|System configuration of the GS 450h. Click to enlarge.|
At last week’s SAE Hybrid Vehicle Technologies Symposium 2007, Akihiro Yamanaka from Toyota provided some details on the hybrid drivetrain for the Lexus GS 450h—the first FR (front-engine, rear-wheel drive) passenger car hybrid on the market.
As it has introduced more hybrid models, Toyota continues to develop enhancements to the original Hybrid Synergy Drive II, which is the core of the second-generation Prius, relevant to various vehicle applications.
The company’s first foray into hybrid systems for vehicles that require more power—such as SUVs—was the system applied in the Rx 400h and Highlander Hybrid SUV. This system adds a conventional planetary gearset as a motor speed reduction gear at the rear of the transmission to increase torque.
By increasing system torque with the reduction gear, Toyota was able to reduce the size of the transmission’s two electric motors, and properly match the hybrid system with a larger engine.
|Schematic of the GS 450h transmission. Click to enlarge.|
The hybrid system in the GS 450h takes a step further, and uses a two-stage motor speed reduction device. Specifically developed for rear-wheel drive systems, the Toyota Hybrid System II with Two-Stage Motor Speed Reduction Device utilizes the motor’s high-efficiency region over a wider range of gear ratios than before. The hybrid system in the GS 450h supports both high output and high vehicle speed.
|Evolution of Toyota Hybrid System|
|System||Toyota Hybrid System (THS)||Hybrid Synergy Drive (THS II)||Lexus Hybrid Drive
w/ motor reduction gear
|Lexus Hybrid Drive FR
two-stage motor reduction
|Lexus Hybrid Drive AWD|
|Vehicle||Prius||Prius||Rx 400h||GS 450h||LS 600h|
|Drive||FF (front-engine, front-wheel drive)||FF||4WD||FR||AWD|
|Motor||33 kW||50 kW||123 kW||147 kW||–|
|System||82 kW||200 kW||254 kW||More than 300 kW|
|High and low gear operation.|
To engage the low gear range, Toyota opens the conventional planetary gearset’s brake 1 and locks brake 2, resulting in a reduction gear ratio of 3.900. For high gear, brake 1 is locked and brake two is open, with a reduction gear ratio of 1.900. (The Rx 400h uses a fixed reduction ratio of 2.478.) The result is that the motor in the 450h acts as a single high-torque and high-speed motor. Motor size can then be reduced enough to fit the transmission in the GS450h floorpan tunnel.
The GS 450h uses a DC boost converter (288 Vdc to 650 V max) to support increased motor speed.
In addition to motor improvements, the GS 450h uses a new compact and high power control unit, commonly referred to as an inverter assembly, that is downsized through an improved cooling system. The new inverter assembly, which is similar to the inverter assembly used in the Camry hybrid, is about the same size as an auxiliary battery, and is located at the passenger side rear of the engine bay. The auxiliary 12V battery is moved to the luggage compartment.
|Acceleration vs. CO2. Click to enlarge.|
Combined with the dual injection (direct and port) 3.5-liter engine, the GS 450h is able to achieve lower carbon dioxide emissions than the 3-liter class diesel engine group, while delivering the acceleration that is an important attribute for this vehicle segment.
The GS 450h uses hydraulic brakes in the front and rear, and regenerative braking on the rear axle only. As front brake energy is unrecoverable, the GS 450h distributes braking force to the rear wheels as much as possible.
The dual injection (direct and port) 3.5-liter engine of the GS 450h produces lower carbon dioxide emissions than the 3-liter class diesel engine group, yet is capable of 0-60 mph acceleration in 5.2 seconds.
Interestingly, a recently published SAE paper on the GS450h transmission compares its form factor to that of the automatic transmission used in the current Toyota Tacoma.