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.
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 Priusit 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.
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:
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.
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.