Subsystem
Side effects
System pressure cycling Increased component loads Component
Pressure Constraints
Failure Modes Hose collapse Pump cavitation Heat exchanger cavitation System leaks
Legend V = Component variability
A = Aging U = Customer usage
E = Environmental
Coolant Pressure
Part 2 - Section 6
Pressure Control Subsystem
equal to the pressure at which the relief valve opens, and that the pressure at the relief valve is the highest pressure in the system. Both of these statements are incorrect.
It is important to remember that the pressure at the relief valve is not always at the maximum limit, and also that the relief valve limits pressures only at the location of the relief valve. Upstream of the cap, pressures will be higher, since this pressure differential is necessary for flow. Ideally the pressure relief valve is very close the pump inlet, so the pressure at the relief valve is closer to the lowest pressure in the system.
ENGINE
The minimum flow required by the engine is a function of subcooling. Subcooling in turn, is a function of pressure, since subcooling increases as the pressure increases (due to the increase in the boiling temperature). Therefore increasing the pressure in the engine will decrease the flow rate required through the engine. However, this increase in boiling temperature will result in an increase in metal temperatures if any amount of nucleate boiling is taking place. This is due to the fact that the metal surface temperature will be very close to the boiling temperature when nucleate boiling occurs. In fact, nucleate boiling can be detected by changing the cooling system pressure and looking for a corresponding change in temperature of the metal surface.
Peak pressures in the engine must also be compared to the pressure limitations of any component in contact with the coolant, such as core plugs, temperature sensors, block heater, etc.
RADIATOR
Pressures in the radiator must be limited to the maximum pressure specified by the radiator manufacturer. If internal pressures exceed the specified limit, the oval tubes can balloon causing distortion of the fins, and reduced heat transfer performance. Pressures inside a component can be decreased by
• increasing hydraulic resistance upstream of the component,
• or by decreasing hydraulic resistance downstream of the component,
• or by decreasing pump output.
Therefore mounting the thermostat upstream of the radiator will reduce pressures inside the radiator, because the thermostat is often the most significant single source of resistance in the system (even while open).
Figure 93, Reduction in flow capacity due to cavitation
PUMP
The minimum fluid energy required by the pump for satisfactory operation is called the Net Positive Suction Head Required, or NPSHR. This minimum pressure, (head) is required at the pump inlet to avoid cavitation.
NPSHR is usually provided by the pump manufacturer.
Net positive suction head available (NPSHA) is the fluid energy available at the pump inlet. NPSHA must always exceed NPSHR. NPSHA can be determined from the following equation:
(63)
Where: Ps = system pressure at cap (Pa, absolute)
= density (kg/m3)
g = gravitational constant (9.8m/s2) Pvp= vapor pressure at pump inlet temperature (absolute pressure)
hfs = friction head losses (m) between the cap and the pump inlet
In words this means that the head at the pump inlet is equal to the head at the pressure cap minus the head losses between the cap and the pump inlet.
The head losses can be calculated by adding up the individual losses associated with each bend, transition, or
Normal Operating Point Normal Pump Curve
Pump Curve with Cavitation
System Resistance Curve
Operating Point with Cavitation
Flow Capacity
Static Pressure (Head)
NPSHA Ps
ρ g× --- Pvp
ρ g× ---– –hfs
=
ρ
other source of hydraulic resistance between the cap and the pump inlet. A one dimensional flow simulation tool provides an easy way to determine these losses.
The head losses between the pressure cap and the pump inlet can be minimized by connecting the outlet of the degas reservoir as close as possible to the pump inlet, and by reducing the resistance of the degas outlet hose (large hose diameter and few bends).
VERIFICATION
A validated one-dimensional computer flow simulation tool can be used to predict peak pressures. When evaluating maximum pressures, assume the pressure at the relief valve to be equal to the relief valve pressure. The relief valve is generally two-way, allowing air to enter back into the cooling system when the pressure falls to just below atmospheric. Use this lower relief valve pressure as the minimum pressure.
To evaluate the potential for cavitation, it is necessary to determine the pressure in the system at a variety of coolant temperatures, since the potential for cavitation is a function of both pressure and temperature. It is necessary for this purpose then, to have a model which predicts pressures as a function of coolant temperature. For the model to accurately predict pressure, it must have an accurate representation of the elasticity in the system. Elasticity is provided by the hoses, and by the vapor volume. The simulation model must therefore have an accurate representation of the vapor volume in the degas reservoir, as well as a representation of the volume change which can be expected due to the expansion of the hoses. Since pressures can be accurately measured in the vehicle, you can use vehicle testing to validate the computer simulation.
Be sure to check high pressures when the system is at the maximum operating temperature, and check for peak pressures in the heater circuit with the thermostat closed.
Check for low pressures at both high and low temperatures, since the vapor pressure will be higher at high temperatures, but the increased viscosity at very low temperatures could also lead to a low pressure problem, such as hose collapse. Remember that the vapor volume will vary depending on what coolant level the customer maintains in the reservoir, so consider a variety of air volumes.
OVERVIEW
This section discusses:
• coolant temperature limits
• and coolant temperature stability.
TERMS USED IN THIS CHAPTER
Bulk Temperature: Average coolant temperature inside a component or cross-section of a component. The term is used to distinguish the temperature of the flowing coolant from local coolant temperatures, which can be at the boiling temperature in a very small layer right next to the metal surface.
Saturation Temperature: Boiling temperature of a fluid at a given pressure.
TEMPERATURE CONTROL REQUIREMENTS
The control factors and sources of noise are shown in the following p-diagram:
Figure 94, Temperature P-Diagram
PRESSURE RELIEF VALVE
In order to increase the margin of safety below boiling, cooling systems have a pressure relief valve. This valve protects components from a catastrophic pressure increase. The valve is set at a pressure well above atmospheric, (typically.5 to 1 bar gauge), and allows the cooling system to pressurize which increases the boiling point. Increasing coolant temperature expands the coolant and results in pressurization.
ONSET OF FLOW INSTABILITY (OFI)
The temperature of the coolant in the engine must not increase to the point where the flow rate is below or even Coolant temperatures within a system must
remain within the following limits:
• Coolant bulk temperature anywhere in the system must not exceed the saturation temperature of the coolant during operating conditions.
• Coolant temperature must remain below that temperature which would cause the coolant flow rate to fall below the onset of flow instability (OFI) (see section 2).
• Coolant temperature must remain below that temperature which would cause the pump to cavitate to the extent that the flow rate is insufficient.
• Coolant temperature must remain below the limiting temperature of any component that is exposed to the coolant.
• Coolant temperature must remain below that temperature that would allow cavitation damage to the cylinder head, and cylinder liners.
These temperature limits must be met under all operating conditions, on all vehicles, throughout the life of the vehicle.
Any oscillations in coolant temperature must not interfere with passenger comfort, prevent emission certification, or cause enough movement of the temperature gauge to cause customer concern.
Noise factors
Thermostat opening temp variation (V) Coolant level in reservoir (U)
Vehicle speed (U) Engine load (U) Loss of thermostat stroke (A)
Ambient temperature (E)
Thermostat hysteresis Bypass design Thermostat valve profile Thermostat throat diameter
Thermostat location Thermostat opening temperature
Control Factors