LO 11-6 Identify and describe the construction features, types, and applications of the diesel engine cooling components

1. Components of Cooling Systems

Components of Cooling Systems

Transferring absorbed heat from the engine to the atmosphere is one of the primary functions of the cooling system. A number of components are involved in this task, including the coolant itself, water pumps, thermostats, various hoses, cooling fans, fan controls, and radiators. Heat is transferred to the atmosphere through radiators, which allow air to pass though cooling fins to release heat. Conventional cooling systems perform the tasks demanded of them by using mostly mechanical devices. Hybridized cooling systems used by the latest energy-efficient GHG-compliant engines do the same jobs but instead have electronic controls applied to the operation of thermostats, water pumps, cooling fans, and other cooling system controls. Water pumps may also be electrically driven to further reduce parasitic power loss, operating intermittently as on-demand-only devices.

1.1. Radiators

Radiators

The radiator is a highly visible cooling system component. Radiators are designed and used to accommodate various heat transfer requirements and chassis applications. Radiator technology continues to change to extend service life and operate more efficiently while reducing construction costs and maintenance requirements.

Radiators are heat-exchanging devices that release the heat absorbed by the coolant circulating around the engine into the air. Coolant can flow through a radiator in two ways. In downflow radiators, the coolant enters through a large hose connected to the top tank of the radiator that carries hot coolant from the thermostat outlet. The coolant then flows downward from the top tank, travels through cooling tubes, and enters the bottom radiator tank at a cooled temperature. Alternatively, in cross flow radiators, coolant flows from a thermostat outlet to a top hose connected to a side-mounted radiator tank. Coolant then flows across cooling tubes in the radiator to another side-mounted tank Figure 38. Cross flow radiators are preferred in vehicles with low-profile, aerodynamic hoods because they can be made wider with a lower height profile. Downflow radiators have a taller but narrower profile that can fit between frame rails Figure 39.

Tubes within the radiator core provide the surface area needed to exchange heat. Heat dissipates from the coolant through the tube wall and then through the fins. Air passing through the fins carries away the heat, thereby allowing tubes and fins to absorb more heat from the coolant Figure 40.

FIGURE 11-38: A. Downflow radiator. B. Cross flow radiator.

FIGURE 11-39: A narrower downflow tank with a replaceable core for a premium-quality vehicle. This is a four-core radiator.

FIGURE 11-40: Tube and fin construction of a radiator.

Copper Versus Aluminum Radiator Material Early radiator construction used copper brass radiator components. Copper brass construction seemed to be the obvious choice for radiators because of its superior heat conductivity, ease of forming, and repair. However, it was learned that increasing the diameter of the tubes in a radiator made radiators more efficient, lighter, and compact. This required thicker tube walls to prevent ballooning of the tubes from internal pressure; the increased weight created unacceptably heavier radiators. Beginning in the 1980s, most manufacturers started to use aluminum tube radiators Figure 41. A two-row aluminum radiator core (also referred to as a double core radiator ) with 1" (25 mm) circumference tubes is equivalent to a five-row, copper brass radiator with 1/2" (13 mm) circumference tubes. The 1" (25 mm) tube size also improves tube-to-fin contact in addition to increasing cooling system capacity by roughly 25%. Copper is a good heat conductor, but the lead solder required to bond the tubes to the fins creates an insulation point that prevents some heat transfer. All the mechanical stresses are borne by the solder in the joints between the radiator tank and the tube core that can weaken and leak over time. Solder can be attacked if the coolant pH level becomes too alkaline through the overuse of DCA or SCA. Aluminum tubes are instead welded or crimped, rather than soldered, to either aluminum or plastic radiator tanks. This inexpensive arrangement provides a more efficient conductor for cooling. Radiator cores are replaceable, and the radiator tanks are reusable in both the aluminum and the brass tube-type designs. However, inexpensive aluminum radiators are usually replaced as a complete unit.

FIGURE 11-41: This radiator is a common design in which plastic tanks are crimped to an aluminum cooling core. A silicone gasket between the tank and the radiator core prevents leaks.

