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

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.