Three key components
Battery-fed power electronics, the e-motor and automatic transmission – these are the three key components of an electric drive train. It makes sense to combine these within a common casing in order to reduce costs and weight. This also makes it possible to supply them more easily with a single fluid, an innovative e-drive fluid. However, this is easier said than done. The gear must be lubricated to ensure low wear and abrasion. The other components especially require heat dissipation. Some renowned lubricant manufacturers have already developed special fluids with a cooling function that also lubricate the gear. These e-drive fluids must be extremely thin as efficient thermal dissipation is needed but also because the input speed of the gear is most often higher than 10,000 revolutions per minute. Its viscosity roughly corresponds to that of diesel fuel.
General information about the formulation of these products cannot be currently provided. Engineers are feverishly working on developing innovative e-drive fluids. However, mineral oils or mineral-oil mixable synthetic oils are hardly used as a basis for pilot projects, but rather water mixtures with more than 50 % consisting of other components, silicone oils or glycols. E-drive fluids must cope with a whole range of electrical, thermal, tribologist and chemical challenges. They must perform under high voltage while directly contacting copper components, elastomers of seals and insulating varnish within the e-motor. These fluids must not absorb water in order to retain a high dielectric strength and thereby prevent electric arcing between live parts.
Special challenges arise with respect to compatibility between liquid and various materials – most of all copper. Copper’s high electric conductivity makes it the most important but also critical component for all live lines as well as for the coils in the e-motor. E-drive fluids are to be highly compatible with copper. Not only the batteries, but also the power electronics and the electric motor must work within moderate temperature range. It is imperative that e-drive fluids provide efficient heat dissipation for temperatures up to 180 °C. Operation above the maximum temperature necessarily reduces the service life, efficiency level and range of the vehicles. However, extremely low-viscous e-drive fluid is not only responsible for the electric motors, but also contributes to securing the power transfer via the transmission. Many requirements must be fulfilled. Reliable lubrication, protection against wear and corrosion, high ageing stability, high material compatibility and a minimal tendency for foam formation must be ensured. Thus far, only lubricating oils whose viscosity is more than 10 times higher than the newly developed e-oils have exhibited these qualities.
Braking saves power
Modern electric vehicles are predominately moved over longer distances with the accelerator pedal. If the driver takes the foot off the pedal, the vehicle brakes automatically as kinetic energy is recovered. Since braking is somewhat time-delayed, e-mobiles are additionally equipped with conventional disc brakes. According to the manufacturer’s specifications, these require the classic brake fluid DOT 4 or DOT 5.1, which mainly consists of temperature-stable polyglycolic compounds. In some case, the silicone-containing brake fluid DOT 5 is to be used, which may not be mixed with other types.