Thermal Management System – Pressure Sensor

Amid the rapid development of new energy vehicles, the Thermal Management System (TMS) has become increasingly crucial as a key system ensuring vehicle performance, safety, and range. Within this complex and sophisticated system, pressure sensors serve as indispensable “nerve endings,” continuously monitoring the dynamic pressure of refrigerants and coolants to provide core data support for system control.

Pressure parameters play an extremely important role in thermal management systems:

In refrigerant circuits, pressure directly determines the saturation temperature of the refrigerant, serving as the key to controlling evaporation and condensation processes, and affecting cooling/heating capacity and efficiency. Abnormal pressure (either too high or too low) serves as a direct warning signal for system failures (such as blockages, leaks, or compressor malfunctions).

How does a pressure sensor transmit pressure values? Taking a certain type of sensor as an example: The ratio between output voltage and supply voltage has the following relationship with the pressure value measured by the pressure sensor:

Vout represents the output voltage, Vc represents the supply voltage, and P represents the gauge pressure (MPaG). The following pressure characteristic curve is observed:

The main types of pressure sensors include:
Silicon piezoresistive (MEMS): Uses the piezoresistive effect of monocrystalline silicon. Pressure acts on the silicon diaphragm, causing stress changes that alter the resistance of the embedded Wheatstone bridge and output a differential voltage signal. Widely used in coolant circuits and medium/low pressure refrigerant circuits (such as the low pressure side of R134a).

Ceramic capacitive: Pressure acts on the ceramic diaphragm, changing the capacitance between it and the fixed base plate. Used in high pressure, high temperature and corrosive environments (such as the high pressure side of R744 CO2 systems and certain high pressure coolant points).

Sputtered thin film: Strain-resistant thin films are sputter deposited on metal (usually stainless steel) elastic elements to form a Wheatstone bridge. Mainly used in extremely demanding, high-reliability applications for premium or commercial vehicles.

As the “pressure eye” of thermal management systems:
Refrigerant high pressure side: Usually located at the compressor outlet and condenser/gas cooler inlet. Monitors compressor discharge pressure and condensing pressure.
Refrigerant low pressure side: Usually located at the evaporator outlet and compressor inlet. Monitors evaporating pressure and compressor suction pressure.

To ensure sufficient refrigerant in the thermal management system for normal cooling/heating operation, how pressure sensors detect refrigerant charge level:

Determine if refrigerant charge is sufficient by checking whether the maximum values of high and low pressure sensors in non-operational state are below a certain rated pressure value. This method is relatively straightforward but crude.

Compare the maximum values of high and low pressure sensors in non-operational state with the saturation pressure value of refrigerant vapor at current external temperature. If the difference exceeds a certain rated value, it indicates low refrigerant charge and triggers a pop-up warning.

Pressure sensors, as core components for sensing fluid pressure in new energy vehicle thermal management systems, are like the system’s “blood pressure monitor.” From being the last line of defense for battery safety to being key enablers for improving heat pump efficiency, from simple threshold alarms to supporting complex intelligent diagnostics and optimal control. In the future, with the emergence of new materials, processes and architectures, pressure sensors will continue to deepen their key role in the “sensing-decision-execution” closed loop of thermal management systems.