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

Site: CDX Learning Systems
Course: DE Section 2-1: Engine Blocks and Cylinder Heads
Book: LO 11-2 Identify and explain the purposes and functions of the cooling system
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Date: Thursday, 5 November 2026, 7:44 PM

Table of contents

1. Purposes and Functions of the Cooling System

LO 11-02: Identify and explain the purposes and functions of the cooling system.

 

Purposes and Functions of the Cooling System

Not all the heat produced from combustion events is converted to mechanical energy. Cylinder components, the block, and the cylinder head are exposed to direct combustion heat exceeding 4000° F (2204° C). Without a method to remove the heat absorbed by these components, an engine is quickly destroyed. It is not only components exposed to combustion heat that require cooling but also other systems that either use coolant or require coolant circulation, all of which add to the heat loads of the cooling system. Additional sources that add to the coolant system heat load include Figure 3:

  • Engine oil
  • Air compressors
  • Pressurized charge air
  • Turbocharger bearings
  • Exhaust gas recirculation (EGR)
  • Transmission oil and fuel cooling
  • Engine control units
  • Electric drive motors, fuel cells, and batteries for electric drive vehicles
   Fig11_3

FIGURE 11-3: Heat-loading components and systems of an engine transferring heat to the cooling system.

 

Some combustion heat is released into the passenger compartment through heat exchangers or to help warm up cold fuel. Whether they add or subtract from the heat loads on the cooling system, these engine support systems depend on its proper operation to enable the engine to perform all its functions well and to deliver long and reliable service. These next sections examine the functions the cooling system performs and outline how each is accomplished in order to provide a better understanding of cooling system maintenance and inspection procedures.

 

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1.1. Video: Cooling System

LO 11-2 Video: Cooling System

 

 

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1.2. Removing Excess Heat

LO 11-02: Identify and explain the purposes and functions of the cooling system.

 

Removing Excess Heat

The flame temperature of diesel fuel is approximately 3900° F (2149° C). Under pressure, the heat of combustion momentarily but regularly exceeds this temperature. Sustained operation of the engine at temperatures like this without some means to remove the heat would quickly result in damaged and destroyed engine components. Operating clearances between moving engine parts would disappear and cause engine seizure. Components such as pistons and valves would burn or soften and would become permanently distorted under the high temperatures. Lubrication oil would also burn upon contact with hot engine components. The oil viscosity would also become too thin to properly support bearing loads and lubricate parts.

Some combustion heat is radiated by hot engine surfaces and removed by air circulation in the engine compartment. A significant amount of heat is also emitted by the exhaust system. However, to prevent engine damage caused by the transfer of heat into engine components, cooling system coolant absorbs heat to enable its transfer from the engine to the atmosphere through the radiator Figure 4. Some of the coolant heat is repurposed to warm up passenger compartments, which is a particularly important function on buses.

   Fig11_4

FIGURE 11-4: Cooling system components and coolant circulation patterns through a typical 16 L, HD diesel engine.

 

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1.3. Calculating Thermal Efficiency

LO 11-02: Identify and explain the purposes and functions of the cooling system.

 

Calculating Thermal Efficiency
In today’s engines, more than half of the heat energy produced from combustion is wasted through heat radiated from engine surfaces and dissipated from the engine through the cooling system and exhaust gases. The measure of an engine’s ability to convert fuel’s energy content into mechanical force is called thermal efficiency Figure 5. Thermal efficiency is measured as a percentage of the potential energy content in fuel compared with the engine’s ability to convert the fuel energy into mechanical energy. A simplified way of stating thermal efficiency would be to compare the mechanical energy available at the flywheel to the cost of fuel. If $100 of fuel is burned by an engine and only $40 of mechanical energy is produced at the flywheel, the engine would have 40% thermal efficiency. Consider another example: In 2010, the best on-highway, HD diesel engines operated at approximately 45% thermal efficiency, meaning 45% of the fuel energy released during combustion was converted into mechanical energy and made available at the flywheel. The remaining energy was absorbed by the cooling system (30–35%), emitted by the exhaust system (20–25%), or radiated to the atmosphere through hot engine parts (5%). Friction and the energy required to turn accessories and move gases into and out of the engine accounted for the remaining losses in thermal efficiency.

