In the recent years, hybrid vehicles have gained traction given to their merits of reducing exhaust gas emissions and improving the vehicles' fuel efficiency. These vehicles thereby help at reducing the carbon foot print by vehicles.
A variety of transmissions have been designed specifically for the hybrid vehicles, with the aim to make the vehicle more efficient for fuel consumption and performance. However, a majority of the typical transmission systems are developed for a niche segment of high-end buyers.
A typical transmission system for a hybrid vehicle comprises an internal combustion engine (ICE) and an electric motor/generator. These transmission systems use similar technology as the ICE driven cars, in which the transmission takes a single input and provides output to the wheels. These vehicles are also commonly known as mild hybrids since the electric motor is of a small capacity. The motor is placed between the engine and the transmission being adapted to add its torque to that of the engine. The combined torque is transmitted to the wheels from the transmission through a differential.
U.S. Pat. No. 5,943,918 discloses one such hybrid powertrain system. The system comprises an internal combustion engine, a transmission including a transmission drive shaft coupled to the internal combustion engine and a transmission driven shaft driven at a plurality of gear ratios, and an electric motor/generator engaged with the transmission drive shaft for synchronizing rotation of the drive shaft with the driven shaft during the shifting of the transmission.
These transmission systems require that the ICE and the electric motor run at the same RPM. This is a compromising design since both the ICE and the electric motor generate their maximum torques at different RPM's. Thus, in this working either the engine or the electric motor can achieve the maximum torque.
Also, the brake energy regeneration is not achieved over a wide range of speeds resulting in limited recharging of the batteries when brakes are applied. The same limitation occurs when the batteries need to be charged while cruising on the ICE. Since, the ICE and the electric motor are running at the same RPM, the speed of the electric motor also drops to lower RPM when the transmission is engaged at higher gears, thereby reducing the efficiency of recharging the batteries. Hence, the said arrangement provides less flexibility in operation.
Another type of transmission system, known as the e-CVT transmission has been developed for full-hybrid or plug-in hybrid vehicles. The said transmission system comprises an ICE and two electric motors. The electric motors are of a higher capacity to drive the car at longer distances. These transmission systems use a power split device, such as a planetary gear set, which is connected to the ICE and the electric motors.
U.S. Patent Application No. 20140378259 discloses one such transmission system for a hybrid electric vehicle. The transmission system comprises an input shaft connected to an engine; first and second motors/generators on a transmission housing; a first planetary gear set on the input shaft having one rotation element connected to the transmission housing, another rotation element connected to the first motor/generator, and the third rotation element connected to the input shaft; and a second planetary gear set on the input shaft having one rotation element connected to the transmission housing, another rotation element is connected to the second motor/generator, and the third rotation element connected to an output gear.
The electric motors are controlled by use of power electronics for generating different torques for driving the vehicle. In such transmissions generally the ICE kicks in at higher speeds irrespective of the actual power needed.
Further, in this case the ICE and the electric motor have to work in tandem, and the transmission cannot be handled by the electric motor or the ICE alone. Further, in the said arrangement two electric motors are to be provided for the transmission to function. Another limitation of these transmissions is the mode used to regenerate the braking energy. The brake energy regeneration is possible only within a small range of vehicle speed. Also, it is not feasible to turn on the ICE for achieving the brake energy regeneration over a broader speed range.
Similar limitations are present in the typical transmission systems used in electric vehicles. Also, the complexities present in both these designs hinder large scale manufacturing, thereby increasing the cost of manufacturing. To be able to fully exploit the advantages of the technology of hybrid vehicles and electric vehicles, it is required to develop a transmission system which overcomes the afore-mentioned drawbacks of the known transmission systems, and which is compatible with a variety of vehicles, including hybrid vehicles and electric vehicles.