1. Series Hybrid Vehicle Cooling System Simulation
Simulink Based Vehicle Cooling System Simulation;
Series Hybrid Vehicle Cooling System Simulation
13th ARC Annual Conference May 16, 2007
SungJin Park, Dohoy Jung, and Dennis N. Assanis
University of Michigan
I’ll present my research on the series hybrid vehicle cooling system simulation.

2. Outline Introduction Configuration of HEV cooling system Summary
Motivation
Objectives
Simulation and Integration
Hybrid vehicle system modeling [VESIM]
Cooling system modeling
Configuration of HEV cooling system
Summary
This is the outline of my presentation. First, I’ll briefly highlight the motivation and the objectives of my research.
And then I’ll show you how the cooling system simulation is conducted.
We used two system models for this study. One is the vehicle system model and the other is cooling system model. The vehicle system model is called VESIM. VESIM has been a key numerical tool for the ARC research activities. We configured a VESIM for a series hybrid electric vehicle.
I’ll present how we integrated these two system models and show you detailed approaches of component model.
By using the models, I’ll show you three configurations for HEV cooling system and the result of configuration comparative study will be presented to highlight the importance of cooling system configuration.
Finally I’ll summarize the results.

3. Vehicle thermal management and cooling system design
Motivation
Additional heat sources (generator, motor, power bus, battery)
Various requirements for different components
Objective
Develop the HEV Cooling System Simulation for the studies on the design and configuration of cooling system
Optimize the design and the configuration of the HEV cooling system
Compared to the conventional vehicle, hybrid vehicles have additional heat sources such as motor, generator, power bus, and battery as shown in the figure on the right.
And these additional heat sources should be operated at different temperatures. For example, motor and generator should be operated under 95 C, power bus under 70, and battery under 45C.
In addition, they don’t operate simultaneously. For example, engine and generator run together in series hybrid vehicle. But, motor operation is directly related to the driving cycle and it is independently.
As a result, hybrid vehicle cooling system is much more complicated than that of conventional vehicle.
So, the objective of this research is to develop the HEV cooling system model to study design and configuration.
The second objective is to optimize the design and configuration of HEV cooling system to minimize components sizes and parasitic losses.
Conventional Cooling System
HEV Cooling System

4. Overview of Cooling System Simulation
Cooling system model use simulation data from the hybrid system model
Minimizes computational cost for optimization of design and configuration
Driving schedule
Let me explain why and how we use the vehicle simulation for cooling system simulation.
To calculate the heat generation from each components, we have to know the operating condition of each component such as engine speed and torque, motor speed and torque, battery voltage, current, and resistance.
This information was obtained from the VESIM and delivered to the cooling system model as input data. and then the cooling system simulation is conducted.
So, even if I change the design of cooling system, I don’t have to run vehicle model again.
This method will minimize the computational cost for design and optimization of cooling system.
Hybrid Propulsion System Model [VESIM]
HEV Cooling System Model

5. Hybrid propulsion system configuration and VESIM
Engine
Generator
Power Bus
Battery
Motor
Wheel
Engine
400 HP
(298 kW)
Motor
2 x 200 HP (149 kW)
Generator
Battery
(lead-acid)
18Ah /
25 modules
Vehicle
20,000 kg (44,090 lbs)
Maximum speed
45 mph
(72 kmph)
Engine
Generator
Vehicle
Motor
Battery
Controller
Power
Bus
This is the schematic of series hybrid electric vehicle that we used for this study.
In a series hybrid vehicle all the engine power is converted to electricity and it is stored to battery or directly used by motor.
The motor is powered by the electricity from engine or battery depending on the driving mode. Motor also functions as a generator in the regenerative braking mode.
The figure on the bottom is the snapshot of VESIM in Matlab Simulink. And the specifications of the vehicle is summarized in the table.

6. Hybrid vehicle power management
Discharging mode
Charging mode
Braking mode
Battery is the primary power source
When power demand exceeds battery capacity, the engine is activated to supplement power demand
Engine / generator is the primary power source
When battery SOC is lower than limit, engine supplies additional power to charge the battery
Once the power demand is determined, engine is operated at most efficient point
Regenerative braking is activated to absorb braking power
When the braking power is larger than motor or battery limits, friction braking is used
Series hybrid vehicle operates in three different driving modes Discharging mode/ Charging mode/Braking mode depending on SOC and driving condition. In discharging mode, battery is the primary power source. But if the power demand from vehicle is larger than the battery capacity, the engine is activated to supplement the power demand. In charging mode, the engine is the primary power source. If the battery SOC is lower than its low limit, engine supplies additional power to charge the battery. The engine is operated at its most efficient point that is shown on the right hand side. In braking mode, the motor works as a generator. But if braking power is larger than the motor or battery’s capacity, friction braking is applied.