1.2. Video: Radiators

1.3. Radiator Caps

Radiator Caps
While early radiator caps simply prevented coolant from spilling out of the cooling system, today’s radiator caps have several additional functions. First, a calibrated pressure valve inside the cap increases the pressure of the cooling system to raise the boiling point temperature of coolant. This feature prevents coolant loss through boil over Figure 42. The pressure valve also allows the coolant to boil over if the temperature of hot coolant exceeds the cap’s pressure rating. Hot coolant escaping as superheated steam will carry heat away, leaving cooler liquid coolant behind in the radiator. To prevent damage to the heater core from excessively pressurized coolant, which can sometimes be pressurized by chassis air, some heater core circuits use a pressure bypass valve. The pressure-regulated valve blocks high-pressure coolant from reaching the core and instead creates a bypass circuit to return coolant back to the engine before it reaches the core Figure 43.

A	B		FIGURE 11-43: Chassis-pressurized cooling systems require a heater core bypass valve to prevent damage to the heater core from excessive cooling system pressures.
FIGURE 11-42: A. The pressure rating of a radiator cap, the point at which the pressure relief valve opens, is stamped on the cap. Radiator pressure cap rating: 7 psi (48 kPa). B. Radiator pressure cap rating: 15 psi (103 kPa).

A second function of the radiator cap is to allow air or coolant to re-enter the cooling system after the engine cools. Another valve in the radiator cap that enables coolant return is the vacuum valve. Coolant volume in the engine contracts when the coolant temperature is lowered, resulting in a pressure drop Figure 44. This allows the vacuum valve to open and draw coolant from the overflow tank back into the engine. If the system has only a surge tank but no additional remote-mounted overflow tank, only air is drawn back into the cooling system through the vacuum valve to prevent negative system pressure from forming. The radiator cap is also important to water pump efficiency. When using the radiator cap to pressurize the cooling system, the pump will operate over the maximum 85% efficiency that is achievable without the cap. Finally, pressurizing the cooling system minimizes cavitation of the entire cooling system because vapor bubbles caused by low pressure are less likely to form.

Faulty vacuum valves are often identified when the upper radiator hose collapses after the engine cools or when coolant is lost Figure 45. If the valve is faulty, coolant will either not be allowed into the system and remain in the surge tank or negative pressure will be applied to the entire cooling system. A coiled spring inside the lower radiator hose, or a solid metal tube, is required to prevent its collapse when the engine is running.

FIGURE 11-44: A. Operation of the pressure-relief function. B. Vacuum valve operation of a radiator cap.

FIGURE 11-45: The upper radiator hose of this engine is collapsed and blocks coolant flow. The collapsed hose is likely due to defective operation of the radiator cap’s vacuum valve.

1.4. Surge Tanks

Surge Tanks

Additional expansion volume for the cooling system is necessary to allow for the expansion of coolant. Surge or overflow tanks provide the space for coolant and vapor to move into when the engine coolant is hot. When the engine cools, the stored overflow volume returns to the radiator. Sometimes large radiator top tanks will accomplish the same purpose as a separate expansion reservoir. Overflow tanks use a line connected to the radiator just below the pressure cap, allowing coolant to move back and forth through the vacuum valve during the cooling system’s thermal cycles.

Surge and overflow tanks provide a coolant reserve for gradual loss of coolant. One study found that new trucks in their first year of service lost almost one gallon of coolant in the first year of operation—or about a drop of coolant per mile. Most of the leakage was determined to come from gear-type radiator hose clamps. Constant torque spring clamps are one of the best clamps to use to minimize coolant loss because they change dimensionally with the expansion and contraction of hoses and connections.

Vent Lines
Coolant vent lines located at the highest points on the engine and radiator bleed steam and air out of the engine and into the surge tank. If these gases are allowed to remain in the engine, they will accumulate in the cylinder head, EGR, or liquid air charge coolers, causing overheating. A water pump housing may even become air-bound, a condition where steam accumulates in the water pump housing and prevents coolant circulation.

1.5. Video: Coolant Recovery

1.6. Water Pumps

Water Pumps
To ensure the positive circulation of coolant, an engine-driven water pump moves coolant through the cooling system. Pumps can be belt- or gear-driven Figure 46. A gear-driven pump does not rely on a belt, which minimizes service requirements, and is also more reliable than a belt for long distances. All water pumps are centrifugal-type turbine pumps, which means they become more efficient the faster they turn. The impeller, which transfers the pump’s energy to the coolant, is located in the coolant and may be made of plastic or metal. Impellers can wear if a significant amount of abrasives circulate through the cooling system Figure 47. Plastic and metal impellers that are pressed onto the water pump shaft can also become separated from the shaft and cause poor coolant flow. This problem is usually identified when the engine overheats but the lower radiator hose, which is normally hot to the touch, is instead cold or lukewarm because radiator coolant is not being pulled by the pump into the engine.

