The two-stroke cycle is so called because it takes two strokes of the piston to complete the processes needed to convert the energy in the fuel into work. Because the engine is reciprocating, the piston must move up and down the cylinder, and therefore the crankshaft must revolve once.
The 4 strokes in the previous cycle are condensed into 2 strokes namely
1. compression stroke
This combines the action of INTAKE and COMPRESSION events.
2. Power stroke
This combines the action of POWER and EXHAUST events.
2-stroke engines carry out useful work
for each revolution of the crankshaft.
This means fuel and exhaust must function each revolution. The camshaft must rotate at the same speed as the engine crankshaft.
COMPRESSION STROKE
The piston would be revolving clockwise.
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| compression stroke |
The air charge would be compressed and thus the energy is transferred to the charge air, increasing its temperature.
POWER STROKE
Just before the piston reaches TDC, fuel is injected in a fine spray and the combustion products push down the piston producing power
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| power stroke |
So now one might wonder how the process of adding air charge and exhausting takes place?
The solution is strategically placing scavenge ports.
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| scavenging process |
once the piston uncovers the scavenge ports at the end of the power stroke with exhaust valves opened, the air charge from the scavenge manifold rushes into the cylinder pushing out the spent gases along with the introduction of fresh air to the cylinder.
thus the process of charge air pushing the spent gases out of the cylinder along with the induction of fresh charge air is known as scavenging.
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