2 stroke or 4 stroke

2 stroke or 4 stroke

A stroke is a complete motion of the piston from the Top Dead Center(TDC) to Bottom Dead Center (BDC) or Vice Versa .
The TDC and BDC are the two extremes of the cylinder . In other words the 180 degree movement of the crankshaft is a stroke.
To get a full understanding please see the below animation.
 
4 stroke engine:
The above is a 4 stroke engine because the crank shaft makes 720 degree rotation to complete one thermodynamic cycle.
The crank is the part which is moving in a circular motion in the lower part of the picture.

 
 
2 stroke engine :
 As you can see the 2 stroke engine makes only 360 rotation of crack shaft to complete the thermodynamic cycle.
 
 

 
To know what is a Thermodynamic cycle and different type of cycles follow the link below.
Thermodynamic cycle

A 2 stroke engine works by having each cylinder (and the piston sliding inside it),  complete 2 strokes (which means one turn for the crankshaft).
Namely:
-Ascending trajectory of the piston, which effectuates both the compression and ignition (when the piston is at it's maximum height).
-Descending trajectory of the piston powered by the combustion started with the spark fo the previous stroke, which effectuates two operations at the same time:

1. The exhaust of combustion remains.
2. The admission of oil-gas-air mixture which also lubricates the cylinder and crankshaft chamber;
   

A 4 stroke engine works by having each cylinder(and the piston sliding inside it) complete -guess what- complete 4 strokes (which means two turns for the crankshaft).
Namely:

1. First ascension: Exhaust of combustion remains.
2. First descent: admission of air and combustion fluid (depends on your engine architecture whether or not these fluids are under high pressure, or at atmospheric pressure).
3. Second ascension: compression of the fluids (with or without a spark at the maximum point whether you are working with a petrol engine or diesel engine).
4. Second descent: Power.

The real power genereated by each cylinder is the difference between the power generated by the combustion, and the power used to make that combustion happen.