Electric and Hybrid Vehicle Gearbox Lubrication

The current state of the art for automotive gearbox and axles is “splash” lubrication. Oil is literally splashed around inside the gearbox by the rotation of the gears providing lubrication. In an electric vehicle, whilst a conventional gearbox with varying ratios is not used, there is still a gearbox and typically a differential to connect the motor to the wheels. The splash lubrication method has been proven to be ineffective, and many electric vehicle projects including some very high profile vehicles have encountered significant gearbox reliability issues.

This is mainly due to the torque delivery characteristics of electric motors. An internal combustion engine delivers no torque if it is not rotating, therefore the load on the gears is very small at low gearbox internal speeds. However, the electric motor has the amazing ability to deliver high torque at low speeds, meaning that the gear and bearing loadings can be very high even when there is no rotation in the gearbox, and therefore no rotation to ensure that lubricating oil is being splash distributed around the gearbox or axle unit.

There is also the added complication of regenerative braking, where the electric motor is used to recover energy as the vehicle is slowing down under braking a large portion of the braking force being used to generate electricity and return electrical power to the batteries. This can lead to rapid changes in load direction through the driveline, small clearances can generate loud clunking noises as the load direction changes on a thrust bearing.

Finally, there are also significant driveline energy loses associated with this churning of gear oil, which is proportional to rotational speed, and not necessarily required flow of oil. [1] In an electric vehicle reducing parasitic loses such as this driveline loss can increase vehicle range without the need for additional battery capacity which is expensive in terms of both cost and weight to add there is significant potential to deploy new methods.

An improved method for lubrication in the electric and hybrid vehicle gearbox is being adopted that uses a controllable electric oil pump to deliver and precisely target pressurised oil flow to the required areas or the driveline independently of rotational speed. This pressurised oil feed can also be used in bearings to dampen out noise generated by changing load directions. A second pump stage is also used to remove excess oil from the gearbox, effectively preventing churning loses from the gears. AVID OP40 dual-stage oil pumps provide a solution to this drivetrain challenge. The OP40 oil pump is powered by a highly efficient brushless DC motor. Pump speed is controllable by CANbus meaning it can be ramped up and down based on lubrication demand and independent of gearbox rotation. The pump has a high-pressure stage to provide lubrication to the gearbox bearings and gears and a low pressure scavenge stage to remove oil from the transmission and prevent parasitic loss caused by oil churning.

[1] Gearbox churning losses review study

[2] Hybrid drivetrain parasitic loss reduction