Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)
1Development of Heat Pump System for xEVs

NOBUYA NAKAGAWA*1TOMOKI HASE*2

TOMOYASU OSAKI*1YASUO KATAYAMA*1

HIDEAKI TATENOI*3

KOJI NAKADO*4

In recent years, xEVs (including BEVs, HEVs, PHEVs, and FCEVs) have rapidly spread
across the globe in an effort to meet stricter environmental regulations. Under such circumstances,
it is required to reduce the amount of power used for air conditioning, which directly affects theirdriving range. For the heat pump systems of xEVs that can reduce the power consumption of airconditioning in winter, we made computational models of the air conditioning cycle and its control
system, and through simulation studied the control of heat pump systems to achieve both powersaving and thermal comfort.Subsequently, we also confirmed that both high efficiency and thermal
comfort can be achieved by controlling the components of the heat pump system properly withvehicle test.

|1. Introduction
In response to the global strengthening of environmental protection (suppression of global
warming gas emissions) and the accompanying vehicle fuel efficiency regulations, xEVs such asbattery electric vehicles and hybrid electric vehicles, have been rapidly spreading. In the case ofxEVs, the electric power used for air conditioning directly affects the range of battery-powered
driving, so demand for energy saving in air conditioning is growing. In particular, heat pumpsystems that can reduce the power used for air conditioning, during heating in winter are effective
technologies for extending the driving range, and are expected to be widely adopted in the future.
However, the realization of the functions essential for ensuring passenger safety, which are
unique to automotive air conditioning, is also necessary when adopting heat pump systems.
Therefore, to achieve both air conditioning power saving and passenger safety and comfort,
Mitsubishi Heavy Industries Thermal Systems, Ltd. used control simulation software as a systemdevelopment tool that controls equipment such as an HVAC (Heating, Ventilation and Air
Conditioning) unit, solenoid valves, and electronic expansion valves (EXVs) that constitute heatpump systems for xEVs in an integrated manner. We also carried out modeling on a computer,examined the optimization of the system control through simulation and implemented verification
using an actual vehicle.

|2. Control of heat pump system for xEVPower saving of heat pump systems for xEVs and ensuring passenger safety and comfort canbe realized simultaneously by controlling the components of the air conditioning system including
the HVAC unit, electric compressor, solenoid valves and EXV appropriately, based on the ambient
temperature, temperature setting and temperature and pressure information of the air conditioning
system obtained from the sensors. To achieve the above, a heat pump system model and its control
logic were programmed into a computer and were used for various preliminary verifications. As a
*1

Engineering Manager, Engineering Department, Automotive Thermal Systems Division, Mitsubishi Heavy Industries
Thermal Systems, Ltd.

*2

Research Manager, Heat Transfer Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*3

Heat Transfer Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*4

Engineering Department, Automotive Thermal Systems Division, Mitsubishi Heavy Industries Thermal Systems, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)2result, practical heat pump control was established without requiring a large amount ofexperimental cost and time.
Controlled objects include the electric compressor, HVAC unit, solenoid valves, EXV andcooling fan. Figure 1 shows the control schematic diagram. The HVAC unit (blower speed, outletmode, outlet air temperature control and air inlet mode) and refrigerant circuit (electric compressor
speed, solenoid valve operation and EXV opening area) are controlled by the ECU (ElectronicControl Unit) based on air conditioning setting information, vehicle sensor information and airconditioning system sensor information. The power saving was evaluated with the ECU, HVAC
unit and refrigerant circuit. The air conditioning function was evaluated with entire the system
including the vehicle.

Figure 1

Control of vehicle heat pump system

Figure 2 shows a specific example of the control logic on a computer. The HVAC unit
control logic controls the outlet mode, outlet air temperature, air flow rate and air inlet mode, etc.,
and refrigerant circuit control logic controls electric compressor speed, solenoid valve operation,
EXV opening area and cooling fan speed, etc.. This report shows the result of confirming thesystem operating state by performing various verifications on the EXV control as an example. After
full verification of the validity of the control logic and control parameters on a computer, theoperation states were confirmed with an actual vehicle.

