The 2-stroke petrol engine is different from the 4-stroke petrol engine. In a 2-stroke engine the inlet and exhaust ports are open and closed by the movement of the piston. But it is still an internal combustion engine and has the 5 events common to all such engines.
Intake occurs when the air-fuel mixture enters the crankcase. It is then transferred from the crankcase to above the piston.
Compression, forcing the mixture into a small volume.
Ignition when the spark from the spark plug ignites the mixture, and it burns.
Power, where energy released by combustion generates the force that turns the crankshaft.
And
Exhaust, removing leftover gases.
As in all internal combustion engines, these 5 events must occur, but not always in the same way.
The whole process is a cycle. A new mixture enters and is ignited. Combustion occurs. Expanding gases drive the piston down and turn the crankshaft which pushes the piston back up the cylinder.
In one 2-stroke cycle, the crankshaft does 1 revolution. In that 1 revolution how many strokes does the piston do? 2 strokes.
Out of those 2 strokes how many actually produce power?
In a 2-stroke cycle, only 1 stroke delivers energy to the crankshaft.
Calling an engine 2-stroke means its engine cycle has 2 strokes. As in a 4-Stroke engine, a stroke is the distance from top dead centre to bottom dead centre.
The piston moves up the cylinder from its lowest point, bottom dead centre, compressing air-fuel mixture already in the combustion chamber. As it rises, the top of the piston covers the exhaust port and the transfer port. This is a third port or passage between crankcase and cylinder. The bottom of the piston uncovers the inlet port. At the same time, the rising piston is increasing the volume of the crankcase below the piston. Pressure in there falls until it is less than outside air pressure. This pressure difference forces more mixture into the crankcase. As the piston reaches top dead centre, mixture already there ignites, and expansion of gases starts to drive the piston downward.
The piston moves down. The bottom of the piston covers the inlet port as the top of the piston uncovers the exhaust port. Opening the exhaust port starts the scavenging process. It releases the pressure above the piston and burned gases escape. At the same time, closing the inlet port means the descending piston is compressing the air-fuel mixture in the crankcase. The top of the piston uncovers the transfer port. Increased pressure in the crankcase forces fresh air-fuel mixture up through the transfer port into the cylinder, replacing the burned gases, and the cycle starts again.
Let’s run the cycle again. The piston moves up the cylinder. The top of the piston covers the exhaust port, the bottom uncovers the inlet port. Air-fuel mixture enters and circulates in the sealed crankcase below the piston. The piston rises and compresses the mixture above it.
A spark plug ignites it and it burns. Pressure from the expanding gases pushes the piston down in a power stroke. The top of the piston uncovers the exhaust port and lets exhaust gases escape. The bottom of the piston covers the inlet port and forces air-fuel mixture up into the now open transfer port. The piston reaches bottom dead centre, and the crankshaft forces it back up to repeat the process.
Notice which events occur above the piston. Compression of the air-fuel mixture into the combustion chamber. Combustion for the power stroke. And scavenging of the exhaust gases.
Notice the events that occur below the piston. Intake, into the crankcase. Pre - compression of the air-fuel mixture - in the crankcase. And the transfer of mixture from the crankcase, through the transfer port and into the combustion chamber.
2-stroke intake system
This intake system is called a piston - port system because the piston acts as a valve to cover and uncover the ports.
This intake system uses a rotary valve. The crankcase has a hole open to the crankcase chamber, and a rotary valve cover attached to the carburetor. The rotary valve is mounted on the crankshaft, between the crankcase and the valve cover. As the crankshaft turns, the rotary valve rotates with it, opening and closing the inlet port as it does so.
Some 2-stroke engines such as outboard motors use an intake system with a reed valve. It is a small flexible metal plate that covers the inlet port, and it can be attached to the crankcase, or to the inlet port. It opens and closes automatically, according to changes in pressure in the crankcase.
The 2-stroke engine makes only 2 strokes, 1 up and 1 down for each time it fires. That’s only 1 revolution of the crankshaft for each cycle.
Compare this with a 4-stroke that needs 2 revolutions of the crankshaft for each engine cycle and only 1 power stroke for each 4 engine strokes.
