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