FIGURE 11-46: A gear-driven water pump.

FIGURE 11-47: Centrifugal metal-type water pump impellers. Abrasives in the cooling system have eroded the tips of the impeller on the left.

Coolant enters the center area of the pump impeller and is flung off the impeller vanes at high speed to create pressure and flow. The inlet to the water pump is the lower radiator hose or some point located near the lower radiator hose inlet. Water drawn into the pump is discharged into the engine. Under rapid acceleration, internal engine block pressures between the pump and the thermostat can reach as high as 40 psi (276 kPa). To reduce energy losses from driving water pumps, manufacturers are now introducing electrically controlled, variable-speed water pumps to help meet GHG targets Figure 48. A common configuration uses a clutch that controls the speed of the water pump and is signalled by the engine control unit (ECU) using a pulse width modulation (PWM) electrical signal. A speed sensor on the pump provides data for closed-loop feedback control of the water pump speed Figure 49.

FIGURE 11-48: An electronically controlled, variable-speed water pump reduces engine drag when coolant flow is minimal.

FIGURE 11-49: This 2015 Paccar MX engine uses an electrically controlled, variable-speed water pump. Note the speed sensor and another connector supplying a PWM signal, which regulates water pump speed to minimize parasitic loads on the engine.

The use of a centrifugal pump means that at low engine speeds with heavy loads, coolant flow can be dangerously slow. Construction equipment engines often encounter these conditions. Conversely, at high engine speeds with low loads, such as those encountered by an empty bus on a highway, coolant flow can be excessively high, consuming power and fuel unnecessarily. Hydraulic or electrically driven pumps turning at speeds independent of the engine are increasingly used to prevent these inefficiencies.

Water Pump Bearing Seals
Separating engine coolant from the water pump shaft support bearings is a seal. Generally made from long-wearing ceramic, failure of the seal can potentially allow a catastrophic internal coolant leak into the engine from gear-driven water pumps. Seal failure in belt-driven pumps causes an external loss of coolant when the cooling system is pressurized, which can result in overheating of the engine. The use of a weep hole in the bearing housing of both types of water pumps can limit the possibility of pressurized coolant leaking past the water pump seal and through the bearing, which can cause catastrophic bearing failure. In the case of gear-driven water pumps, the weep hole prevents coolant from leaking into the engine, outside of and away from the pump. If coolant reaches the bearings, a major failure of the water pump will occur Figure 50. To prevent this from happening, manufacturers add a weep hole between the seal and the bearings Figure 51. If the seal is worn and damaged, the weep hole allows coolant to leak from the cavity inside the water pump to prevent bearing damage. Some coolant leakage from a weep hole is normal, and some water pumps hold any small leakage of coolant in a small reservoir, where it evaporates.

FIGURE 11-50: The water pump seal prevents coolant from contacting the pump shaft bearings.

FIGURE 11-51: The weep hole of a water pump on a Mack E7.

1.7. Video: Centrifugal Force

1.8. Video: Water Pump

1.9. Water Filters

Water Filters
Water filters on diesel engines, if equipped, perform two primary purposes. First, they remove sediment, rust, and dissolved particles, which cause abrasive damage to the cooling system. Removal contributes to longer water pump life and minimizes cooling system deposits. Second, they supply DCAs to the coolant. Filters contain pucks of DCA additive, which is dissolved within minutes of coolant circulating through the filter. The quantity of additive required for the maintenance of the cooling system is adjusted by selecting the appropriate filter. For example, some filters are prepared with no DCA while others may have 2, 4, or 18 units of DCA. When servicing the vehicle, the technician must select the appropriate filter for the cooling system capacity and the DCA requirement. This information is located in the vehicle service manual.

Diesel engines without water filters usually use SCA in LLC that does not require additives when it is fully formulated. SCA is a liquid used to condition the cooling system if required. With the increased use of ELC, which may only require the use of an extender halfway through the coolant’s service life, if at all, coolant filters are generally not installed in order to prevent the contamination of the coolant with powdered DCA.

1.10. Thermostats

Thermostats
Thermostats regulate the temperature of the engine coolant by modulating or controlling the volume of coolant circulated through the radiator. When the engine is below its normal operating temperature, the thermostat blocks coolant flow to the radiator. At a predetermined temperature (i.e., 190° F [88° C]), the thermostat begins to open, and it opens further if the coolant temperature continues to increase. A thermostat may be fully open at 195° F (91° C).

