LO 11-2 Identify and explain the purposes and functions of the cooling system

Minimizing Cavitation Erosion

Diesel engine cooling systems must function to minimize the effects of a phenomenon called cavitation erosion . Cavitation erosion is a condition unique to diesel engines that causes destructive pitting of the cylinder walls and wet-type cylinder liners Figure 19. Cavitation erosion is caused by the collapse of tiny water vapor bubbles formed when coolant rapidly heats next to hot surfaces or when the coolant is subjected to a rapid drop in pressure. Liquid vaporizes in both of these situations, producing air bubbles. This phenomenon is easily observed while watching and listening to a pot of water sitting on the heating element of a stove. In a pot of cold water, bubbles first appear on the bottom of the pot where it is in contact with the hot heating element. Rising vapor bubbles collapse back into a liquid state when cooled by the surrounding cold water or when pressure increases around the vapor bubble. The snapping and popping sound initially heard from the cold pot on a hot element is evidence of vapor bubble collapse, which is the same process that can destroy cylinders in a diesel engine. Pressure exerted by the force of bubble collapse is reported to be as high as 60,000 psi (4137 bar). The force of implosion against cylinder walls, cylinder liners, water pump inlets, injector tubes, and other such components easily blasts holes into the components and eventual destroys them Figure 20. Cylinder liners have an exaggerated pattern of cavitation erosion on the major thrust surfaces because that particular liner surface flexes and relaxes during and after the pistons’ power stroke. The longer throw of a diesel engine crankshaft combined with higher cylinder pressure multiplies the pistons’ side thrust force, distorting cylinder liners and walls. This expansion–contraction vibration on the coolant side of a liner or cylinder wall creates a temporary low-pressure area, producing even more vapor bubbles and thus more cavitation erosion Figure 21. Chemical treatment of the cooling system is the preferred method for minimizing cavitation erosion. Both new and old formulations of antifreeze for diesel engines use nitrite, a critical coolant additive to minimize cavitation erosion. Today, most OEMs use and recommend the use of fully formulated long-life antifreeze containing nitrite at the point of production.

FIGURE 11-19: Cavitation erosion is unique to diesel engines. The right cylinder liner has holes in the major thrust side of the liner due to cavitation erosion.		FIGURE 11-20: Hot coolant vaporizes, forming a bubble on the side of the cylinder wall. The force of a collapsing vapor bubble is great enough to remove material from the cylinder wall.		FIGURE 11-21: Cavitation erosion is unique to diesel engines because of higher cylinder pressures and longer stroke lengths. Movement of the cylinder wall due to high combustion pressure and the thrust angle of the connecting rod create a momentary void between the coolant and the cylinder wall.