Two-stroke Engine Basic Operation

The two-stroke engine is simple in construction, but complex dynamics are employed in its operation. A typical simple two-stroke contains a piston whose face is shaped, an exhaust port on one side of the cylinder, and a transfer port on the other side. The downward movement of the piston first uncovers the exhaust port, allowing most of the exhaust to be expelled, and then uncovers the transfer port through which an air-fuel mixture (the fuel normally has some oil mixed in) is let into the cylinder. The piston then moves upwards, compressing the mixture which is ignited by a spark plug, driving the piston back down.


In more detail:

Intake and compression

The rising piston creates a partial vacuum in the sealed crankcase. A connection (inlet port) between the crankcase and the carburetor is uncovered by the piston as it rises, and the air-fuel mixture is sucked into the crankcase. At the same time, the air-fuel mixture already in the cylinder is being compressed as the piston gradually moves up.


Steps of two-stroke cycle:

Expansion stroke:

The piston is at Top Dead Center (TDC)
Crank is at 0 or 360°.

In real engines the process is completed from 0 to 150° but in this model it is completed at 120°.


Intake/Compression stroke:

The piston moves from Bottom Dead Center (BDC) to TDC.

The intake port is opened and working substance flows in.

Intake gases move inside due to partial vacuum; also, blowers are used to push intake gases in.

The vacuum opens the reed valve (thin flexible sheets made of steel, glass fiber or even carbon fiber) allowing the mixture to enter the crankcase.

The air-fuel mixture already in the cylinder is compressed.
As the piston nears the top of the stroke, the ignition system ignites the charge in the combustion chamber.

In diesel engines, at 11-13° before TDC fuel is injected. Before that point, only air is compressed. Fuel is injected only in the last stage of compression.



Exhaust and scavenging process:

The piston moves from TDC to BDC.

At 120°, the exhaust port is opened and exhaust gases move out of the cylinder due to gas pressure.

After 10-40°, fresh scavenging gases are then let into the cylinder through the transfer port(s).

The air/fuel/oil mixture that was let into the cylinder pushes the exhaust out the exhaust port.

The piston, then, compresses the air/fuel/oil mixture and lets left over exhaust out.



Power and exhaust

When the piston reaches the top of its stroke, the mixture is ignited, and the piston is forced down by the rapidly expanding combustion gases.

As the piston descends, a hole in the side of the cylinder connected to the exhaust pipe (exhaust port) is opened, allowing the burned gases to escape.

Furthermore, the descending piston closes the inlet port and pressurizes the crankcase. This also pushes some mixture from the crankcase back to the inlet tract, causing the reed valve to close and preventing the mixture from entering the air filter.

The air fuel mixture is forced into passageways that connect the crankcase to the cylinder. Holes connecting these passages to the upper cylinder (transfer ports) are uncovered by the descending piston and air-fuel mixture is forced into the upper cylinder. The transfer ports are just a bit lower than the top of the exhaust port, so there is a period of time when fresh air-fuel mixture is coming in while exhaust is leaving. The incoming fresh charge assists in forcing the exhaust gas out.

As the piston reaches the bottom and then starts to rise again, the transfer ports are closed by the piston and the air/fuel mixture is compressed. The next cycle starts.



Design issues

A major problem with the two-stroke engine has been the short-circuiting of fresh charge from intake to exhaust which increases fuel consumption and emissions of unburned hydrocarbons. The cylinder ports and piston top are shaped to minimize this mixing of the intake and exhaust flows. Furthermore, a tuned pipe with an expansion chamber provides back pressure at just the right time to push fresh air-fuel mixture sneaking out the exhaust back in again.

The major components of two-stroke engines are tuned so that optimum airflow results. Intake and exhaust pipes are tuned so that resonances in airflow give better flow.

Two-stroke engines typically mix lubricants, two-stroke oil, with their fuel (either manually at refueling or by injecting oil into the fuel stream); this mixture lubricates the cylinder, crankshaft and connecting rod bearings. The lubricant is subsequently burned, resulting in undesirable emissions. An independent lubrication system from below, as is used in four-stroke designs, cannot be used in the above-described engine design, since the crankcase is being used to hold the air-fuel mixture.

This problem has been addressed in newer engines which employ gasoline direct injection, similar to diesel two-strokes.