   Fig11_5

FIGURE 11-5: Thermal efficiency is highest in a diesel engine. Newer diesels convert as much as 45–50% of their heat into force. Since 2016, diesel engines have been expected to meet phased-in targets for GHG emissions standards but will eventually need to achieve more than 60% thermal efficiency.

 

Larger, slow-speed, industrial diesel engines used on ships and for electric power generation can operate at more than 50% thermal efficiency. GHG emissions standards for HD diesels increase engine efficiency beyond this by requiring a 17% increase in thermal efficiency for diesel engines by 2025 from the 2010 base levels. This stretch goal of 17% is not set because diesels are inefficient. The thermal efficiency of today’s diesel engines is almost double that of spark ignition (SI) gasoline engines, which typically operate at up to approximately 25–30% thermal efficiency. Higher compression ratios, turbocharging, the absence of a throttle valve, and other mechanical factors also help diesels achieve better thermal efficiency. Recapturing waste heat energy that would normally be lost can potentially increase the thermal efficiency of a diesel engine by a significant amount. Downspeeding of engines, when the engine operates at lower revolutions per minute (rpm) at cruising speeds than traditional engines, is another strategy to improve thermal efficiency. With fewer combustion events for each mile or kilometre traveled, less heat is ultimately transferred to the cooling system.

 

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1.4. Diesel Engine Heat Loads

LO 11-02: Identify and explain the purposes and functions of the cooling system.

 

Diesel Engine Heat Loads

While compression ignition (CI) combustion is one of the most thermally efficient processes for converting fuel energy into mechanical energy, it still involves the release of a substantial amount of heat to the cooling system. The release of heat to the cooling system from the combustion chamber is called heat rejection. Diesel engines have several exclusive, or diesel-only, systems and components that are not found in automotive engines but that require additional cooling. These systems use engine coolant to transfer combustion heat away from the engine and into the atmosphere.

Exhaust Gas Recirculation
Cooled EGR, which has been used by HD diesels in North American since 2002, places an even larger heat load on the cooling system Figure 6. Up to 30% of intake air mass in most current diesels includes exhaust gases, which can reach temperatures over 1200° F (649° C). These temperatures are reduced to 400° F (204° C) or less by a coolant-type heat exchanger before being mixed with intake air originating from the outlet of the air charge cooler. This means the diesel-powered vehicle’s cooling system needs to dissipate 15–40% more heat absorbed from the exhaust gas. Until model year 2002, when cooled EGR was introduced, most diesels transferred 100% of exhaust heat directly into the atmosphere. Now larger heat loads are absorbed by the cooling system to accommodate EGR cooling. Engine modifications are needed to remove the additional heat from the cooling system and include larger water pumps, cooling fans, and radiator cross-sectional areas combined with increased underhood airflow Figure 7.

  Fig11_6

FIGURE 11-6: Diesel cooling systems absorb additional heat loads from components. High-capacity EGR coolers, air compressors, oil coolers, and turbochargers represent just a few additional heat loads.

   Fig11_7

FIGURE 11-7: Larger frontal air intakes, such as this truck, are required on EGR-equipped engines to help remove additional underhood heat loads generated by hot surfaces.

  

 

Lubrication Oil Heat Loads
In diesels, heat generated by combustion is rejected and transferred to engine parts and lubrication oil. Because every turbocharged engine cools the underside of the piston with engine oil, more heat is transferred to the lubrication system. If excess heat is not removed from the oil, it can lose its viscosity and thin too much to support bearing loads and properly lubricate parts. At temperatures above 250° F (121° C), conventional lubricating oil can quickly oxidize and even burn. To remove this heat from the oil, virtually all diesel engines use oil coolers that circulate engine coolant to remove heat from the engine oil Figure 8.

