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Lexus HS 250h Extends Electrification of Drivetrain to Transmission Fluid Pump

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.

—Jack Rosebro



Now that the HS250 has been reviewed by a few car magazines I wonder who the customer will be?

Ford has shown that they are the technology leader when a purpose built Lexus hybrid with similar displacement engine, total weight, and Cd gets lower fuel economy. The Lexus HS250 is expected to achieve 35city/34highway while the Fusion hits 41city/36highway.

From what I have read the Lexus refinement only equals a well equipped Fusion hybrid.

...even the images I have seen of the production HS250 bear a striking resemblence to the Fusion Hybrid from the front.



We had Ford's and Toyota's and like most who had similar experiences, the inherent quality of a Lexus is generations ahead of a Ford Fusion. They are not even comparable quality wise) even if they are about the same size.

As far as mpg, it remains to be seen which car will be best. For 50+ mpg, buy the Prius III.


It looks a lot like a Camry with a Lexus badge. Quality will be determined over time by agencies like J.D. Powers and others that collect data. Ford will keep an eye on quality going forward, their future depends on it.


Um, does the mechanical pump run too slow and too fast as engine or vehicle speeds change?

Does it run when it does not need to?

If not, then I look forard to Lexus encorporating mechanical headlights.

My daughter has a Lexus RS350 - it is very nice.
At 90kMi it accepted a transmission rebuild for $6000, notvery nice.

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