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

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

Piston Cooling

The high cylinder temperatures and pressures in diesel engines place high heat loads on pistons. To prevent piston failure, several mechanisms are used to transfer heat away from the pistons. First, piston rings can transfer some heat to the cylinder walls. As much as one-third of the heat can be removed from pistons in this way. Heat is also removed from pistons during the valve overlap period. During the valve overlap period, when the intake and exhaust valves are both open at the end of the exhaust stroke and the beginning of intake, the fresh, cool air mass flowing out of the exhaust valve also passes over and cools the piston crown. An engine with a restricted air filter or an inadequate valve overlap period will cause the crown to crack from excessive heat Figure 23. Oil sprayed at the bottom surface of the crown provides the most cooling of diesel engine pistons. Passageways formed in the pistons also help transfer more heat to the oil Figure 24. Heat absorbed by oil is transferred to the cooling system through an oil cooler, and the directional aim of the oil cooling nozzles to the piston is critical to ensure adequate heat transfer Figure 25. One or two cooling nozzles may be used. If only one piston in an engine overheats, it may indicate a misaligned, damaged, or blocked cooling nozzle.

FIGURE 07-23: This piston crown cracked due to inadequate cooling during valve overlap. The engine this piston was in had a restricted air filter.

FIGURE 07-24: Cavities and passageways inside the piston allow oil to collect and circulate to remove heat from the piston.

FIGURE 07-25: Oil cooler nozzles spray lubricating oil to the underside of the piston crown, removing excess heat.

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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.

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

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