7. Vehicle simulation Vehicle simulation model [VESIM]
Vehicle driving cycle
Cycle simulation results ( engine / generator / motor / battery)
Engine Speed
Generator Speed
Motor Speed
Battery SOC
This slide shows the driving cycle simulation result from VESIM.
The simulation results are used for the calculation of heat generation in the cooing system simulation.
As you can see, the motor speed follows the vehicle speed but other components are controlled by the power management controller based on the driving mode.
Engine BMEP
Generator Torque
Motor Torque

8. Cooling system modeling; Configurations
Motor
Generator
PowerBus
Radiator1
Radiator2
T/S
Electric
Pump
Engine
CAC1
Parallel
Circuit
Mech.
Block
Fan
Turbo
Charger
Configuration A
Radiator
A/C
Condenser
T/S
CAC2
Mech.
Pump
Fan
Oil
Cooler
Now let me switch to the cooling system modeling.
The first step of the cooling system modeling is to configure the system.
I configured a relatively simple notional configuration as shown in this slide.
In the configuration, all the electric components are in one cooling circuit.
However this doesn’t look so reasonable because the electric components such as motor, generator and power bus don’t operate simultaneously and have different operating temperatures.
So, We decided to develop new configurations more systematically.
HEV Cooling System Model in Matlab Simulink

9. Guide Lines of Cooling system configuration
Criteria for system configuration
Radiators for different heat source components are allocated in two towers based on operation group
The radiators are arranged in the order of maximum operating temperature
Electric pumps are used for electric heat sources
The A/C condenser is placed in the cooling tower where the heat load is relatively small
Battery is assumed to be cooled by the compartment A/C system due to its low operating temperature (Lead-acid: 45oC)
Component
Heat generation (kW) *
Control Target T (oC)
Operation group**
Engine
190
120 A
Motor / controller 27 95 B
Generator / controller 65
Charge air cooler 13 -
Oil cooler 40
125
Power bus (DC/DC converter)
5.9 70 C
Battery*** 12 45 D
* Grade Load condition
** The heat sources that generate heat simultaneously during driving cycle are grouped together.
*** Maximum speed condition / Lead-acid
To develop the cooling system configuration I made some guidelines.
To develop the guide lines for the cooling system configuration, we summarized the heat generations under grade load condition, target operating temperature of each components ,and groups of components that operate simultaneously.
These are shown in the table. The grade load condition will be explained in later slide.
Based on the results, I developed several guidelines of configuring the cooling system .
- radiators for different heat source components are allocated in two towers based on operating group. This guide line is made to minimize the fan power consumption by removing redundant operation of fan during driving cycle.
-The radiators are arranged in the order of maximum operating temperature
- Electric pumps are used for electric heat sources .
- The A/C condenser is placed in the cooling tower where the heat load is relatively small
- Battery is assumed to be cooled by the compartment A/C system due to its low operating temperature (Lead-acid: 45oC)

10. Configurations Configuration C Configuration B Power Generation
Based on the guidelines in previous slide, I configured two cooling systems
In config B, every electric component has their own cooling circuit because the target temperature and the operating mode is different from each other.
All the components related to the power generation are in one cooling tower and all the components related to the vehicle propulsion are in another cooling tower.
Config. C is modified from Config B by adding a by-pass line to the oil cooler. This is because the heat load from oil cooler is much smaller than that of engine.
Vehicle Propulsion