  Fig11_8

FIGURE 11-8: Oil cooler nozzles spray the underside of all engine piston crowns with oil to remove heat. The heat absorbed by the oil is removed by the cooling system through the oil cooler.

   Fig11_9

FIGURE 11-9: The cooling of compressed intake air is sometimes performed by liquid heat exchangers. This MaxxForce DT engine uses jacket water aftercooling engine coolant between the low- and high-pressure turbochargers. ATAAC is used for the final stage of cooling.

  

 

Air Intake System Heat Loads
Intake air compressed by the turbocharger easily heats to over 400° F (204° C). Temperatures are even higher than this in engines using series turbocharging. The cooling of heated intake air is necessary to reduce emissions and prevent the thermal overloading of cylinder components such as piston crowns and valves. While most engines use air-to-air aftercooling (ATAAC), engine coolant is often used to help reduce air charge temperatures. Liquid heat exchangers, air charge coolers, or aftercoolers may be located in the intake manifold to circulate coolant, which lowers air inlet temperatures Figure 9. More commonly, liquid interstage coolers are used on series turbocharged engines to reduce air temperatures between the low- and high-pressure turbochargers. Heat removed from intake air is simply transferred to the atmosphere through the radiator.

 

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1.5. Temperature Regulation

LO 11-02: Identify and explain the purposes and functions of the cooling system.

 

Temperature Regulation

Another important job of the cooling system is to raise engine coolant temperature quickly to keep engine component wear to a minimum by circulating it through the engine Figure 10. Machined parts operating with close tolerances are designed for consistent operating temperatures of between 180–200° F (82–93° C), with one exception being emergency and military engines that are designed to withstand as much as 230° F (110° C).

Higher cooling system operating temperatures lower the amount of heat rejected, but if temperatures climb even as little as 12° F (−11° C) beyond normal range, cylinder wall wear begins to dramatically increase. Conversely, cold cylinders prevent engine parts from operating at optimal clearances and lead to excessive oil consumption, carbon deposits on pistons, poor combustion quality, and oil sludging due to water condensation. It is important to remember that in current diesels, the coolant must reach a minimum temperature necessary within a reasonable time to enable other on-board diagnostic (OBD) monitors to run. This explains why fault codes for the cooling system are set if the engine does not quickly reach operating temperature.

  Technicians Tip  
  Short-run operation of any engine causes sludging of the engine oil. If oil is not allowed to properly heat up, water condensation from blowby gases contaminate the oil, which can eventually clog the oil pickup tube for the oil pump and other oil passages in the engine. Crankcase ventilation system filters can also clog with sludge and heavy blowby particulate. They may even freeze due to extended cold-weather, low-temperature engine operation. Technicians may need to verify that engine oil is frequently brought to operating temperature and maintained in the correct operating range by the cooling system to vaporize any water condensation in the lubrication oil.  
  Fig11_10

FIGURE 11-10: Typical coolant circulation pattern in a V8 diesel.

 

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1.6. Coolant Expansion in Cooling Systems

LO 11-02: Identify and explain the purposes and functions of the cooling system.

 

Coolant Expansion in Cooling Systems

Cooling systems must provide for the expansion of liquid coolant. As engine coolant warms, it expands to occupy a larger volume. An overflow reservoir is needed to contain hot coolant that is pushed out of the radiator and engine during warm-up. Surge tanks, overflow reservoirs, and large radiator top tanks are used to buffer coolant expansion Figure 11. Some coolants are made with propylene glycol, which is nontoxic and environmentally friendly; however, most antifreezes are made from a base chemical called ethylene glycol, which is considered toxic and must not be allowed to escape into the environment through overflow hoses. When the engine cools, coolant is drawn back into the radiator through negative pressure caused by the contraction of cooling coolant in the closed cooling system Figure 12.

