Subsystem
Side effects
System temperature cycling
Heat to Coolant
Failure Modes
Heat exchanger thermal stress Poor heater performance Engine overheat
Temperature gauge instability Legend
V = Component variability A = Aging U = Customer usage
E = Environmental
Coolant in Liquid Phase
Part 2 - Section 7
Temperature Control Subsystem
close to the onset of flow instability (OFI). Recall from Section 2 that the flow rate through the engine must always be well above OFI, and also that OFI is a function of subcooling (see figure 28 in section 2). Since subcooling decreases as temperature is increased, and critical flow rate increases as subcooling decreases, the flow rate at which OFI occurs is higher at a higher temperature.
Temperature therefore must be limited in order to maintain a satisfactory margin above OFI. Or in other words, a higher coolant temperature may mean higher flow rates are required through the engine.
For example, assume we are controlling the coolant temperature at 105 degrees C and 1.4 bar pressure. Since boiling temperature of 50/50 coolant at 1.4 bar = 132 degrees C, then subcooling = 132 - 105 = 27 degrees C If we are using the engine with the critical flow rate map shown in the section 2, and the system is providing 30 liters/min, then the critical flow rate under these conditions is 21 liters/min, and the margin of safety is:
30 - 21 = 9 liters/min (about 30%).
Let’s assume we increase the allowable operating temperature to 120 degrees.
• Subcooling = 132 - 120 = 12 degrees C
• The critical flow rate is now 25 liters/min. The margin of safety is now reduced from 9 to 5 liters/min (about 17% margin).
SUBCOOLING AND OFI
Temperature and pressure can both be changed to increase subcooling. Lowering the coolant temperature is the best alternative because it will increase the margin of safety above OFI without increasing metal temperatures at all.
Increases in pressure will increase engine wall temperatures if the coolant is in a state of nucleate boiling at the metal surface in the engine. When the metal surface is exposed to nucleate boiling, the metal will be very close to the boiling point of the fluid. If the coolant boiling point is increased through increased pressurization, then the metal surface temperature will also be increased. This increase in surface temperature will not be as great as the temperature increase that would result from vapor blocking, however.
You can determine whether nucleate boiling is occurring in an engine if the metal temperatures at the coolant interface are thermocoupled and the metal temperatures are monitored. If a metal temperature increase occurs when an increase in pressure is imposed on the cooling system (and no other operating conditions are altered), then some amount of nucleate boiling is occurring.
In the past engine designers deliberately used nucleate boiling to minimize the power required by the water pump.
Currently designers are choosing to avoid nucleate boiling altogether, and to stay well within the convective heat transfer mode to increase the safety margin from OFI.
SUBCOOLING AND CAVITATION
In engines with high cylinder pressures, particularly diesels, erosion of the coolant side of the cylinder wall can occur due to a combination of insufficient subcooling and surface vibration.
As the piston passes over top-dead-center on the firing stroke, gas forces acting on the piston cause it to swap its load bearing face from the anti thrust side to the thrust side of the cylinder wall. As it moves from one side to the other an impact occurs.
This impact causes a shock wave (a wave of very high pressure) to be emitted from the cylinder wall. Behind this shock wave is a rarefaction wave (a wave of very low pressure) which can cause the local pressure to fall below the saturated vapor pressure of the coolant, causing it to boil or cavitate.
As the pressure wave is reflected back off the opposite side of the coolant jacket, the vapor bubbles formed collapse dissipating their energy in very narrow points of contact.
This implosion has sufficient force to destroy the layer of oxide that protects the surface from corrosion. In engines fitted with machined cast iron cylinder liners, this can lead to severe pitting.
Increasing the level of subcooling may significantly contribute to the prevention of cavitation damage. It should therefore be a goal of the design engineer to maintain a high degree of subcooling, in any region that is susceptible to cavitation damage.
PUMP CAVITATION
When the coolant temperature approaches its maximum value, the flow rate through the engine will be closest to the critical value (safety margin is at its smallest).
Unfortunately, if we exceed the temperature at which the pump begins to cavitate, then the flow rate will decrease, just as it is needed the most. Therefore it is necessary to choose a maximum coolant temperature at which there is both sufficient margin in engine flow rate, and also in water pump inlet pressure.
