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Electric Vehicle Machines and Drives
– Design, Analysis and Application
Part III
Machine Systems for Hybrid Electric
Vehicles
K. T. Chau
The University of Hong Kong
Hong Kong, China
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Machine Systems for Hybrid Electric Vehicles
(HEVs) – Content
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Integrated-Starter-Generator (ISG) Systems
Planetary-Geared Electric Variable Transmission (PG EVT)
Systems
Double-Rotor Electric Variable Transmission (DR EVT) Systems
Magnetic-Geared Electric Variable Transmission (MG EVT)
Systems
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ISG Systems – Independent Starter-Generator
System
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In conventional vehicle, the starter motor and generator are
separately coupled with the engine, hence providing high starting
torque for cranking and generating electricity for battery charging.
It takes the advantage of simplicity, but suffers from poor
utilization of both machines and thus resulting in heavy weight
and bulky size.
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ISG Systems – Belt-Driven ISG System
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It is mounted in the conventional location of the starter motor
or generator to couple with the flywheel and engine via the belt.
It is simple and inexpensive. No significant modifications are
required for the conventional vehicle chassis to accommodate
this belt-driven ISG system.
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ISG Systems – Crankshaft ISG System
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It is mounted onto the crankshaft so that the ISG rotor can serve
as the flywheel inertia.
It can offer direct control for both cranking and generation, and
can avoid the belt-driven mechanism, hence reducing the
construction cost and complexity. But, it needs additional weight
of copper and iron, and should withstand severe vibration.
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ISG Systems – ISG Operations
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Cranking: It works as a starter motor spinning the engine to
initiate the engine operation under its own power.
Electricity Generation: It serves as a generator which converts
the mechanical energy of the spinning engine into electrical
energy for battery charging.
Idle Stop-Start: It allows the engine to turn off and hence save
fuel at stops, and instantly restart the engine upon pressing of
the accelerator pedal.
Regenerative Braking: It works as a generator which captures
the vehicle kinetic energy and turns it into electricity to
recharge the battery.
Power Assistance: It works as a motor which can provide
additional power to assist the engine for fast acceleration.
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ISG Systems – ISG Criteria
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The starting torque should be able to achieve three to four
times the rated torque to satisfy the cranking requirement.
It needs to have a long service life, over a million times of idle
stop-start in ten years.
The constant-power operation range should be able to achieve
three to four times the base speed in order to facilitate widespeed electricity generation for battery charging.
The regenerative braking should be energy-efficient.
It should offer high torque at speeds below the base speed.
It should offer high efficiency over wide torque-speed ranges.
It should be able to withstand working vibrations.
It should work well over a wide range of temperatures.
It should offer good serviceability, and maintenance free.
The system cost should be acceptable.
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PG EVT Systems – Input-Split System
It is composed of a power-split device, two machines, an energy
storage device and a final drive. The planetary gear serves to
split the engine power into two paths: one is a mechanical path
from the engine to the wheels via the power-split device and
final drive; another is an electrical path from the engine to the
wheels via the power split-device, machine 1, machine 2 and
final drive.
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PG EVT Systems – Compound-Split System
It involves two power-split devices: one located at the input and
another located at the output. In one mode, the power-split
device at the input serves for torque differential and the power
split-device at the output serves as a torque multiplier. In
another mode, it operates both the input and output power-split
devices serving for torque differential.
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PG EVT Systems – Key Features of Input-Split PG
EVT Systems for HEVs
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Because of the absence of clutches or shifting gears, it can
improve the transmission efficiency and reduce the overall size.
In the presence of continuously-variable ratio between the
engine speed and wheel speed, the engine can always operate
at its most energy-efficient or optimal operation line (OOL).
The system can offer the idle stop-start feature, thus
significantly saving the fuel consumption.
It can offer the electric launch feature, thus significantly
improving the startup response time.
The system can fully enable regenerative braking when the
vehicle is slowing or downhill coasting.
The system facilitates full-throttle acceleration where both the
engine and motor operate simultaneously.
There are two main input-split PG EVT systems: one developed
by Toyota; another developed by Ford.
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PG EVT Systems – Input-Split Toyota Hybrid
System
In this system, the planetary gear set plays a key role to split the
engine power into two paths of power flow: mechanical and
electrical. The mechanical power produced by the engine is
transferred to the driveline via the ring gear. The sun gear is
attached to the machine 1, which converts a portion of the
engine power to electrical power to feed the machine 2.
