Thermal Management Plays Critical Role in Electric Vehicle Performance
Thermal management plays a critical role in the performance of electric vehicles and, when harnessed with the right technology, even holds the potential to give electric vehicles more mileage per charge. But EV thermal management comes with its own challenges.
To keep up with changing flow requirements in an EV cooling system and to maintain optimal component temperatures, the cooling system ideally needs proportional control. A valve to control this proportional flow would need to feature zero hysteresis and zero steady-state power at any setpoint, include a leak-free off state, and also contain a fail-safe condition for when power is lost.
No valve featuring all of these capabilities existed until last year. In early 2019, TLX Technologies, a creator of customized electro-mechanical solenoid products, designed and introduced the Discrete Proportional Valve (DPV).
The DPV is a simple combination of two or more binary (on/off) valves with different flow coefficients which, when combined, approximate proportional control. The number of possible flow states is 2n, where n is the number of valves. In practice a two-valve system with four flow states is all that is needed.
Figure 1 shows the performance comparison between a continuous proportional valve (orange) and a three-valve DPV (blue). At 0% duty, the continuous valve has some bypass flow. The continuous valve also requires lower and upper deadbands to accommodate part tolerances. Finally, the flow for a given input in a continuous valve is not the same when moving in an increasing direction as opposed to a decreasing direction (hysteresis).
In contrast, the DPV does not need to have bypass flow (but can be designed with bypass flow as desired). Additionally, it has no need for deadbands and features zero hysteresis. The DPV also uses latching (bistable) valve actuation to reduce power consumption.
Controls for DPV systems can be implemented either on the valve or on a remote. Integrated control may be preferred in some systems, since it reduces wiring to two power wires and a signal line. Remote electronics may be preferred when incorporating controls on the valve is undesirable due to its environment. In this case, the valve system needs two power wires per valve.
The control requirements for DPV-based valving are simpler than they are for many other solutions. PID control is not necessary. It only needs a mapping of the valve state to target flow conditions. If using on-valve electronics, the valve system’s mapping can be provided in the control circuitry, and the vehicle’s control electronics only needs to communicate a flow target.
This DPV design also features two built-in fault-tolerant features. The first is that the control system can be designed with a specific fail-safe position. The second is that the DPV system does not have any single point of failure that can disable the entire system. A failure in one valve does not completely disable the entire valve system.
The DPV concept provides advantages that match the unique needs of EV cooling and energy management systems, such as enabling rapid changes in coolant flow to instantly meet demands while enabling power-efficient management of coolant flow and potentially reducing the overall weight and cost of the cooling system.
***First published in Engine + Powertrain Technology International magazine, March 2020***