1. > UNIVERSITY OF DURHAM - M.ENG RESEARCH PAPER APRIL 2015 < 1
The Electric Gearbox Project
Joshua Horton supervised by George Carter
Abstract—This report explores a method of reducing the high
weight and cost of current hybrid electric buses (hybrid buses)
and improving their overall efficiency. Using an adaptation of
a series hybrid design, the electric gearbox system (EGBS),
reduces the number of powertrain components by almost half.
A demonstrator, purpose built to validate such claims, clearly
presents the EGBS as a single controllable unit and a probable
design for future development.
Index Terms—Hybrid vehicles, Power electronics, Urban buses.
ABBREVIATIONS
BEV Battery Electric Vehicle
EGBS Electric Gearbox System
Engine Internal Combustion Engine
Hybrid Hybrid Electric Vehicle
Hybrid Bus Hybrid Electric Bus
IM Induction Machine
Machine Electric Machine
Non − Hybrid Non-Hybrid Internal Combustion
Engine Vehicle
PCB Printed Circuit Board
PM Permanent Magnet
PMM Permanent Magnet Machine
PWM Pulse-Width Modulation
SRM Switched Reluctance Machine
I. INTRODUCTION
THE general requirement to improve motor vehicle fuel
efficiency and reduce urban pollution has led to the
commercial development of hybrid electric vehicles (hybrids).
A hybrid in its simplest form is a vehicle that is driven by
two or more sources of propulsion, one of which is electrical.
This is commonly an internal combustion engine (engine) and
an electric machine (machine).
This report explores current hybrid powertrains, and through
the design of a simple experimental demonstrator, seeks to
provide further improvements by means of the electric gearbox
system (EGBS). The EGBS is specifically designed to improve
the urban hybrid electric buses' (hybrid buses) powertrain.
II. DRIVE SYSTEMS
A. Electric Machines
Developments in semiconductor technologies during the
1980's and 1990's led to high-power, high-frequency electronic
switches that allow voltage-frequency control [1]; allowing
machines to be designed to have near ideal power, torque and
efficiency profiles. However, at high speeds the efficiency of
these machines fall.
Fig. 1: Battery electric vehicle powertrain.
Fig. 2: Non-hybrid internal combustion engine vehicle power-
train.
Voltage-frequency control encouraged the advancement of
hybrids and battery electric vehicles (BEVs). For comparison,
a BEV powertrain is shown in Figure 1.
B. Internal Combustion Engines
The majority of road vehicles are non-hybrid internal com-
bustion engine vehicles (non-hybrids) and are powered by
either a petrol or diesel engine; the powertrain for such vehi-
cles is displayed in Figure 2. Both engines use the explosive
combustion of high energy density hydrocarbons to drive the
vehicle.
Diesel engines have higher fuel efficiency as they are capa-
ble of a higher compression ratio due to the direct injection
of the diesel into the cylinders. However, petrol engines have
fewer limitations on how fast the spark plug can fire compared
to the fuel injectors of a diesel engine.
Engines have a power, torque and efficiency curve plotted
against speed; all of which have a peak, and either side of
which performance significantly deteriorates. This has resulted
in the conventional clutch, multi-ratio gearbox powertrain

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Fig. 3: Series hybrid vehicle Powertrain.
which rationalises the torque requirement but exacerbates the
problem of fuel efficiency.
The hybrid powertrain seeks to use the engine at best
efficiency, eliminating the need for a clutch and a complex
multi-ratio gearbox resulting in overall better performance.
Depending on the duty cycle of the vehicle, hybrid drives are
already claiming fuel savings of 40% [5].
III. POWERTRAIN CONFIGURATIONS
A. Series Hybrid
A series hybrid connects the engine to a generator, which
in turn provides electricity that is used to charge the battery,
and in turn power a motor; as presented in Figure 3. Given
that machines are more efficient at low speeds, the series
powertrain is best suited to urban driving, whilst at high speed
it becomes less efficient.
There is no direct mechanical link between the engine and
the drive wheels in this setup, allowing the engine to run
at a constant speed at its rated power output for optimal
fuel efficiency [2]. The engine is mechanically linked to the
generator only, providing greater flexibility of its placement
within the vehicle.