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PG EVT Systems – Input-Split Ford Hybrid
System
Similar to the Toyota hybrid system, it has a planetary gear to
perform the desired power-split. Additionally, it incorporates
output gearing between the motor and final drive.
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PG EVT Systems – Key Features of CompoundSplit PG EVT Systems for HEVs
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The compound-split PG EVT system is more complicated than
the aforementioned input-split system.
The key is that it incorporates the feature of variable structure
control. With the use of mechanical disconnect clutches, the
system can offer various structures.
The representative compound-split PG EVT system is the GM
two-mode hybrid system. Consequently, there are various
variants such as the Renault and Timken systems.
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PG EVT Systems – Compound-Split GM Two-Mode
Hybrid System
It is mainly composed of three clutches, two planetary gears, two
machines, two converters and a battery pack.
Ring
Clutch 3
Ring
Planet
Wheel
Planet
Clutch 2
Engine
Sun
Machine2
Machine1
Sun
Clutch 1
Planet
Ring
Planet
Ring
Converter1
Wheel
Converter2
Battery
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DR EVT Systems – DR EVT System
By allowing double rotors to rotate freely, it can simultaneously
offer the planetary gear function to split the engine power, and
the generator function to produce electricity.
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DR EVT Systems – DR Machines
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A basic DR EVT system is mainly based on the induction DR
machine.
Advanced DR EVT systems can be based on the permanent
magnet (PM) synchronous DR machine which can improve the
efficiency and power density, and the switched reluctance (SR)
DR machine which can offer the definite advantages of simple
structure, high reliability and low manufacturing cost.
There are the axial-flux DR machine which has higher torque
density, higher power density and better heat dissipation, and
the advanced magnetless DR machine which does not suffer
from the use of expensive and limited supply of PM material.
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DR EVT Systems – Induction DR EVT System
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DR EVT Systems – PM DR EVT System
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DR EVT Systems – SR DR EVT System
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DR EVT Systems – Axial-Flux PM DR EVT System
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Although having the
advantages of higher
torque density, higher
power density and better
heat dissipation, it suffers
from a mounting problem
in bearings due to the
attractive forces among
the stator and two rotors.
Additional rotors or stators
can alleviate such problem,
but significantly increasing
the system complexity.
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DR EVT Systems – Advanced Magnetless DR EVT
System
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Viable advanced magnetless
machines are the doublysalient DC (DSDC) machine,
flux-switching DC (FSDC)
machine, vernier reluctance
(VR) machine and vernier
reluctance DC (VRDC)
machine.
The DSDC machine takes
the advantages of higher
torque density and higher
power density while
retaining the merits of
simple structure, high
reliability and low
manufacturing cost.
DSDC DR EVT System
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MG EVT Systems – Magnetic planetary gear
By applying the concept of magnetic gearing to a mechanical
planetary gear, the resulting magnetic planetary gear can inherit the
multi-port feature from the mechanical planetary gear as well as the
advantages of low transmission loss, silent operation and
maintenance free from the magnetic gear.
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MG EVT Systems – Magnetic Planetary-Geared
EVT System
By replacing the mechanical planetary gear with the magnetic
planetary gear, the magnetic planetary-geared (MPG) EVT can
readily be deduced.
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MG EVT Systems – Magnetic Concentric Gear
When the ferromagnetic ring of the magnetic coaxial gear is
rotatable, it is dubbed the magnetic concentric gear which can be
considered as a multi-port magnetic gear.
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MG EVT Systems – Magnetic Concentric-Geared
EVT System
By replacing the mechanical planetary gear with the magnetic
concentric gear, the magnetic concentric-geared (MCG) EVT system
can be deduced.
Gear modulation ring
Machine 2 rotor
( Gear outer rotor)
Machine 2 stator
Machine 1 stator
PM pole
PM
pole
Driveline
Engine
shaft
Machine 1
rotor
Gear inner
rotor
PM pole
Armature windings
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Converter 2
Battery
Converter 1
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MG EVT Systems – Integrated Magnetic
Concentric-Geared EVT System
Two electric machines are integrated into the magnetic concentric
gear to form a single machine.
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