2 stroke spark ignition cycle or 2 stroke petrol engine

Two / 2 Stroke Spark Ignition Cycle (Petrol Engine) Principle & Working Explained

What is 2 Stroke Spark Ignition Cycle?

Although, 2 stroke spark ignition cycle (2 stroke petrol) engines are not much in use in most of the countries, they are still part of the legacy of types of engines world over. A 2 stroke spark ignition cycle or 2 stroke petrol engine differs from a 4 stroke petrol engine by the way it generates power in number of revolutions of the crankshaft. Unlike in a 4 stroke petrol engine, the 2 stroke spark ignition cycle produces power in each of its revolution. The 2 stroke spark ignition gives way to the traditional four strokes to run its complete cycle. Instead, it merges two strokes into one; thereby making one upward stroke and one downward stroke in every revolution of the crankshaft.

Hence, the 2 stroke spark ignition cycle engine generates power during each downward stroke of the piston. Thus, it produces double the power output as compared to a 4 stroke cycle engine of the same size. However, the efficiency of the 2 stroke cycle petrol engine is lower as compared to a 4 stroke petrol engine of the same size. The 2 stroke spark ignition cycle eliminates the suction and exhaust strokes. It uses the only two remaining strokes which are the compression stroke and power stroke. These are known as the upward stroke and downward stroke respectively.




Upward Stroke in 2 Stroke Spark Ignition Cycle:

During upward stroke of a 2 stroke spark ignition cycle, the piston moves upward i.e. from the bottom dead centre to the top dead centre. While moving up, it compresses the air-fuel mixture in the combustion chamber. The upward movement of the piston creates a partial vacuum in the crank case. This results in a fresh charge (air-fuel mixture) to come into the crank case thru’ the open inlet port.

Upward Stroke in 2 Stroke Spark Ignition Cycle
Upward Stroke in 2 Stroke Spark Ignition Cycle

When the piston is at the top dead centre, it covers / closes the exhaust and transfer ports. The spark plug ignites the compressed charge in the combustion chamber and produces the power stroke. The power stroke pushes the piston downward and rotates the crank shaft.




Downward Stroke in 2 Stroke Spark Ignition Cycle:

As soon as the spark plug ignites the charge in a 2 stroke cycle, the hot gases expand and push the piston downward; rotating the crankshaft. During this stroke, the piston covers the inlet port and compresses the new charge (air-fuel mixture) in crank case.  Further downward movement of the piston first uncovers the exhaust port followed by the transfer port. This allows the exhaust gases to escape through the open exhaust port.

Downward Stroke in 2 Stroke Spark Ignition Cycle
Downward Stroke in 2 Stroke Spark Ignition Cycle

No sooner the transfer port opens; it forces the fresh charge into the cylinder. The charge first strikes the deflector on the piston crown, rises to the top of the cylinder and pushes out the remaining exhaust gases. Some part of the fresh charge also escapes through the exhaust during this process. Because of this reason, the 2 stroke spark ignition cycle does not meet with the stringent emission norms. Hence, it was later discontinued and / or replaced with 4 stroke cycle.

Now, the piston is at the bottom dead centre. The cylinder is completely filled with fresh charge and somewhat diluted with the remaining exhaust gases. The engine burns this charge during the next upward stroke. This cycle repeats itself and the piston makes two strokes for each revolution of the crankshaft.

Most earlier-generation motorcycles, bikes, scooters and small gen-sets used the 2 stroke spark ignition cycle engine.

Watch a 2 stroke spark ignition cycle engine in action here:

Continue Reading: How a Four Stroke Petrol Engine Works.>>

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