The battery acts as a buffer between the motor and the
engine, therefore allowing the maximum power output of the
engine to be less than that of the motor.
B. Parallel Hybrid
A parallel hybrid mechanically connects both the machine
and the engine directly to the drive wheels via a gearbox, as
outlined in Figure 4. This configuration is more efficient at
high speeds compared to the series hybrid; however it is also
less efficient at lower speeds. Therefore the parallel hybrid is
better suited to driving at a constant high speed with minimal
starting and stopping.
C. Series-Parallel Hybrid
A series-parallel hybrid, shown in Figure 5, enables the
vehicle to utilise the advantages of both the series and parallel
powertrains. The series-parallel hybrid is capable of running
as a series hybrid during urban, low speed driving; before
switching to a parallel system once at higher speeds.
Fig. 4: Parallel hybrid vehicle powertrain.
Fig. 5: Series-parallel hybrid vehicle powertrain.
Toyota and Honda have spent over a decade in the commer-
cial hybrid market, and have made extensive developments
in their series-parallel designs. Toyota adopts a planetary
gearbox design in the Toyota Prius, whereas Honda has its new
Intelligent Multi-Mode Drive (i-MMD) system incorporated
into the Honda Accord [3].
IV. HYBRID ELECTRIC VEHCILE TECHNOLOGIES
A. Regenerative braking
Developments in power electronics have had a further effect
of allowing four quadrant drive electronics to come to fruition -
a single motor can now operate as both a motor and generator
bi-directionally. Hence a single machine can drive a hybrid
forwards, backwards, and also provide regenerative braking -

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transferring the car's kinetic energy into electrical energy using
the machine (generating).
B. Integrated Starter Motors
Integrated starter motors have also developed from four
quadrant drive motors. A machine connected to an engine is
used to generate electricity; however, this machine can also be
operated as the engine's starter motor; reducing the weight of
the hybrid, as no separate starter motor is now required.
C. Wheel-Hub Machines
A machine can either connect to a differential (in lieu of
the gearbox in a non-hybrid), or be placed inside the wheel
hubs achieving fewer mechanical losses. Wheel-hub machines
are expensive [2]; this is because the machine must endure
up to 20G within the wheel-hub compared to the 5G of a
machine in the conventional setup [4]. The wheel-hub motor
must allow steering, braking and suspension to fit into the
confined volume. Hence safety is also an issue; if the machine
is damaged it can cause further damage to the steering and
braking systems. Wheel-hub motors are in their infancy and
require further research before they can compete with other
designs [4].
V. HYBRID ELECTRIC BUSES
A. Current Market
TABLE I: Major manufacturers of hybrid electric buses [2].
Table I displays an overview of the top seven hybrid bus
powertrain providers as of 2013 [2].
The most recent addition to the UK's hybrid transport
network is the new London Routemaster (otherwise known
as the Boris Bus) which made its first appearance in London,
in February 2012. Its red iconic design has been retained, as
can be seen in Figure 6, and the new hybrid bus is said to be
40% more fuel efficient compared to its predecessor. During
2011 London buses traveled 486 million kilometres [5]. A
40% saving in 2011 would have saved over £88 million in
fuel costs [6].
The powertrain of the new Routemaster is the Siemens
ELFA, as displayed in Table I; and the diesel engine is
a Cummins 4.5 Litre ISBe turbodiesel engine [5]. The 4.5
Litre engine runs at a constant speed with maximum power
Fig. 6: The new London hybrid electric Routemaster bus [8].
output due to its series hybrid design. As well as generating
electricity, the diesel engine is used to compress air and
support the hydraulic system. Due to the demand on the engine
to open and close the doors; raise and lower the bus; assist the
steering and provide electronics to the buses' accessories; the
diesel engine runs the majority of the time. Thus the operation
of the 4.5 Litre engine isn't entirely symbolic of the intended
power-on-demand application within series hybrids. On the
other hand, Cummins later suggested that in hindsight, the
new London Routemaster's power demands could have been
realised with a smaller 2.7 Litre turbodiesel [7].
