There are several different types of hybrid drives available today. The Toyota Hybrid Synergy Drive (used in the Prius) and the variation used by the Ford Escape hybrid are defined as “full hybrids,” meaning that they present all the advantages that hybrid technology makes available. These advantages include the efficiency gains from regenerative braking, idle stop technology, electric launch, and the ability to utilize a downsized engine, which needs only be sized to the average vehicle load. The electric drive can add its additional torque for above-average power demands when needed, such as full throttle acceleration.

However, another hybrid technology is now available to the consumer. That technology is known as the “power hybrid” and is different from full hybrids in only one major detail: The engine is not downsized to average load capability. Instead, the engine is left large and powerful so that its full force 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 its power hybrid concepts. These vehicles are the Toyota Highlander and the Lexus R400h, both sport utility vehicles (SUVs).

The Lexus R400h has a combined horsepower output of 268 HP from the 3.3 litre V-6 engine and hybrid-electric drive system, enabling this 4400 pound SUV to go from 0 to 60 mph in about 7 seconds. According to Lexus, by combining the 208 HP from the 3.3 litre V-6 with the power produced by the hybrid-electric drive, the total available horsepower produced is 268 HP. This is equivalent to the acceleration performance of a 4.0 litre V-8.

Toyota highlander hybrid

The Toyota Highlander hybrid is 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 engineering these larger vehicles, it was necessary to improve upon the first generation hybrid drive transaxle developed for the Prius. A new electric drive transaxle that could handle Toyota’s heaviest vehicles was required. The Toyota engineers wanted to improve performance and fuel economy 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 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.

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

The first generation unit has a single planetary gear set compared to the second generation unit that incorporates two planetaries into a single housing called the “compound gear.” This compound gear includes front and rear planetary ring gears, counter drive gear (including the new “rear planetary”), and the parking gear.

The front planetary functions like in the Prius—it is a power split device that 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 (connected to the gasoline engine) and the ring gear (directly connected to the drive wheels) can be continuously adjusted by controlling the speed of the sun gear. This is why Toyota refers to its transaxle as having a continuously variable gear ratio.

Epicyclic gear ratios

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 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

The rear planetary unit can be thought of 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. This accomplishes several things:

  • An effective gear reduction of about 2.47/1 is created 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, meaning 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 67 HP to 165 HP.
  • The MG2 is required to operate at much higher speeds than the first generation design. In fact, while the maximum 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.
P130 transaxle

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, an increase of 150V (30 percent) over the first generation design. The 288V direct current battery voltage is converted to alternating current and inverted up to 650V as needed by the vehicle. These functions are all accomplished by the motor controller mounted atop the transaxle unit.

Highlander drivetrain

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.

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