Lube-Tech
churning losses, reduced gear protection, and increased fluid compressibility. While many tests are available to assess a fluid’s foaming tendency (e.g. ASTM D892, ISO12152, ASTM D3427), the number of true aeration tests is limited. The DEXRON® Aeration Test, developed by GM and Southwest Research Institute (SwRI), determines the degree to which a transmission fluid will aerate, as well as the speed at which both aeration and de-aeration occur. As with other conventional fluid tests, there is some question regarding applicability to the unique operating conditions presented by current and future EV drive units. Smaller sump sizes result in increasingly limited residence times, which in turn require reduced aeration levels and faster air release. SwRI has developed a test to quantify increased aeration levels caused by the higher operating speeds of modern electric drive units.
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.141 page 4
Aging effects There is currently limited data on how EV fluid properties change as they age. To address this, mileage accumulation efforts are underway at SwRI using heavily-instrumented battery electric and hybrid electric vehicles. These projects assess the degree to which chemical properties, electrical properties, and performance change over the life of a fluid in an electrified system. The studies also seek to determine the effects of aging on both fluid and full-vehicle efficiency.
Direct liquid battery cooling Immersive coolants are used in EV applications to conduct heat away from battery cells in the absence of a cold plate. This allows the battery pack to operate at lower temperatures, which improve its charging and discharging performance. It also provides a safety benefit. Without direct liquid cooling, a battery cell failure may lead to thermal runaway, which can spread to adjacent cells and cause a fire. Direct cooling contains the thermal runaway to individual cells, potentially allowing the battery pack to continue to operate safely with individual failed cells. The advantages of immersive cooling should be considered against associated weight and packaging concerns.
Testing is required to determine a fluid’s suitability for use as an immersive battery coolant. Material compatibility tests assess the ability to prevent corrosive degradation in contact with battery cell materials such as copper, aluminum, nickel, and polypropylene. Tests designed to determine fluid heat transfer properties must be applicable to the battery environment.
Abuse testing
Figure 2: A test has been developed to quantify increased aeration levels caused by the higher operating speeds in modern electric drive units.
Battery abuse testing is necessary to determine if the immersion fluid can mitigate thermal runaway and limit propagation to adjacent cells in extreme conditions. This type of testing includes evaluations of combustibility, resistance to failure induced by physical damage, and heat induced failures.
LUBE MAGAZINE NO.170 AUGUST 2022 27
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