FIGURE 11-52: Operation of a blocking thermostat. When closed, coolant recirculates through a bypass hose back into the water pump inlet. When open, coolant flows to the radiator and the bypass passage is blocked.

FIGURE 11-53: A blocking thermostat has a hollow center. Coolant flow is regulated by movement of the outer sleeve, which blocks or opens passageways to either the radiator or the bypass hose.

A variety of thermostats are commonly used in diesel engines. Blocking or partial blocking thermostats stop the flow of coolant through the top radiator hose, causing coolant to be redirected through a bypass passageway and back to the water pump inlet for recirculation into the engine Figure 52 and Figure 53. A damaged or missing seal located around the circumference of these types of thermostats causes overcooling or results in the failure of the engine to warm up because cold coolant has a flow path around the seal to the upper radiator hose Figure 54. In engines not equipped with vent lines, a small jiggle valve or venting valve is located in a choke-type thermostat to allow air and steam to escape past the thermostat and into the radiator Figure 55.

FIGURE 11-54: Coolant flow through a cooling system using a blocking thermostat.

FIGURE 11-55: A jiggle valve in a choke thermostat enables trapped steam to vent through the closed thermostat.

Choke- and blocking-type thermostats operate by using a special wax pellet combined with a powdered metal tightly squeezed into a copper cup that is equipped with a piston inside a rubber boot Figure 56. Heat causes the wax pellet to expand, which pushes the piston upward, opening the valve Figure 57.

FIGURE 11-56: A cross section of a choking thermostat.

FIGURE 11-57: The operation of an engine choke thermostat to regulate the engine temperature.

Faulty thermostats cause overcooling of the engine by partially staying open. An engine that will not warm up to operating temperatures, is very slow to warm up, or warms only under heavy load likely has a thermostat that is partially stuck open or a blocking thermostat with a leaking seal. Thermostats may occasionally fail to fully open or be fully open at a particular temperature. This condition causes an engine to overheat when under load.

1.11. Video: Thermostat

1.12. Anim: Thermostat

1.13. Coolant Hoses, Hose Seals, and Hose Valves

Coolant Hoses, Hose Seals, and Hose Valves
The upper and lower radiator hoses and heater hoses require flexible connections between the engine radiator and the heater core due to engine vibration and torque flexion. These hoses must be able to withstand the hot under-hood environment as well as up to 40 psi (2.7 bar) or more of internal coolant pressure. Because cooling systems operate under a wide range of temperatures, hoses must be compatible with operating conditions from well below zero to over 250° F (121° C) without failing. A silicone-based material and an ethylene propylene rubber compound known for its resilience and flexibility are two common hose materials. Silicone-based hoses are more chemically resistant to deterioration and are able to withstand wider temperature variations, which makes them premium quality. Silicone hoses, however, are generally not compatible with oil and will soften if exposed to fuel and lubrication oil. Also, silicone hoses are not compatible with all types of long-life antifreeze either, particularly some OATs, and may quickly deteriorate if the wrong type of coolant is used.

Hose clamps help the hose seal at its connections. Constant-torque spring clamps allow hose ends to thermally expand and contract, which minimizes coolant loss from loose clamps Figure 58. Some vehicles are manufactured with shrink bands, which perform like electrical shrink tube instead of hose clamps. Shrink bands are designed to prevent cold-weather leaks, and they do not require any adjustment after installation. These bands are not reusable and are cut off an engine hose during service.

MD vehicles are increasingly using an electrically operated, three-way coolant control valve instead of a coolant bypass passageway. The purpose of a three-way control valve is to open or close coolant flow to the heater core. By shutting off coolant flow to the cab heater core when only the air-conditioning (A/C) is demanded or when the engine is warming up, the engine warm-up time is shortened. Because warm coolant flow is not directed to the cab, warm-up time is shortened to reduce fuel consumption and GHG emissions Figure 59.

FIGURE 11-58: Constant-torque hose clamps use a spring to maintain continual torque around the hose clamp. Tightening the clamp increases the torque applied continually by the clamp. A spring hose clamp uses spring pressure to maintain a continual pressure. Tension is not adjustable.

FIGURE 11-59: An electrically operated three-way valve controls the direction of coolant flow through the radiator and engine block or the block and heater core. The valve helps speed up engine warm-up.