Most reciprocating piston internal combustion engines work on one of two mechanical cycles—the four-stroke cycle or the two-stroke cycle. Marine diesel engines work in the same manner. The cycles designate, in correct sequence, the mechanical actions by which (a) the fuel and air gain access to the engine cylinder, (b) the gas pressure (due to combustion) is converted to power, and (c) the burnt gas is expelled from the engine cylinder.

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The basic four-stroke cycle marine diesel engine

From the name, it is obvious there are four strokes in one complete engine cycle. A stroke is the movement of the piston through the full length of the cylinder, and, since one such movement causes the crankshaft to rotate half a turn, it follows that there are two crankshaft revolutions in one complete engine cycle. The four strokes, in correct order, are:

  1. The inlet stroke. With the inlet valve open and the exhaust valve closed, the piston moves from top dead center (TDC) to bottom dead center (BDC), creating a low-pressure area in the cylinder. Clean, filtered are rushes through the open inlet valve to relieve this low-pressure area, and the cylinder fills with air.
  2. The compression stroke. With both valves closed, the piston moves from BDC to TDC, compressing the air. During this stroke the air becomes heated to a temperature sufficiently high to ignite the fuel.
  3. The power stroke. At approximately TDC, the fuel is injected, or sprayed, into the hot, compressed air, where it ignites, burns and expands. Both valves remain closed, and the pressure acts on the piston crown, forcing it down the cylinder from TDC to BDC.
  4. The exhaust stroke. At approximately BDC the exhaust valve opens and the piston starts to move from BDC to TDC, driving the burnt gas out of the cylinder through the open exhaust valve.

For more help with your marine diesel engine grab your copy of “Marine Diesel Engines For Beginners

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