11. Modeling Approach Component Approach Implementation Heat Exchanger
Thermal resistance concept 2-D FDM
Fortran (S-Function)
Pump
Performance data-based model
Matlab/Simulink
Cooling fan
Thermostat
Modeled by a pair of valves
Engine
Map-based performance model
Engine block
Lumped thermal mass model
Generator
Power bus
Motor
Oil cooler
Heat exchanger model (NTU method)
Turbocharger
Condenser
Heat addition model
Charge air cooler
Each cooling system configuration has many components models as listed in this table.
Each component was carefully modeled with different fidelity depending on its influence and sensitivity to the cooling performance.
Let me go over the main sub models.
Heat exchanger is radiator. -> HX slide
Pump model is performance data based model. . -> PUMP slide
Cooling fan model is similar to pump model. -> FAN slide
Thermostat is a pair of valve. -> TS slide
For electric heat sources, Lumped thermal mass model is used.
For example, generator is biggest electric power source. -> GEN slide
Oil cooler is one of heat exchanger model. -> OC slide

12. Modeling Approach: Heat source
Heat Input and Exchange Model for Engine Block and Electric Components
Lumped thermal mass model
Heat transfer to cooling path (Qint) and to outer surface (Qext; radiation and natural convection)
Engine
Map based engine performance model
Heat rejection rate as a function of speed and load is provided by map
Turbo Charger
Map base turbo charger performance model
The temperature and flow rate of the charge air as functions of speed and load are provided by map
Schematic of Heat Exchange Model at Engine and Electric components
Engine heat rejection rate

13. Modeling Approach: Heat sources (cont.)
Oil Cooling Circuit
Heat addition model : heat is directly added to the oil
Heat rejection rate as a function of speed and load is provided by map
Condenser
Heat addition model: heat is directly added to the cooling air
Constant value is used for heat rejection rate
Charge air coolers
2-D FDM-based model
In contrast to radiator, heat transfer occurs from air to coolant
Generator
Heat generation is calculated using a 2D look-up table indexed by speed and input torque
Lumped thermal mass model
Heat generation from generator is calculated using two dimensional lookup table for the efficiency indexed by the rotor speed and input torque.

14. Modeling Approach: Heat sources (cont.)
Motors
Heat generation is calculated using a 2D look-up table indexed by speed and input torque
Lumped thermal mass model
Power bus
Power bus regulates the power from electric power sources and supply the power to electric power sink
Heat generation is determined by battery and motor power

15. Modeling Approach: Heat sinks
Heat exchanger (radiator)
Design variables
Core size
Water tube : depth, height, thickness
Fin : depth, length, pitch, thickness
Louver : length, height, angle, pitch
Based on thermal resistance concept
2-D Finite Difference Method
Structure of a typical CHE
The radiator is modeled based on thermal resistance concept using two dimensional finite difference method. It can predict the radiator performance as a function of design parameters such as core size, tube size design, fin design and louver design.
Design parameters of CHE core
Empirical correlation for ha
(by Chang and Wang)
Staggered grid system for FDM

16. Modeling Approach: Heat sinks(cont.)
Oil cooler
Finned concentric pipe heat exchanger model for Oil Cooler
Counter flow setup
NTU approach is used to calculate the exit temperature of two fluids
Water cooled oil cooler is modeled. In order to calculate the exit temperature of coolant and oil, NTU method is used.
Schematic of Heat Exchange at Engine and Electric components
NTU Method

17. Modeling Approach: Delivery media (Coolant)
Coolant Pumps
The coolant flow rate is calculated with calculated total pressure drop by cooling system components and the pump operating speed
Performance map is used to calculate the coolant flow rate
The mechanical pump is driven by engine and electric pump is driven by electric motor
In the model, the coolant flow rate is calculated by total pressure drop across the cooling circuit and the pump speed.
Performance map is used to calculate the coolant flow rate.
Flow rate
Efficiency
Flow rate
Efficiency
Performance Maps of Mechanical Pump
Performance Maps of Electric Pump

18. Modeling Approach: Delivery media (Coolant)
Thermostats
Two way valve with Hysteresis characteristics
Coolant flow rate to re-circulate circuit and radiator are determined by the pressure drops in each circuit
Valve lift curve of T/S
T/S valve lift with hysteresis
As you know, thermostat prevent overcooling of engine by re-circulating the coolant .
The flow rate to re-circulate circuit and to radiator circuit are determined by pressure drops in each circuit.
Coolant flow calculation based on pressure drop

19. Modeling Approach: Delivery media (Oil/Air)
Oil Pump
Map based gear pump model for Oil Pump
Cooling fans
Total pressure drop is calculated from the air duct system model based on system resistance concept
Performance map is used to calculate the air flow rate
Map Based Gear Pump Model
Condenser
Fan & Shroud
Total air flow rate is calculated by pressure drop across grille, condenser, radiators, and fan and shroud and pump speed.
Performance map is used to calculate the sir flow rate.
Grille
Radiator 1,2