Figure 2

Control logic

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)3|3. Power saving
To save power, it is important to control the refrigerant circuit appropriately. This section
describes the approach to power saving with electric compressor speed control and the EXV
opening area control that greatly affect air conditioning performance.
To select control parameters which are assumed to be correlated to air conditioning
performance and to be able to acquire temperature and pressure information via each sensor in the
refrigerant circuit, the air conditioning performance coefficient (COP) and outlet air temperature
were obtained with various compressor speeds and EXV opening areas through experiments. The
evaluation was carried out under the three electric compressor speed conditions of low, mediumand high with a constant ambient temperature, cooling fan speed, HVAC outlet mode and airflow
rate. Figure 3 (a) presents changes in the COP and outlet air temperature in response to the EXV
opening area during heating operation as an example. Since the EXV opening area at which theCOP and the outlet air temperature are maximized differs for each compressor speed, theexperimental results were rearranged using control parameters A and B. The results are shown inFigure 3 (b) and (c). In the case of control parameter A, the control parameter that maximizes theCOP regardless of whether the electric compressor speed is low, medium or high exists in a
relatively wide range. On the other hand, in the case of control parameter B, the range in which theCOP is maximized is limited and varies depending on the electric compressor speed. In both casesof control parameter A and B, the outlet air temperature tends to increase as the parameter valueincreases.
From the above, it can be seen that EXV control using an appropriate control parameter isnecessary to achieve both power saving and the specified air conditioning performance.

Figure 3 (a) Changes in COP and outlet airtemperature depending on EXVopening area

Figure 3 (c) Relationship between control
parameter and COP, outlet air
temperature in the case of control
parameter B

Figure 3 (b) Relationship between control
parameter and COP, outlet air
temperature in the case of control
parameter AMitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)4|4. Dehumidification heating
One of the necessary functions of heat pump systems for xEVs is dehumidification heating.
This function dehumidifies the outlet air with an evaporator before heating the air in the HVAC
unit under a certain ambient temperature range to prevent the inside of vehicle windows fromfogging. For dehumidification heating, air is cooled and dehumidified while passing through theevaporator in the HVAC unit, and then reheated while passing through the heater, and relatively hot
air is blown into the vehicle cabin.
Since the required amounts of dehumidification and reheating vary depending on the ambient
temperature and temperature setting, it is necessary to control the evaporation temperature andrefrigerant flow rate by controlling EXV opening areas, as well as the electric compressor speed.
We established refrigerant circuit characteristics and multiple control logic plans on a computer inadvance, performed simulations, examined the feasibility of the dehumidification heating function
and confirmed the effect.
Figure 4 gives a typical example of the simulation results. The air conditioning operation
was set to “Auto” with a constant ambient temperature and vehicle speed. This shows the transitionof the “Foot” outlet air temperature, the representative temperature in the vehicle cabin and theoutlet temperature of the evaporator in the HVAC unit. In the case of control plan I, the outlet
temperature of the evaporator in the HVAC unit was lower than the ambient temperature. Thismeans that the dehumidification function worked, and the outlet air temperature was stable. On theother hand, in the case of control plan II, dehumidification functioned, but the outlet airtemperature was unstable.

Figure 4

Simulation results of control plans I and II

We verified the controllability of control plan I (which had been confirmed by the simulation
to have better controllability) with an actual vehicle test. The verification results are given in
Figure 5. The vehicle test conditions were the same as the simulation conditions. It was verified
that the dehumidification heating function worked, and outlet the air temperature was stably
controlled. The vehicle test results were consistent with the simulation results.

Figure 5 Results of dehumidification heating
test of control plan I with actual vehicle

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)5In addition, to confirm the controllability of control plan I under a wide range of operating
conditions, the ambient temperature and the temperature setting were changed with the air
conditioning operation set to “Auto.” The results are presented in Figure 6. Even when thetemperature setting was raised or lowered under different ambient temperature conditions, theaverage temperature in the vehicle cabin followed the temperature setting and satisfactory airconditioning controllability was obtained.

Figure 6
Controllability under conditions
temperature setting was changed

where|5. Conclusion
To achieve both air conditioning power saving and passenger safety and comfort, we used
control simulation software as a system development tool that controls the components of a heatpump system for xEVs. Preliminary verifications were performed with this tool, and actual vehicle
testing was implemented as an experimental verification.
In the development of a heat pump system for xEVs, higher efficiency and a
dehumidification heating function were realized by appropriately setting and controlling theparameters that control the electric compressor speed, EXV opening area and HVAC unit.
We will proceed with the development of systems targeting not only higher efficiency andthermal comfort, but also a larger heating capacity for cold regions, and contribute to the progressof achieving a sustainable society by improving the marketability of the heat pump systems for
xEVs and reducing power consumption to suppress carbon dioxide emissions.

References(1)(2)

A. Natsume,et.al. Validation and Verification Technology of the Control System Software for High
Reliability Product, Mitsubishi Juko Giho Vol.43 No.2 (2006)
Y. Katayama, et.al. Development of Heat Pump System for Plug-in Hybrid Vehicles，Mitsubishi HeavyIndustries Technical Review Vol. 54 No. 2 (2017)