  Fig11_11

FIGURE 11-11: The surge tank and overflow reservoir are contained in a single unit.

   Fig11_12

FIGURE 11-12: When coolant is heated, it expands. The overflow reservoir or surge tank collects coolant leaving the cooling system through the overflow pipe below the neck of the radiator cap. After the engine cools, coolant is drawn back into the cooling system.

  

 

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1.7. Pressurization of Engine Coolant

LO 11-02: Identify and explain the purposes and functions of the cooling system.

 

Pressurization of Engine Coolant

Cooling systems are pressurized for two important reasons. The first is to maintain pressure greater than atmospheric pressure at the water pump inlet. Without pressure to the water pump inlet, pumps operate less efficiently. If the pump inlet is not under pressure, the cooling system can become air-bound due to coolant that vaporizes under low pressure and can prevent coolant flow. Related to this is cavitation, which is a major problem in diesel cooling systems. Cavitation is erosion in cylinder block walls, heads, injector tubes, and liner sleeves caused by the vaporization of coolant produced by the combination of heat and low coolant pressure. Cavitation occurs more easily if cooling system pressures are abnormally low.

The second important reason for pressurizing the cooling system is that it increases the boiling point of water. Permitting water to turn to vapor or steam near the cylinders and in the cylinder heads dramatically lowers the ability of the cooling system to absorb heat. Even momentary loss of direct contact with coolant causes severe thermal stress on these components, resulting in the cracking of heads and engine blocks. Increasing cooling system pressure prevents engine damage and loss of coolant due to boiling by increasing the boiling point of coolant. Every 1 pound per square inch (psi; 7 kilopascal [kPa]) increase in cooling system pressure increases the boiling point by approximately 3° F (−16° C). This means 10 psi (69 kPa) of pressure on the cooling system raises its boiling point to 242° F (117° C) Figure 13. Pressurizing the cooling system also prevents water from boiling away from the cooling system. The cooling system is pressurized by both coolant expansion and trapped air, including steam trapped in the radiator during heating.

   Fig11_13

FIGURE 11-13: Increasing the pressure of the cooling system increases the boiling point of the coolant. Adding antifreeze to the coolant further increases the boiling point.

 

  Technicians Tip  
  Diesel engine cooling systems have traditionally operated at pressures between 7–10 psi (48–69 kPa), which is lower than gasoline engines. The primary reason for this lower pressure is to prevent overheating the oil in the oil cooler. At higher coolant temperatures, the lubricating oil loses its viscosity, which can potentially cause a loss in lubricating abilities. At a lower boiling point, coolant escaping as steam carries heat away from the engine and keeps the oil temperature in the oil coolers lower. Higher operating temperatures allow the oil to potentially overheat. Newer engines using cooled EGR have higher cooling system pressures to prevent coolant from boiling in the EGR cooler. Some manufacturers, such as Volvo, pressurize the cooling system using chassis air pressure.  

 

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1.8. Video: Pressure verses Boiling Point

LO 11-2 Video: Pressure verses Boiling Point

 

 

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1.9. Anim: Coolant Pressure & Boiling Point

LO 11-2 Animation: Coolant Pressure & Boiling Point (FULL SCREEN)

 

 

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1.10. Deaeration of Coolant

LO 11-02: Identify and explain the purposes and functions of the cooling system.

 

Deaeration of Coolant

Aeration is the condition in which excessive amounts of air or steam bubbles are dissolved in coolant, which reduces the coolant’s effectiveness. Cooling systems must provide for deaeration of the coolant to prevent pumps and coolant passageways in the cylinder head from becoming air-bound or blocked by an accumulation of vapor or steam Figure 14. As coolant warms, some vaporization occurs, forming bubbles and vapor pockets. Even when coolant is relatively cold, coolant next to hot internal engine surfaces becomes hotter than surrounding coolant and vaporizes. If the vapor collects into large steam pockets, the engine cannot transfer heat to the coolant, causing localized hot spots and potential damage from overheating Figure 15. Additives called wetting agents are sometimes added to the cooling system as a chemical supplement or as part of the additive package in antifreeze. Wetting agents are essentially chemicals that reduce the surface tension of bubbles, which causes them to quickly collapse as they grow.

