Containment Coolant
Expansion
Part 2 - Section 11
Coolant Containment & Sealing Subsystem
OVERVIEW
This chapter discusses the requirements for both heater and engine warm-up, and design strategies for improved warm-up.
TERMS USED IN THIS CHAPTER
Heat capacity: The amount of heat required to change the temperature of a volume of material.
There are two sometimes competing requirements for warm-up, the requirement to provide timely heat to the heater, and the requirement for timely heat to the engine (including the engine oil and exhaust).
HEATER REQUIREMENTS
ENGINE WARM-UP REQUIREMENTS
Some of the control factors which affect heater warm-up positively can affect engine warm-up negatively, such as coolant flow rate through the engine. Higher coolant flow rates, for example, will increase the heat transfer from the engine metal to the coolant and thus to the heater (with less heat available to the engine).
However, there are control factors which can positively influence both the engine up, and the heater warm-up. They are:
• Coolant volume
• Thermostat leakage
• Degas line routing
• Engine thermal mass
Figure 119, P-diagram for heat to heater
The rate of coolant temperature increase during warm-up is primarily influenced by the rate of heat transfer from the engine, the system heat capacity and the rate of leakage of cold coolant into the hot side of the system (the hot side of the system refers to that volume of coolant which is heated when the thermostat is closed).
The rate of heat transfer to the coolant can be increased by increasing the heat transfer coefficient (i.e., increasing flow through the engine), or by increasing the heat output of the engine. The overall heat output of the engine is sometimes increased for the purpose of improved warm-up by deliberately making the engine run more inefficiently.
Adjusting the spark, increasing engine load (by turning on accessories), and choosing a lower gear, are all ways of increasing the heat generated by the engine.
Supplemental heating and heat storage devices can also be used to improve heater warm-up. Supplemental heating devices include fuel fired heaters and electrically heated glow plugs and resistance heaters (the use of these devices will increase fuel consumption). Heat can also be recovered from exhaust gases, but this process must not Sufficient heat must be delivered to the heater via
the coolant for adequate heater steady state and warm-up performance. Additionally, the heat to the heater must be sufficient to defrost the windshield within the required time period.
The engine combustion surfaces and engine oil must warm-up at a rate sufficient for adequate exhaust emissions and fuel consumption levels.
Furthermore, the rate of warm-up of the exhaust gases must be sufficient to allow the exhaust aftertreatment devices to function as required.
Noise Factors Pump variation (V)
Heater fouling (A) Coolant concentration (U)
Vehicle load (U) Ambient temperature (E)
Heater Subsystem
Heat to Heater Heat from
Engine
Heater flow rate Thermostat opening temperature Recommended coolant concentration Coolant volume Degas line routing Control Factors
SC and FM Power consumption Corrosion
Heat loss
Legend:
V Component variability A Aging
U Customer Usage E Environment FM Failure Modes SE Side Effects
Part 2 - Section 12
Heater Subsystem
interfere with exhaust aftertreatment devices that require a minimum temperature to function as required.
Heat batteries have also been used, the concept behind them is to capture the heat once the system is fully warmed up and hold the heat energy until the next cold start. One method of heat storage uses a latent heat storage device that stores heat energy in the form of a phase change. The device consists of a volume of material that will become liquid when exposed to temperatures that the coolant will reach when the engine is fully warmed up. Then, as the vehicle cools after then engine is shut off, the material slowly solidifies, releasing heat energy into the coolant.
Another heat storage concept involves capturing the hot coolant and storing it into a separate insulated volume.
Upon start-up, the cold coolant is replaced with the hot, stored coolant.
The amount of heat transferred to the coolant can also be increased by reducing the amount of heat lost to the air flow through the use of grill shutters and winter-fronts.
The cooling system heat capacity is a measure of the amount of heat required to change the temperature of the coolant in the system. This will be the sum of the heat capacity of the metal which must be heated in order to heat the coolant, and the heat capacity of the coolant. Heat capacity of a volume of material is a function of the material and the mass. The cooling system designer can reduce the heat capacity of the coolant by reducing as much as possible the mass of coolant in the circuit. Minimizing the mass of the coolant in turn will maximize the amount of heat available to the heater and the engine.
To minimize the heat capacity of the coolant, the cooling system design engineer needs to evaluate the system for any opportunity to reduce coolant volume from the hoses and the degas reservoir. Avoid long hose runs whenever possible, and be sure to size the degas reservoir appropriately. With gas engines, it may be possible to route the degas lines so that the heated coolant does not pass through the reservoir when the thermostat is cold. Diesel engines however, require degassing at all times regardless of thermostat position.
It is possible to estimate the effect of a change in coolant volume on the warm-up time, if initial warm-up data is available.
Temperature versus time can be predicted from the following equation:
(64) Where: T = Temperature of the fluid (°F)
A = empirically derived gain (°F) τnew = modified time constant (min)
Procedure is as follows:
1. Determine the gain and time constant from warm-up data of a comparable vehicle (example shown in figure 120). Gain is steady state operating temperature of the coolant at 0°F. Time constant is time to reach 63% of steady state operating temperature (this is the old time constant ).
Figure 120, Warm-up data of baseline system
2. Plot
This equation will approximately fit the experimental data.
3. To determine warm-up response with the volume of coolant increased or decreased (due to the presence of a degas system, for instance), modify the time constant as follows:
4. Plot:
Compare time to various temperatures as in figure 121.
Effective separation of the hot and cold sides of the cooling system will also improve warm-up. Coolant leaking past the thermostat through the radiator and back into the hot side of the system will degrade heater performance. If the vent line to the degas reservoir is connected to the radiator, and the outlet of the degas reservoir is connected to the hot side of the system, this will allow a significant amount of leakage of cold coolant, and would likely result in unacceptable heater performance.
T = A 1( –e( ) τ–t ⁄( new))
τold
0 50 100 150 200 250
0 10 20 30 40
Elapsed Time (m in)
Coolant Temperature (deg F)
Gain
63% Gain
Time Constant
T = A 1 e( – ( ) τ–t ⁄ old)
τnew = τold(Vnew⁄Vold)
T = A 1 e( – ( ) τ–t ⁄ new)
Figure 121, Comparison of warm-up time
VERIFICATION
Since the warm-up performance of a vehicle can only be measured after both the powertrain and the heater system hardware is available, actual vehicle testing can only take place very late in the development process. Decreasing product development cycle times will require that the up performance be analytically predicted. However, warm-up models which predict not only the steady state heat to coolant but also the rate of warm-up can be complex, incorporating thermostat, heat exchanger, flow and heat release sub-models.
0 50 100 150 200 250
0 5 10 15 20 25 30
time (min)
Coolant Temperature (deg F)
Predicted value without degas Predicted value with degas