COMPONENT REQUIREMENTS
It is important to verify that all components that are exposed to coolant have limiting temperatures above that
of the local coolant temperature. There may be no mechanism to cool the coolant after the engine is shutdown, and some portion of the coolant will vaporize and be heated to a temperature above the boiling temperature.
Those components through which vapor is likely to pass (such as the degas reservoir, and those hoses which pass coolant and gasses to the degas reservoir) will be exposed to temperatures higher than the bulk coolant temperature during operating conditions.
THERMOSTATIC CONTROL
The purpose of the thermostat is to control the temperature of the coolant for optimum heater performance, and reduced vehicle emissions. The typical engine cooling system is controlled with a wax pellet thermostat.
The wax pellet thermostat valve is activated by a temperature sensitive wax power element containing a mixture of wax and various other substances. The power element expands as it heats up due to the melting of the wax. The expansion of the power element opens the valve and allows fluid to flow to the radiator. When the coolant temperature reaches the desired operating temperature, ideally the thermostat would open just enough to maintain the desired coolant temperature. In reality the thermostat, and the vehicle cooling system cannot respond immediately and the result is an imperfect control mechanism. Thus the coolant may not always approach the desired operating temperature in a stable manner.
Many times the coolant temperature initially overshoots and then oscillates around the desired operating temperature as shown in figure 90.
Figure 95, Coolant temperature overshoot and oscillation
TEMPERATURE OVERSHOOT AND OSCILLATION Overshoot and oscillation are undesirable because they can result in:
• temperature gauge oscillation which may cause the customer to bring the vehicle in for service,
• unnecessary fan operation,
• engine overheating in the event of severe overshoot,
• thermal cycling of components,
• excessive opening and closing of the thermostat which will lead to reduced thermostat service life,
• excessive hydrocarbon emissions (in extreme circumstances),
• and heater discharge air temperature cycling
It is essential to control the amplitude of oscillation in order to achieve required component life, emissions, acceptable gauge operation and desired heater performance.
There are three causes of the temperature overshoot and oscillation;
• delay in the response of the thermostat (due to the time required to melt the wax in the thermostat),
• or delay in the response of the cooling system (due to the time required for the coolant to travel through the radiator and back to the engine),
• or excessive changes in coolant temperature resulting from small changes in the opening of the thermostat.
When a change in the flow rate is imposed on the cooling system, the coolant temperature will change, and this new temperature is not achieved instantaneously, but takes a few seconds. Furthermore, when a change in the coolant temperature is imposed on the thermostat, there is a few seconds delay before the thermostat responds with a new valve position. These delays are called transport delays in control system theory. When a transport delay exists in both the system and the controller, i.e., an engine cooling system and thermostat, then oscillation will always result.
When small changes in thermostat position result in large changes in coolant temperature, the amplitude of oscillation will be greater, because the system changes even more before a correction is applied (it is if the delay were longer). Certain thermostat valve profiles, excessive radiator size, low ambient temperatures and dual-inlet or dual-outlet water pumps can cause these excessive system responses.
Temperature
Time
Desired Temperature
THERMOSTAT VALVE PROFILE
The coolant flow rate changes as the thermostat starts to open. The rate of this change affects the rate of response of the system. The flow rate is a function of flow area, so the rate at which the flow area changes as the valve opens affects the response of the system. Figure 91 shows valve opening versus temperature for two thermostats. This graph shows similar valve opening characteristics for these two thermostats.
Figure 96, Thermostat Opening versus Temperature
Figure 97, Radiator Coolant Flow Rate versus Thermostat Opening
FLUSH VS. RECESSED VALVE PROFILES
The flush valve is characterized by the flow area being directly proportional to the opening lift. The recessed valve has an initial increase in flow area at lower valve openings, followed by an area that remains almost constant over a range of valve openings before once again becoming a linear function of opening lift, see figure 92. It is possible to develop a thermostat with any desired flow versus opening lift characteristic by adjusting the shape of the flank profile, thereby making it possible to control the system transient response, see figure 93.
Figure 98, Thermostat Opening vs. Lift for Typical Designs of Valve Profile
RADIATOR SIZE AND AMBIENT TEMPERATURE Both radiator size and ambient temperature can significantly contribute to the amplitude of coolant temperature oscillation. When the radiator is excessively large and/or the ambient temperature sufficiently low, the coolant temperature will change rapidly with small changes 0
2 4 6 8 10 12
66 71 76 81 86
Coolant Temperature (°C)