The hybrid transaxle in the upcoming Lexus HS 250h hybrid sedan (earlier post) will use an electric transmission oil pump in place of a mechanical pump, further shifting ancillary loads from mechanical to electrical propulsion, according to Toyota.
Toyota and Lexus hybrid transaxles have up to now used low-pressure mechanical oil pumps that operate whenever the powertrain’s IC engine is running. An electric pump allows a more refined control strategy while reducing parasitic losses. Fluid paths have also been changed to flow transmission oil over the vehicle’s drive motor, increasing its efficiency.
Previous iterations of Toyota and Lexus hybrid transaxles have used transmission fluid primarily for lubricating rather than cooling purposes, with motor-generators cooled by a mixture of air circulation inside the transaxle, a water-based coolant loop shared with the vehicle’s inverter, and splash distribution of transmission fluid.
Improvements in recent years have included the optimization of oil catch tanks at the top of the transaxles to channel splashed fluid to various internal components.
The HS 250h refinement follows close on the heels of the release of the 2010 Prius, which uses an electric engine coolant pump. Although previous generations of the Prius had used secondary electric engine coolant pumps, their primary engine coolant pump was still mechanical.
Many automotive electric fluid pumps are brushless DC (BLDC) designs, which offer higher efficiency and more precise control than brushed motors. The cost of BLDC motors and their electronic circuits, which has been relatively high in the past as compared to brushed motors, has steadily dropped in recent years.