EV and hybrid applications

Thursday, May 08, 2014

Tags :  oil-system

Electric and hybrid electric/internal combustion technology is finally starting to make headway in the racing world. Such powertrain solutions present a host of challenges relating to electrical energy storage and motor technology, and they also require a reassessment of lubrication and cooling systems. Electric motors and energy stores need both lubrication and cooling, with the two tasks increasingly being combined through the use of shared lubricating oil/coolant.

While not yet common in racing machinery, significant savings in packaging and weight can be made by sharing lubricating/cooling oil between the electric motors and any transmission systems used. (There is a possibility that such an approach is taken on Audi’s current R18 LM P1, where the front-mounted MGU features a dedicated gearbox/differential unit.) For example, General Electric unveiled a motor in 2013 that uses regular automotive transmission fluid that can be shared with a vehicle’s transmission.

Such sharing considerably reduces the complexity of cooling and lubrication systems on hybrid vehicles, and in a racing context that means less pipework and thus less weight and a smaller packaging envelope. If the same fluids can also be used for cooling energy stores, then evidently further savings can be made. Simplifying these cooling circuits could also have potential benefits in relation to aerodynamic performance. Beyond simply reducing the number of pipes under a car’s body, allowing for tighter body packaging, individual coolers for hybrid and other systems can also be eliminated. Although the same levels of heat rejection will still need to be accommodated, the use of a single cooler allows for more efficient packaging and less ductwork, potential reducing overall vehicle drag.

While not directly related to lubrication, another factor that needs to be considered is the impact of electricity on the life of bearings. Most electric motors used in motorsport are ac, three-phase traction motor/generators. Since batteries provide dc, inverters (also known as variable frequency drives, or VFDs) are needed to convert the dc to ac. These have an unfortunate side effect – they induce unwanted voltages on motor shafts. Without effective, long-term electrical grounding, this shaft voltage can erode and eventually destroy motor bearings. The bearings can be insulated, but this can simply move the problem elsewhere.

The effect is exactly the same as that found in an electrical discharge machine, with the current moving from the motor’s shaft to the bearing, removing material. The need to find ways of mitigating this effect is not a new one, and electric motor manufacturers for sectors such as mass transport and heavy industry have developed various solutions. For example, one system being investigated by some automotive manufacturers is the use of a conductive fibre ring around the motor shaft, which provides an earth path that bypasses the bearings.

The point illustrated here only scratches the surface of what is a new and emerging area of vehicle lubrication. As mixed and pure EV powertrains appear in increasing numbers, efficiency gains relating to lubrication improvements will no doubt be an area of great interest to those involved in racing them.  

Written by Lawrence Butcher

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