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Precise Control for Thermal Management Systems

December 10, 2018

Managing temperature is one of the most important considerations in optimizing vehicle efficiency and performance. The use of a custom, precision-controlled proportional valve or a valve module system can assist in achieving these objectives. A predictable linear control band alongside a coolant system enables maximum control of the vehicle’s temperature. Quick optimization of temperatures helps reduce CO2 emissions as well as response times, resulting in increased efficiency.

To manage temperature in a vehicle application fluid is typically used to dissipate heat. Coolant, which has long been been utilized for mechanical systems, is now also used to manage the temperature of electrical systems. For these applications, coolant can be redirected using a custom proportional valve assembly to help regulate the temperature for batteries, starters, electrical systems and transmissions. Some applications that require thermal control have two inlet fluid temperatures.

There are many different configurations and designs for proportional valves. A typical valve design incorporates ferrous alloy components, copper wire (coil) and a variable orifice designed to meet the necessary requirements of an application.

Figure 1
Proportional valves can control fluid based on heat feedback

Maintenance of Solenoid Valves

Depending on the fluid application, material selection is critical to a solenoid valve’s life and operation. Diverse ferrous alloys are required for solenoid performance. Some of these materials can be more susceptible to corrosion than non-ferrous alloys. If a thermal management system requires fluids similar to water or antifreeze it is appropriate to use a material that has minimal degradation due to oxidization or use a plating process to protect the base material.

There is a trade-off in using plating rather than a less corrosive material. The plating process has a lower cost but is less reliable than a more expensive, less corrosive material. Plating on a component of this sort can be degraded over time and compromise the durability of the solenoid valve in corrosive environments. The decision on what material should be used is a balance between application requirements and overall cost.

Ferrous alloys are used as both static and dynamic components to complete a magnetic circuit. The magnetic flux, generated from the coil, attracts the dynamic component (armature) toward the static component (pole) creating a positional change interacting with the orifice. With highly engineered valves, intricate shapes can be used for the orifice, other than a simple circle, to create more desirable flow curves and performance. These shapes can help meet the requirements of maximum flow at the minimum supply pressure as well as achieving minimum flow at maximum supply pressure. However, with every incremental increase in performance there is another hurdle to overcome. In the case of proportional valves this hurdle is achieving zero leakage in the closed position without losing the desired proportional control.

One way TLX Technologies has achieved zero leakage is to pair a binary valve, or on-off valve, with a proportional valve into one module. With the binary valve able to completely restrict flow, the proportional valve will not sacrifice performance. This combination of valves can achieve fast response times with flow control accuracy up to 2%.

Figure 2
Combining the binary and proportional valve restricts leakages but not performance

The key to achieving this accuracy range is to reduce hysteresis. There are three main forces that cause valve hysteresis - friction, fluid dynamics and residual magnetism. Through development and testing these effects can be minimized and optimized to meet the customer’s requirements. Additionally, if a flow feedback sensor is added to the valve assembly, the valve can adapt in real time to the system and can provide accurate temperature and flow control within milliseconds.

Valve Control Strategies

The valve module can be controlled by either a DC voltage or a pulse-width modulation (PWM) signal. The ideal control strategy to reduce power consumption for a module containing both valves requires a 'peak' (operating at 95-100%) and 'hold' (a much lower output percentage) signal for the binary and a PWM signal to vary the duty cycle in the proportional. If the duty cycle is varied, the position of the armature will change. The hold will depend on the forces created from the valve design. For a proportional valve, if you increase the PWM duty cycle it will result in increased or reduced orifice area exposure. The valve will then increase or reduce the operating fluid flow.

Proportional Performance Graph
The control band will vary depending on controller limitations

To further decrease power consumption a latching solenoid valve may be used in place of the binary valve. A typical latching solenoid is configured similarly to a binary valve, but with the addition of permanent magnets. The permanent magnets generate their own magnetic circuit within the assembly, attracting the armature to the pole. A pulse of current brings the armature in contact with the pole, where it then remains 'latched' by the force from the permanent magnets, requiring no power. A pulse with an opposite polarity can be introduced to cancel the force from the permanent magnets, causing the armature to return to its initial position, again requiring no power.

Adjustable and Predictable

When in search of a device that needs adjustable and predictable flow rates, look for a proportional valve. When zero flow is also required the proportional valve should be paired with a binary or latching valve. These valves should utilize a PWM input signal to significantly reduce the power consumption of the entire module. If DC voltage control is the only signal available, power not used is wasted and lost as heat. Solenoid valves can be designed to operate under extreme conditions and maintain performance for millions of actuation cycles with very little power consumption. Overall, this technology will provide benefits to many different thermal management applications for automotive and industrial vehicles.

This article was originally published by IVT International in December 2018

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