When THT Co. refers to its TH-Mission, on display at the April 12-14 SAE 2011 World Congress in Detroit, as fifth-generation technology, it is in a way paying homage to what came before it, even though “it is superior to even the latest transmission in terms of the structure and performance. TH-Mission does not only inherit all the pros of former generations but also solves the problems that the former generations had.”
For example, while continuously variable transmissions (CVTs), which at their most common consist of two variable-size pulleys linked with a belt, offer good fuel efficiency and convenient operation, disadvantages include high manufacturing costs and belt skidding at rapid acceleration. And while dual-clutch transmissions (DCTs), in which one clutch for odd number gears and the other one for even number gears operate alternatively, also offer fuel efficiency and convenience, they have the disadvantage of high manufacturing costs and a somewhat complicated structure.
By contrast, the company says that its continuously gear-meshed variable transmission (CGVT) technology that makes up TH-Mission is “extremely simple in structure” and requires no advanced technology to manufacture. Also, its gears are always meshed so loss from power delivery is minimized for better performance, allowing it to be made very small and light. In fact, THT says that TH-Mission “can be small as a thumb and indefinitely large as long as building technology allows, so that it can apply to all kinds of existing driving machines,” including automobiles, electric vehicles, electric carts, motorcycles, trucks, tractors, submarines, agricultural equipment, and even industrial power plants.
While the device may look unassuming, THT says the mechanism that controls speed and torque output “is not as simple as the structure. Basically, the main motor supplies primary power to the whole mission, and the control motor decides how to use the power.”
THT describes the operation of TH-Mission as being “characterized by moving smoothly from one extreme state to another” or, essentially, gear shifting. Those two extreme states can be compared to the lowest and highest gear of other transmissions; the lowest gear corresponds to the “torque-oriented” state and the highest gear to the “speed-oriented” state. THT says that it refers to the conditions as states instead of gears because TH-Mission is not designed with multiple numbers of different gears to shift. Rather, the control motor functions so that even though gears are always meshed, gradual shifting from torque- to speed-oriented state is possible. Moreover, the state shifting is so gradual that shift shock is never experienced in TH-Mission, claims the company.
Torque-oriented state provides the highest output torque and lowest speed, very similar to the first gear of other transmissions. But, according to THT, torque-oriented state is unique in that it maximizes output torque close to 100% and minimizes the speed almost to zero. In other words, TH-Mission converts power from the main power source almost completely to torque, which is beneficial since it requires a great amount of torque for a vehicle to overcome static friction that occurs when trying to move from standstill.
In terms of its make-up, in the torque-oriented state, the rotation of the ring gear and the carrier (pinion gear revolution) is subtracted to decrease the sun gear (output) rotation.
By contrast, speed-oriented state provides the highest output speed and the lowest torque. As a vehicle starts moving, static friction disappears and dynamic friction becomes the only force to overcome, which is much less than the static friction. Therefore, the output torque starts to be wasted.
Although other transmissions are capable of shifting to higher gears as the dynamic friction decreases, output torque is still wasted to some extent because the ratio gap between the gears is so large that it is expressed as shift shock. THT says that TH-Mission “gradually converts the output torque to speed as if it has thousands of gears to shift.” As a result, TH-Mission goes to maximum speed “with minimum waste of energy.”
In the speed-oriented state, the rotation of the ring gear and carrier is added to increase the sun gear rotation.
The engine (or motor) braking system of TH-Mission is controlled electrically. Releasing the accelerator and stepping on the brake switches from speed- to torque-oriented state by both stopping the main power source and gradually increasing the carrier rotation to predetermined rotation rate. As the carrier rotation rate increases, the sun gear rotation rate exponentially decreases, thus the speed also decreases. The stepping strength of the brake pedal determines how fast the rotation rate of the control motor increases to differentiate braking intensity.
During deceleration, the sun gear run by inertia provides power to the mission so the direction of power changes toward the input; as a result, the sun gear rotates the input gear that is connected to the main power source. Therefore, if the main power source is a motor, it becomes a generator during braking.
In addition, because braking shock increases the workload of the control motor, important for the braking system of TH-Mission, workload to the worm gear should be reduced. The bevel gear of TH-Mission takes the half of braking shock. In that way, the worm gear works more easily.