The 4 Stroke Engine Cycle - EngineKnowHow

The 4 Stroke Engine is the most common form of engine today and occurs over four “strokes” of the cylinder by the piston. These four strokes also result in two rotations of the crankshaft per delivery of power. The 4 Stroke Cycle is also known as the Otto Cycle after Nikolaus Otto who sold the first engine operating with these principals.

 

The 4 Strokes
The four strokes of the piston within the cylinder comprise the:

Richard Wheeler (Zephyris), https://commons.wikimedia.org/wiki/File:4StrokeEngine_Ortho_3D_Small.gif

The 4 Stroke Engine Cycle

  1.  Intake Stroke – The piston travels down the cylinder drawing a fresh charge of air, or air mixed with fuel into the cylinder. This fresh charge enters the cylinder via an inlet valve which opens with the piston near the top of the cylinder and closes with the piston near the bottom of the stroke.
  2. Compression Stroke – The piston travels back up the cylinder with the intake and exhaust valves closed compressing the fuel / air mixture until ignition, either via spark or compression. The beginning of combustion occurs prior to the piston reaching the top and cylinder pressure begins to increase rapidly.
  3. Power / Expansion Stroke – The piston starts to travel down the cylinder under force from the increasing combustion pressures and temperatures until it nears the bottom of the cylinder and the exhaust valve opens. This valve opening allows the combustion products to begin to exhaust from the cylinder with this phenomena known as “Blow Down”.
  4. Exhaust Stroke – As the piston starts to travel back up the cylinder the combustion products exit the cylinder due to their high pressures and the upward movement of the piston continues to force the exhaust gases out of the cylinder. With the piston near the top of the cylinder the exhaust valve closes and the 4 Stroke Cycle repeats itself.

 

The benefits of the 4 stroke cycle are:

  • Low cost
  • Robust and reliable
  • High power density
  • Relatively high thermal efficiency

 

The 2 Stroke Engine Cycle

Also enjoying worldwide use is the two stroke engine cycle where the intake / compression strokes and the expansion / exhaust strokes are combined and power delivery occurs every crankshaft revolution.

 

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The 2 Stroke Engine Cycle

 

An alternative internal combustion engines which operates on the 4 Stroke Engine Cycle is the Wankel engine, a type of rotary internal combustion engine.

Source: Y tambe, https://upload.wikimedia.org/wikipedia/commons/f/fc/Wankel_Cycle_anim_en.gif

The Wankel Engine