Ventilation solutions boost safety and extend service life of vehicle electronics
Equipping electronics for extreme conditions
Whether on the underbody of the car or under the hood, electronic components such as engines, control units, sensors, compressors or pumps are exposed to extreme variations in temperature and must be protected from dirt or liquids getting in. At the very least, they should be protected in line with the IP6k9k standard. Electronics housings that meet this standard offer reliable protection against dust particles, brief immersion and jets of steam.
Electronic components in vehicles are exposed to major fluctuations between the operating temperature and the cooler ambient temperature. When the vehicle is in use, the components become very hot; when confronted with something such as cold road spray, they are then subjected to rapid cooling. This generates a great deal of negative pressure in the electronics housings, sucking in air from the outside through the seals. Over time, this unwanted pressure equalization puts such stress on the seals that dirt particles and liquids get in, corroding the electronics and potentially shortening the component’s service life.
A particular challenge: protecting electronics in electric and hybrid vehicles
Because of their extremely high operating temperatures and the above-average size of their electronics housings, hybrid and electric vehicles present automotive manufacturers and suppliers with an even greater challenge when it comes to the issue of temperature and pressure equalization. When hybrid or electric vehicles are in operation, significant power dissipation heats up their sensitive high-performance components to a far greater extent than it would the electronics in a vehicle with a combustion engine.
To protect the electronics from damage caused by extreme temperature fluctuations and keep them in the optimal temperature range, manufacturers most often turn to liquid cooling. However, this still carries the danger of condensation forming at the coldest point in the housing – corroding and possibly even destroying the electronics.
These high-performance electronics must also be protected against the pressure spikes that occur due to the difference between the internal and external temperatures. In large battery housings, these pressure variations reach an extent where they are almost insurmountable without effective temperature and pressure equalization solutions. Even small differences in temperature can exert enough pressure on large housings to deform them.
Example: pressure equalization in a high-voltage battery housing
The following example demonstrates this phenomenon using the pressure changes observed in an electric battery with a volume of 150 l measuring 100 cm x 50 cm x 30 cm. Let us assume that the volume of free air within the housing is 50 l. In the 30-minute journey from Innsbruck (570 m above sea level) to the Brenner Pass (1370 m above sea level), the electric vehicle will have gained 800 m in altitude. In an electric battery with no ventilation, this leads to a positive pressure of 90 mbar, a difference that cannot be equalized even over a 15-minute break at a service station and that puts constant pressure on the seals. (Fig. 2) This 90 mbar of positive pressure is equivalent to around 450 kg acting on a surface of 0.5 m2. No housing built using lightweight engineering techniques will be able to withstand that sort of force in the long term. And while the seals are designed to cope with high levels of stress, this sort of extreme strain will eventually lead to a porous and inadequate seal for the housing.
Much more dangerous than the positive pressure generated on the way up is the negative pressure of 90 mbar that builds up in the housing on the way back down from the Brenner Pass to Innsbruck. To equalize the pressure, air is drawn in through the stressed seals. This is how dirt particles and liquids get inside the housing, where they condense and contaminate the electronics, potentially even destroying them over time. A ventilated battery housing, on the other hand, is subject to a negligible negative pressure of around 15 mbar. This sort of pressure does not put any unnecessary stress on the seals and can be fully equalized in the course of a 15-minute break.
Membrane technology equalizes pressure
OEMs currently use a variety of methods to overcome the challenge presented by pressure differences. One way to protect electronics units from dirt and liquids is to mold them into the housing. While this does create a closed system, the weight added is not insignificant. What’s more, it can’t be opened up and repaired if a defect arises. Other ways of achieving a hermetically sealed system include the use of high-end seals and thick walls for the housing. The disadvantage is that these components are more expensive and unnecessarily heavy. A much more sensible and commonly used option is to employ a membrane that ensures the equalization of pressure within closed housings while preventing liquids and dirt particles from getting inside. This membrane must fulfill different requirements depending on its intended application – whether in control units, engines, sensors or batteries, or in halogen, xenon or LED headlamps.
The most important properties of any membrane are airflow and water entry pressure. Airflow determines how much air passes through the membrane in a set period given a specific pressure difference. In other words, it defines the time it will take to equalize a given difference in pressure, whether positive or negative. The water entry pressure is the minimum hydrostatic pressure required to force water through the membrane and cause a leak. Both properties are affected by the membrane’s pore size.
Breathable and resistant: the right ventilation for every application
Membrane suppliers have to find the right balance between airflow and water entry pressure for each individual ventilation application. For instance, a large battery housing needs to exchange a large volume of air within a short space of time, but is less demanding when it comes to the IP protection class. In this case, it makes sense to use a membrane with a high airflow. Electronics housings located under the hood, on the other hand, often have to contend with extreme spikes in temperature – and so the usual choice is to go with a membrane that is highly temperature resistant.
Given that the challenges vary dramatically from application to application, it’s advisable for automotive manufacturers and suppliers to choose the right solution for them in close consultation with the membrane supplier.
ePTFE (expanded polytetrafluoroethylene) is a material with a fine-pore microstructure that makes it ideal for use in the engine compartment and powertrain – resistant to chemicals, intense heat and extreme weather conditions. Due to the small amount of free surface energy, ePTFE displays exemplary water-repellent properties, so water that beads on the surface cannot penetrate the membrane structure. In the case of automotive applications, the membrane ought to be oil-repellent as well, since it’s extremely likely that components will come into contact with motor oil, cleaning agents or similar car fluids. However, achieving this quality calls for further work on the membrane. Another advantage of ePTFE is that it is extremely resistant to temperatures from -150 °C to 240 °C, a characteristic that is particularly important in light of the trend towards engines that deliver more performance but are more compact in design. This downsizing means that temperatures of 150 °C and more in the engine compartment are not infrequent.
All in all, membranes are a way of prolonging the service life of electronic components both in current and future vehicle concepts. With their help, automotive manufacturers and suppliers will be able to continue pushing forward with the electrification of the motor car.
About the author:
Rainer Enggruber is Product Line Manager for Automotive Electronics at W. L. Gore & Associates GmbH.
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