2-Stroke Engine

Above: Timing Diagram for a 2-stroke Engine Cycle 

Stages	Piston Motion	Temp. Of Compressed Air (°C)	Pressure of Compressed Air (Bar)	Exhaust Valve	Scavenge Port
Compression	Upwards	550	80-90	Closed	Covered
Combustion	Downwards	-	-	Closed	Covered
Expansion	Upwards	1700	130-160	Closed	Covered
Exhaust & Scavenge	Downwards	-	-	Open	Uncovered

Compression Stroke: 

·         Fresh air is compressed from BDC to TDC (80-90bar) with its temperature raised to around 550°C.

·         Scavenge port are just covered, exhaust valve is closed and fuel injector is shut.

Combustion stroke:

·         Fuel oil is injected into the hot air, igniting it.

Combustion Period

  Rapid Combustion Period:

·    Between injection and the start of ignition.

·    Initial droplets enters the cylinder are being heated up by the surrounding charge air and starts to vapourise and ignite.

    Rapid Combustion Period:

·    Fuel that was accumulated during the ignition delay period starts to burn rapidly due to the pre-mixed combustion.

·    This period will be accompanied by a sharp rise in cylinder pressure and fuel injection still continues during this period.

    Steady Burning Period:

·    Combustion has been achieved and the fuel entering the cylinder will burn as soon as they have penetrated, heated, mixed and vapourised.

·    Cylinder pressure rises to its maximum value just after TDC, near the middle of the steady combustion period, and decreases gradually before expansion stroke begins.

   After Burning Period:

·    Combustion continues until the remaining fuel is burnt despite the fact that the injection stops.

Expansion Stroke:

·         After combustion, the air pressure goes up to 130-160bar with its temperature reaching to about 1700°C.

·         Piston is pushed downwards by the force from the expansion of gases.

·         Gases fall in pressure as they expand.

Exhaust & Scavenge:

·         Exhaust valve open towards the end of expansion stroke.

·         Due to differential pressure during blow-down period, exhaust gases are pushed out of the cylinder into exhaust manifold.

·         Once scavenge ports are uncovered, the scavenge air pushes the remaining  gases out of cylinder and filling it up with fresh air for the next cycle.