  Fig11_14

FIGURE 11-14: Vent lines from the engine block and the radiator connect to the deaeration tank, also referred to as the surge tank . Without deaeration, the engine coolant flow becomes blocked.

   Fig11_15

FIGURE 11-15: Two surge tanks that deaerate coolant are used by this prototype Freightliner eM2 chassis. It is critical that vapor is removed from coolant that circulates through the electric motors, batteries, and power electronics.

  

 

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1.11. Minimizing Engine Corrosion

LO 11-02: Identify and explain the purposes and functions of the cooling system.

 

Minimizing Engine Corrosion

Dissimilar metals in contact with engine coolant cause electrochemical reactions Figure 16. The coolant is an electrolyte, which means it can conduct electrical current. In a sense, the cooling system can behave like a primary battery in which dissimilar metals in the presence of an electrolyte produce a flow of electric current. This principle of inducing current flow by using dissimilar metals and an electrolyte is called the galvanic effect . Unless some strategy is used to prevent these reactions, some of the metals in contact with the coolant will become severely corroded through these chemical reactions. For this reason, antifreeze contains a number of additives that minimize electrochemical reactions, prolonging the durability of engine and cooling system components. It also explains why it is important to clean and properly maintain the multiple engine electrical wiring grounds, especially those dedicated for the starting motor and alternator. Engine coolant should never be allowed to become a ground pathway for damaging electrical current. Instead, lower-resistance cable grounds should conduct current flow.

   Fig11_16

FIGURE 11-16: This injector tube, which is surrounded by coolant, transfers heat away from the injector. The brass in tubes like this—along with other metals, such as aluminum and iron in the cooling system—produces galvanic electrochemical reactions that lead to corrosion.

 

  Technicians Tip  
  Galvanic activity in a cooling system is easily measured with a digital multimeter. After setting the meter to a low-voltage, direct current (DC) scale, place one lead in the coolant. Place the other test lead anywhere on the engine where there is metal exposed for a good ground connection. A cooling system with a level of additives that is adequate to protect metals from corrosion tests at 0.2 volts or less. Coolant that has become depleted of additives or highly acidic or alkaline (basic) will produce more voltage. If the voltage exceeds 2 volts, the cooling system should be drained and flushed and new coolant containing the recommended level of corrosion-inhibiting conditioners should be added.  

 

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1.12. Video: Electrolysis

LO 11-2 Video: Electrolysis

 

 

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1.13. Minimizing Scaling and Deposits

LO 11-02: Identify and explain the purposes and functions of the cooling system.

 

Minimizing Scaling and Deposits

Scale and mineral deposits on internal surfaces within the engine and cooling system can sharply reduce heat transfer. In fact, a 0.06" (1.6 mm) deposit will reduce heat transfer by 40%. Scale can originate from calcium and magnesium in the water used for cooling, but some scale is produced by additives in the antifreeze, such as phosphate, silicate, and nitrite. Scaling and deposit formation commonly occur on hot engine surfaces where coolant turbulence is low. Maintenance practices, such as following prescribed replacement schedules for coolant and using recommended formulations of engine coolant, can help minimize the formation of these engine-damaging deposits Figure 17.

   Fig11_17

FIGURE 11-17: Scale buildup in diesel engines can be prevented with the use of nonmineralized water and specialized coolant conditioners.

 

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1.14. Freezing and Boiling Protection

LO 11-02: Identify and explain the purposes and functions of the cooling system.