B. Advantages and Disadvantages
The high capital costs and weight of hybrid buses are
their primary drawback. The new London Routemaster costs
£330,000 and weighs almost 18,000 kg [5]. Compare this with
the previous non-hybrid Routemaster: costing £190,000 [9]
and weighing 7,500 kg [10].
The cost savings during the operation of hybrid buses are
expected to recuperate their high capital costs. Hybrid buses
show increased fuel economy; increased brake life - due to
regenerative braking; little, if any, gearbox servicing; less
mechanical parts to service; less engine wear - due to reduced
load on the engine; and a less expensive engine - due to
the small power output required [2]. Due to these factors,
hybrid buses have a longer life expectancy compared to a
conventional non-hybrid bus, and are less prone to unexpected
downtime.
The only exception to the low operational costs is the battery
replacement. Batteries are currently underdeveloped compared
to engine technologies, and rarely last the lifetime of an urban
bus. An urban bus is expected to operate for a minimum of 12
years; during this time the battery in a hybrid bus may have
to be replaced several times [2].
Recent studies are stating that improved battery storage
and electrically-driven accessories are required to ensure the
widespread uptake of hybrid buses [2]. If battery storage is
improved, batteries will provide power to the machine for
longer; reduce the amount of time the engine needs to charge
the battery; and reduce the size and weight of the battery.

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Fig. 7: Electric gearbox vehicle powertrain.
VI. THE ELECTRIC GEARBOX SYSTEM
A. Task
The EGBS is a series hybrid design; and can provide inte-
grated starting; generating; motoring; and regenerative brak-
ing. The unit has two controllable rotational inputs that both
allow four quadrant operation. The unit converts a constant
speed input into a variable torque and speed output; thus the
name, EGBS. Figure 7 displays the schematic of the concept;
and when compared to Figure 3 the reduction of component
duplication can be observed.
B. Benefits
The systems integrated into the EGBS are intuitive in
comparison to current series hybrids. The power-on-demand
functionality is driven heavily by a hybrid buses' duty cycle,
rather than the current state of the battery. Urban buses operate
on the same bus routes continuously. The EGBS is linked to
GPS and traffic data feeds to understand when best to activate
the engine; when significant regenerative braking energy will
be received; and allow the battery capacity, and therefore
weight, to be better suited to a specific bus/bus route.
The EGBS provides hybrid buses with the following bene-
fits:
Weight saving -
• Saving weight by the elimination of any mechanical gear-
box and by combining the motor and generator concept
into one electrical machine.
• Minimising the necessary battery capacity to match the
expected duty cycle.
• Reducing the maximum power output of the engine to
equal the average power demand rather than the maxi-
mum power demand, thus reducing the size of the engine
and saving weight.
Efficiency -
• Designing the associated engine to only run at a high
and efficient speed, unrestricted by road or gearbox
limitations; further reducing weight for a required power
output.
Cost -
• Less material and components required.
• Smaller engine power required; therefore simpler fuel
injection systems and cheaper manufacture.
• The design is more compact; therefore smaller hybrid
buses, with the same passenger carrying capability, can
be designed.
Pollutants -
• It is possible that running the engine at one speed and one
torque, and the addition of an advanced power-on-demand
function will allow the engine to be better designed to
reduce gaseous pollution.
The EGBS does place added limitations on the placement
of the engine compared to the conventional series hybrid
powertrain shown in Figure 3. Similar to that of the gearbox
in a non-hybrid, the EGBS must be connected directly onto
the drive axle, and thus the engine is also placed in close
proximity to connect directly to the EGBS.
VII. ELECTRIC GEARBOX MACHINE DESIGN
A. Machine Type
All machines are variations of either an induction machine
(IM), a permanent magnet machine (PMM) or a switched re-
luctance machine (SRM). SRMs are a type of stepper machine,
recent developments mean they can now be considered for
industrial applications.
The manufacturing costs of the PMM are greatest due to
the rare-earth materials required for the permanent magnets
(PMs) and the difficulty in fitting them. The PMM is also
the largest of the three motors and is unable to free-wheel
without cutting flux paths due to the PMs; whereas IMs and
SRMs are capable of coasting due to their controllable flux.
