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

Cavitation and Engine Damage
Cavitation erosion is particularly destructive to diesel engines because it can produce pin-size holes in the cylinder wall, injector tubes, and other coolant surfaces Figure 22. Cylinder wall destruction is a serious problem because it allows coolant to leak into the cylinders or compression gases to leak into the cooling system. Hydrostatic lock, a condition in which the upward movement of a piston is stopped due to the presence of liquids in a cylinder, is often caused when pressurized coolant is pushed through these holes and into the cylinder during the engine’s hot-soak period (the short time period when a hot engine is cooling down but has increased coolant temperature and pressure after it is shut off due to a loss of coolant circulation). If the cavitation erosion creates a hole in the cylinder below the point where the piston normally rests, coolant leaks into the crankcase, causing an internal leak and the complaint of coolant loss.

FIGURE 11-22: An injector tube damaged by cavitation erosion.

FIGURE 11-23: Diesel engine coolant is made up of water, antifreeze, and supplemental coolant additive or diesel coolant additive, which contains nitrite.

FIGURE 11-24: A coolant filter that uses powdered diesel coolant additive containing nitrite.

Parent bore engines, or engines without liners, are not immune to the problem of cavitation erosion contrary to some OEM claims. Sharp increases in combustion pressure in parent bore engines create the same temporary pressure voids on the coolant side of the block’s cylinder walls as wet sleeve engines, resulting in cavitation erosion. For this reason, all diesel engine cooling systems need to use special conditioners to prevent or minimize cavitation erosion. There are two categories of coolant additives, also called conditioners: organic acid technology (OAT) and inorganic additive technology (IAT) Figure 23. IAT is commonly marketed using the terms supplemental coolant additive (SCA) and diesel coolant additive (DCA). Both OAT and IAT were developed primarily to treat or condition the cooling system to prevent cavitation erosion. The most effective additive that prevents cavitation erosion is nitrite. Nitrite, when added to coolant, forms a thin, protective film on the coolant side of the engine cylinder walls and liners, as well as other surfaces, to reduce cavitation erosion. Vapor implosion in an engine protected by nitrite will only damage the nitrite layer, which can be replenished from the nitrite dissolved in the coolant Figure 24. Nitrite also combines with the iron in liners and block walls to form very hard, micro-thin iron oxide alloys that minimize damage from vapor implosion. Regardless of the formulation of the antifreeze used in a diesel engine, cooling system maintenance must regularly schedule the measurement of the level of nitrite to evaluate the level of this important additive. In older green-colored type I antifreeze, there is in fact a recommended service interval for testing and adding a prescribed amount of DCA or SCA. Newer antifreeze formulations do not need additions of nitrite as frequently, if at all, as type I antifreeze.