the operation.
These types of engines are used in cars, buses, trucks, etc.