Toyota engineers sought to improve performance and fuel economy over the Prius unit while reducing noise and maintaining a compact size. The result is a second generation hybrid drive system centered around a greatly improved hybrid transaxle.

A New Generation Hybrid Transaxle for Toyota's Power Hybrids

Rich Cregar | NAFTC

A New Generation Hybrid Transaxl
Toyota engineers sought to improve performance and fuel economy over the Prius unit while reducing noise and maintaining a compact size. The result is a second generation hybrid drive system centered around a greatly improved hybrid transaxle.
A New Generation Hybrid Transaxle for Toyota's Power Hybrids
By: Rich Cregar, Instructor, NAFTC

As Earthtoys readers are aware, there are several different types of hybrid drives available today. As discussed in our previous articles (Earthtoys June 2005), the Toyota Hybrid Synergy Drive as used in the Prius and the variation utilized by the Ford Escape Hybrid are defined as "Full hybrids" in that they present all the advantages that hybrid technology makes available. These include the efficiency gains from regenerative braking, idle stop technology, electric launch and the ability to utilize a downsized engine which need only be sized to the average vehicle load. The electric drive can be counted on to add its additional torque for above average power demands when needed, such as full throttle acceleration.

However, as discussed previously, another hybrid technology is now available to the consumer. That technology is known as the "power hybrid" and differs from full hybrids in only one major detail. The engine is not downsized to average load capability in a power hybrid. The engine is left large and powerful so that its full power can be added to the additional torque available from the hybrid-electric drive to provide performance beyond what is possible from a conventional automobile. Toyota Motor Co. recently introduced their power hybrid vehicles. These vehicles are the Toyota Highlander and the Lexus R400h, both SUV's.

Make no mistake, these are power hybrids. The R400h Lexus claims a combined horsepower output of 268 HP from the V-6 engine and hybrid electric drive system which can hurtle this 4400 pound SUV from zero to sixty mph in about 7 seconds. The "400" in R400h typically refers to engine displacement. Therefore, you might think it means the Lexus has a 4.0 litre V-8 motor. This is not the case however. The R400h is equipped with a 3.3 litre V-6. According to Lexus, by combining the 208 HP from the 3.3 litre V-6 with the power produced by the electric hybrid drive, the total available horsepower produced is 268 HP and this total output is equivalent to what would be produced by a 4 litre V-8 engine. Or as stated in their literature, the R400h provides "The acceleration performance of a 4.0 litre V-8".

The Toyota Highlander hybrid. Powered by a 3.3L V-6 and the Toyota Hybrid System ll (THSll) hybrid transaxle. This drivetrain is shared by the Lexus R400h. Photo courtesy of Toyota Motor Co.

In the course of engineering these larger vehicles, it was necessary to improve upon the first generation hybrid drive transaxle that had been developed for the Prius.

A new electric drive transaxle that could handle Toyota's heaviest vehicles was required. Toyota engineers sought to improve performance and fuel economy over the Prius unit while reducing noise and maintaining a compact size. The result is a second generation hybrid drive system centered around a greatly improved hybrid transaxle known as the P310. (The unit found in a Prius is designated P112). The P310 hybrid transaxle is capable of handling more than twice the power output of the Prius while maintaining nearly the same size and only 36 pounds of additional weight.

The Prius, or first generation transaxle, basic components include two electric motor generators (MG1 & MG2), a damper plate which acts as the connection to the gasoline engine, a final drive (differential) which connects the unit to the wheels and a planetary gear unit.

The first generation unit has a single planetary gear set. The second generation unit incorporates two planetaries into a single housing which is called the "compound gear". This compound gear includes front and rear planetary ring gears, counter drive gear (which includes the new "rear planetary") and the parking gear. Think of it like this: The front planetary functions just as it does in the Prius. It is a power split device which uses the variable load produced by running MG1 as a generator as an electronic "clutch". The variable electronic "friction" produced by MG1 can control the speed of its sun gear. The gear ratio between the planetary gears (which are connected to the gasoline engine) and the ring gear (which is directly connected to the drive wheels) can be continuously adjusted by controlling the speed of the sun gear. This is why Toyota refers to their transaxle as having a continuously variable gear ratio.

Two views of a planetary gear set which can be used as a power split device or for speed reduction and torque multiplication. The components of a planetary include the sun gear (yellow), the carrier (green), the planetary gears which are attached to the carrier (blue) and the ring gear (pink). If any one component of the planetary is "held", power can be transferred through the other two components with a multiplication of torque. Note the alignment of the red index marks in the first diagram. In the second diagram, the sun gear has rotated 180 degrees, yet the planetary carrier has only rotated about 45 degrees.
Graphic courtesy of Wikipedia, the free encyclopedia

Question: If a Prius is at rest and the gas engine is running, is MG1 rotating? Is MG2 rotating?

Remember, the ring gear is directly connected to both the wheels and MG2. Therefore, if the car is at rest MG2 cannot be rotating! So, the ring gear cannot rotate (the vehicle is at rest), yet we know that the planetary gears are rotating because they are connected to the gas engine and it is running. This means that the sun gear (With MG1) must be rotating as well! This coupling between MG1 and the gas engine is how MG1, acting as a motor, is used as a starter motor for the gas engine.

Now let's look at the function of the rear planetary unit. Think of this new device simply as a reduction gear for MG2. The first generation unit coupled MG2, the primary drive motor, directly to the wheels through the ring gear of the planetary. Within the newly designed compound gear, MG2 is connected to the rear unit sun gear. The planet carrier is locked to the case. The ring gear, as before, drives the wheels. What does this accomplish?

  • An effective gear reduction of about 2.47/1 between MG2 and the output of the ring gear.
  • Since the ring gear now rotates more slowly than MG2, a multiplication of torque is achieved. this means the torque available at the ring gear is greater than the torque produced by MG2 at the sun gear!
  • This design allows the output torque of MG2 to be reduced while at the same time increasing the overall output of the transaxle. In fact, while the torque of MG2 was reduced by almost 50 ft lbs, output is more than doubled: from 67hp to 165hp.
  • This design requires MG2 to operate at much higher speeds than the first generation design. In fact, while the max. speed of MG2 in a Prius is limited to 6000 RPM, the new MG2 operates at speeds up to 12,400 RPM!
  • An electric motor running at higher speed yet producing less torque allows the size of MG2 to be reduced. The result is more power, with essentially the same size and weight of the previous unit!

 Cutaway detail of the P310 transaxle showing the details of the "compound gear", the front planetary "power split device" and the rear planetary "reduction gear". The field windings of MG1 (to the left of the compound gear) and MG2 (on the right of the compound gear) are also visible. NAFTC File Photo

Battery voltage and operating voltage are also both increased in this design. The battery now operates at 288V (an increase of about 80V) and maximum voltage utilized by MG2 now stands at 650V, in increase of 150 volts (30%) over the first generation design. The 288V direct current (DC) battery voltage is converted to alternating current (AC) and inverted up to 650V as needed by the vehicle. These functions are all accomplished by the motor controller which is mounted atop the transaxle unit.

Cutaway view of the entire highlander drive train, including a view of the V-6 engine. NAFTC File Photo

Toyota and Lexus now have a much more efficient hybrid transaxle that can be sized to handle its entire range of passenger vehicles, This new unit is also more efficient and powerful than the hybrid transaxle used in the only other hybrid SUV, the Ford Escape. I expect to see this new generation unit replacing the original design within the near future.

For more information on hybrid technology, or one of our other training topics, please visit our website: The NAFTC has training courses and workshops covering every facet of alternative fuels and advanced technology vehicles.


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