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LO 7-4 Explain the purpose and methods for cooling engine pistons

1. Piston Cooling

1.1. Piston Cooling

LO 7-04: Explain the purpose and methods for cooling engine pistons.

 

More pistons fail due to heat damage when an engine overheats because of a lack of coolant. In that instance, the piston skirt and rings cannot transfer heat from the crown to a cooler cylinder wall. Excessive skirt expansion causes the piston operating clearances to disappear, which then causes the piston to seize in the cylinder bore or score the cylinder wall. Metal-to-metal transfer takes place between the cylinder wall and the piston during an overheating failure, and only the skirt is damaged, not the crown Figure 26.

  Technicians Tip  
  Whenever cylinder components are removed, always remove the piston cooling nozzle first. Failure to first remove these nozzles may allow damage or misalignment of the nozzle to take place during service, resulting in an improperly cooled piston. Incorrectly aimed nozzles and plugged or restricted nozzles will result in an overheated piston. In fact, operating an engine with a misaligned or damaged piston cooling nozzle results in piston failure in under 20 minutes when an engine is operated under load.  
  Fig07_26

FIGURE 07-26: This piston skirt was scored due to a coolant overheating failure.

 

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