Novel Hybrid Electric Vehicle Transmission

Introduction

The sharply rising crude oil prices have put a technical challenge to the automotive sector to reduce the fuel consumption. Automotive business is looking towards electric vehicles to reduce the dependence on oil. However, battery technology development has been found deficient to support electric vehicles. Fuel Cell powered vehicles are being sought as permanent solution but infrastructural demands of transporting hydrogen currently cloud their progress. Hybrids - assimilation of electric vehicles and current IC Engine based vehicles - have been shown to reduce fuel consumption by Toyota (Prius, Lexus RX400h), Honda (Insight, Civic) and Ford (Escape).

The fundamental difference between a hybrid vehicle and an IC Engine based (conventional) vehicle lies in their powertrain. A hybrid uses at least two prime-movers whereas a conventional vehicle uses only one. This implies that the transmission gearbox be able to accept two inputs rather than one i.e. it needs to be a two degrees-of-freedom mechanism. Additionally, the gearbox needs to be compact. A planetary gear train (PGT) is the most basic two degrees-of-freedom mechanism. But a single PGT gearbox (as in Toyota Prius) means that the wheels never receive the full torque of the prime-mover, whenever the vehicle is being run by a single prime-mover. A compound PGT - with two such basic PGTs interconnected - then becomes a natural solution.

The functional desirables of a compound PGT transmission are

• Add up the speeds / torques of both the prime-movers while motoring;
• Operate the engine in favourable region of its torque-speed map;
• Allow a good torque multiplication at low-end speed - a characteristic extremely important considering the increasing vehicular traffic density around the globe;
• Fool-proof i.e. allow the vehicle to be operated by any of the prime-mover (operating alone), should the other energy arm of the powertrain fail for any reason;
• Ensure that the torque-speed addition from both the prime-movers is harmonious at all times.

Further the transmission mechanism needs to be augmented by a tailored control architecture that is

• Implementable and real-time i.e. one devoid of any a priori knowledge of drive cycle.
• Scalable - increasing the motor or engine torque or any other system variable must not negatively affect its output.
• Non-myopic - A myopic operation of vehicle misleads into notions of lower fuel consumption based on one round of a drive cycle.
• Balanced - none of the two energy paths i.e. electric motor and IC Engine is overloaded at any time, whereby health of both the energy paths is taken care of, at all times.

Technology being offered

The current invention relates to one such compound PGT meeting all the desirable features listed above, and harmoniously adds up the power of both the prime-movers i.e. the recirculating power - that plagues all of two degrees-of-freedom mechanisms - never goes to disastrous levels. The engine is connected to the transmission via an auxiliary gearset that lends it the scalability for various types of engines (gasoline, diesel) or for various categories of vehicles (hatchbacks, SUVs etc.). Additionally, this auxiliary gearset ensures that speed longitude (on torquespeed map) in which the engine operates is narrow and is at the lower end of speeds (1200-3000 RPM), a characteristic extremely important for current (as well as those in future) Lean Burn SI Engines and also Common-Rail Diesels. Apart from the desirables above, the invented compound PGT allows fool-proof vehicle operation with varying ratios also.

Status

Technology available for licensing.

For further information please contact

Prof.B.Seth, Dept. of Mechanical Engineering
Indian Institute of Technology Bombay
Powai Mumbai 400 076