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Tuesday, October 23, 2012

TRUNK PISTON VERSUS CROSS HEAD ENGINE

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Most medium and small size engines use trunk pistons. Resulting side thrust causes the piston to press against the cylinder wall, first on one side, then on the other. At the top of stroke, when the gas pressure is greatest, side thrust is negligible (due to small connecting rod angle). So most of wear takes place at the middle of stroke: making piston skirt increases thrust-bearing area, and hence reduces wear. In medium and small size engines, due to lower gas pressure, units’ side pressure is so small that neither piston nor liner wears much.
In crosshead engines, crosshead takes the side thrust, which will be high in large engines. So, crosshead engines have the following advantages:

  1. Easier lubrication.
  2. Reduced liner wear.
  3. Uniformly distributed clearance around piston.
  4. Simpler piston construction because the ‘Gudgeon pin’ and its bearing are eliminated.

However these advantages of cross head engines are offset by:
  1. Greater complication.
  2. Added weight.
  3. Added height.
  4. Careful adjustments. 

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