 

Freezing and Boiling Protection

Because water is the primary ingredient of engine coolant, the engine requires protection from frozen and boiling coolant. Frozen coolant easily breaks and cracks cylinder heads, blocks, water pumps, and radiators. Additionally, it pushes out core plugs, sometimes called frost plugs , which causes a major loss of coolant. It should be noted that core plugs are not designed to relieve pressure resulting from frozen coolant in order to prevent engine damage. When coolant is frozen in areas such as the radiator or a hose, it will not allow system flow, which causes the cylinder components to rapidly overheat, thereby thermally stressing the cylinder block and head to the point of cracking. Conversely, boiling coolant is not able to effectively absorb heat because it is not in a liquid state. Protection against freezing and boiling are both provided to the engine by the use of antifreeze TABLE 11-1. Additional protection against boiling is accomplished by pressurizing the cooling system.

  TABLE 11-1: Freezing Points of Ethylene Glycol and Propylene Glycol Antifreeze  
Concentrations of Antifreeze by % Volume Freeze Point of Coolant
Ethylene Glycol
 Propylene Glycol
0% (Water only) 32° F 0° C 32° F 0° C
20% 16° F –0° C 19° F –7° C
30% 4° F –16° C 10° F –12° C
40% –12° F –24° C –6° F –21° C
50% –34° F –37° C –27° F –33° C
60% –62° F –52° C –56° F –49° C
80% –57° F –49° C –71° F –57° C
100% –5° F –22° C –76° F –60° C

 

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1.15. Vacuum Method for Refilling Engine Coolant

LO 11-02: Identify and explain the purposes and functions of the cooling system.

 

Vacuum Method for Refilling Engine Coolant
A coolant refill is an event during which the protection of components from damage caused by boiling coolant is easily overlooked. It is important to understand that the procedure used to refill the system after draining the coolant can create air pockets. Air pockets commonly form in the EGR coolers, which results in portions of the coolers sustaining damage from hot spots where coolant has boiled or there is no coolant. Original equipment manufacturers (OEMs) recommend the use of coolant refill equipment, which completely evacuates the cooling system of air and pulls coolant back into the engine using negative pressure Figure 18. A coolant refill/air purge tool is typically air operated. The tool first uses a venturi and compressed shop air to draw a vacuum in the entire cooling system. A vacuum ensures that no air pockets are in the cooling system while refilling. A coolant reservoir holding the coolant charge is then used to draw only coolant into the cooling system using the service tool, blocking unwanted air from entering the system. A pair of hand-operated valves are used to switch the tool from drawing out air and establishing a negative pressure in the cooling system to filling the system with liquid coolant.

  Technicians Tip  
  Core plugs—or frost plugs—are used to seal passageways in cylinder blocks and head castings designed to flush sand from casting after manufacturing. While these plugs may be displaced by frozen coolant, they are not designed to prevent engine damage.  
  Fig11_18

FIGURE 11-18: A coolant refill/air purge tool uses shop air and a venturi to pull a vacuum into the cooling system. Next, a hose is dropped into a container of coolant, and the cooling system’s negative pressure draws in coolant, which fills all spaces and voids with coolant.

 

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1.16. Minimizing Cavitation Erosion

LO 11-02: 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.

Fig11_19

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.

   Fig11_20

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.

   Fig11_21

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.

 

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1.17. Cavitation and Engine Damage

LO 11-02: 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.

Fig11_22

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

   Fig11_23

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

   Fig11_34

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.

  Technicians Tip  
  Coolant in the cylinder of a diesel locks the engine because cylinders have very little clearance volume. Coolant may enter through a leaking cylinder head gasket, the intake air system, or a pinhole leak path in the cylinder wall caused by cavitation erosion. An engine turned in the correct direction of rotation will lock up if liquids are in the cylinder. To diagnose a hydrostatically locked engine, remember that turning the engine in the opposite direction of rotation pushes liquids out of the intake valve so that the engine can rotate freely.  

 

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