20. Test conditions
Test condition for sizing components and evaluating cooling system configuration
The thermal management system should be capable of removing the waste heat generated by the hardware under extreme operating condition
Grade load condition is found to be most severe condition for cooling system
Grade Load
Maximum Speed
Off-Road
Once the model development is completed, test condition of cooling system is needed for sizing components and evaluating the cooling system configuration.
Since the thermal management system should be capable of removing the waste heat generated by the hardware under these extreme operating conditions, I evaluated the three extreme conditions.
These three conditions are Grade load, which is up-hill driving schedule, at 35mi/h and 7% grade up-hill road. Maximum speed condition, at 45mi/h and flat road and the Off-road driving at 35mi/h on prescribed off-road profile.
Among the three driving conditions, Grade Load condition is found to be the most severe condition to cooling system. So, I selected the driving condition for cooling system design and evaluation.
Ambient Temperature 40 oC
Road profile of off-road condition

21. Configuration test; Grade Load (30 MPH, 7 %)
Engine Speed
Engine BMEP
Battery SOC
Grade Load
These plots are the results from VESIM under grade load driving condition.
The result shows that the vehicle is switching between the charging mode and the discharging mode.
The simulation is conducted for 30 minutes to get stabilized results of all cooling system components.
These results are fed into the cooling system model to study the design and the configuration of cooling system.
Max. SOC: 0.7
Min. SOC: 0.6
Initial SOC: 0.6

22. Configuration A and B
Config. A could not meet the cooling requirements of electric components
Configuration A
Configuration B
Generator
Generator
Motor
Motor
I compared configuration A and B first.
As you can see in the plots, Configuration A couldn’t meet the cooling requirements of electric components. As you can see, motor is over cooled but power bus is under cooled.
This is because the heat sources are connected in parallel in one circuit but their target temperatures are different from each other.
But configuration B could satisfy all the cooling requirements of electric components.
From these result, it can be concluded that proper configuration of cooling system is important for hybrid vehicle components, because the electric components work independently and have different target operating temperatures
PowerBus
PowerBus

23. Configuration A and B
Configuration A
Configuration B
Performance of one CAC in Config. B was better than that of two CAC in Config. A
CAC1
CAC
CAC2
And then I compared the performance of CAC in Config A and Config B.
The result showed that the performance of one CAC in Config B was better than that of two CAC in Config A.
As you can see in the plots, The charge air temperature at the exit of CAC in Config B is lower that that of Config A even though it has two CAC.
It’s because the CAC in Config B is in independent circuit.

24. Configuration B and C
Configuration B
Configuration C
Config. C is designed by adding a coolant by-pass line to Oil Cooler in Config. B
Power consumption of pump is reduced by 5% adding the bypass circuit
I compared configuration B and C.
Configuration C is modified configuration from Config B by adding by-pass circuit to oil cooler because the heat load of oil cooler is much smaller than that of engine.
The results shows that the power consumption of the pump in the engine cooling circuit is reduced by 5%.
This is because by-pass circuit reduces the coolant flow rate to the oil cooler, and , as a result, the pressure drop across the oil cooler decreases.
This shows that parasitic loss of cooling system can be saved by optimal configuration of cooling system.

25. Summary
The HEV Cooling System Simulation is developed for the studies of the cooling system design and configuration
The HEV cooling systems are configured using the simulation
In hybrid vehicle, the heat rejection from electric components is considerable compared with the heat from the engine ( Grade Load : heat from electric components ≈ 98kW, heat from engine module ≈ 240kW)
Proper configuration of cooling system is important for hybrid vehicle components, because the electric components work independently and have different target operating temperatures
Parasitic power consumption by the cooling components can be reduced by optimal configuration design
Optimization study of cooling system is conducted using developed model (Symposium II, “Optimal design of electric-hybrid powertrain cooling system”)
Let me summarize this study.
The optimization study of cooling system for hybrid vehicle will be presented in Symposium 2 room this after noon.

26. Acknowledgement General Dynamics, Land Systems (GDLS)
I’d like to thanks to general dynamics land system. This study has been partly supported by GDLS.
Thank you.

27. Thank you!