The PMM does however present the highest absolute efficiency
at maximum torque. Conversely the low speed, freewheeling,
or light/heavy loading duty cycle of a hybrid bus, may present
its efficiency as less impressive [11].
The IM requires more copper compared to the other two,
whereas the SRM offers minimal cost but requires low toler-
ances in its construction. The SRM's primary drawback is that
it suffers from high copper losses at low frequencies (heat
losses) and high iron losses at high frequencies (hysteresis
losses). Nonetheless IMs and SRMs are more robust compared
to the PMM.
The proposal specifies control as the objective for the
EGBS. IMs offer limited control at lower speeds compared
to the PMM and SRM, which are known for their precise
control across their speed ranges. Therefore the IM cannot
be considered for the demonstrator. The relatively immature
data surrounding SRMs and its need for precision manufacture
means the hand-built demonstrator will adopt a PMM.
B. Overexcitation and Underexcitation
Figure 8 shows an open circuit, per-phase equivalent circuit
of a PMM in both (a) motoring and (b) generating modes;
where E represents the EMF excited by the PMs. Rs and Xs
represent the resistance and reactance of the copper wire. The

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Fig. 8: Permanent magnet machine per-phase open circuit in
(a) motoring and (b) generating [12].
EMF has a time dependent magnitude and phase angle relative
to the terminal voltage Vt defined by |E|ejδ
. During generation
the EMF is larger than the terminal voltage, and is defined to
be overexcited; whereas when the terminal voltage is larger
than the EMF, the PMM is motoring and is defined to be
underexcited [12].
C. Axial and Radial Machines
There are two designs of machine: axial and radial flux. The
radial flux machine is cylindrical and has a diameter smaller
than the length of the machine, the flux lines flow radially to
the rotating axis. An axial machine is again cylindrical, but its
diameter is larger than the length of the machine and the flux
flows axially. It comprises of two discs that are placed normal
to one another.
Axial machines are not a new design. They were previously
overlooked due to the small air gap required by induction
machines, which proved hard to mass manufacture. Only
recently have axial machines become more developed. The
development of more advanced PMMs allows axial machines
to reappear in machine design. Larger air gaps are now
possible due to rare permanent magnets and flux vectoring,
reducing their cost of manufacture [13].
The EGBS uses an axial machine design; this is because:
• Large tolerance in manufacture to demonstrate EGBS
principal.
• Aesthetically demonstrates the EGBS idea more clearly.
• Modularity; allowing simple component replacement or
addition once manufactured and constructed.
• Allows for future designs; additional discs and alterations.
D. Machine Windings
The windings in the PMM provide the excitation.
Fig. 9: Solenoid flux lines [14].
There are many ways to
manufacture windings;
however in industry,
continuous windings
are commonly used. A
continuous wire is wrapped
around the machine to
produce a singular phase.
The windings can be
manufactured in a variety
of designs.
Fig. 10: Flux path through demonstrator's windings, permanent
magnets and steel core.
The use of printed circuit boards (PCBs) to create windings
is an area of current research. Using the etched copper to act
as the windings allows accurate and cheap manufacture, and
in principal mimics that of a continuous winding.
Discrete windings are also a method of creating the flux vec-
tor of a PMM, using singularly excited windings. The EGBS
uses discrete windings to excite the PMM. The continuous
windings are harder to accurately manufacture by hand and
require lower tolerances; they are also less modular and thus
failure is hard to locate and replace. PCBs are not considered;
whilst they offer easy alterations and quick replacement, they
are underdeveloped and not completely understood.
The type of winding adopted by the EGBS is a solenoid
(helix) winding as shown in Figure 9. The flux paths resemble
that of a conventional North-South PM.
Figure 10 presents a diagram that shows how the magnetic
flux ideally passes through the windings, PMs and steel core.
E. Machine Phases
A PMM can be operated using any number of phases.
The more phases adopted by a machine, the smoother the
power delivery. For example, if only one phase is adopted,
the instantaneous power equals zero twice a cycle (sinusoidal
wave); however when this is increased to three phases and
there is no instantaneous power that equals zero. The same
principal applies when increasing the number of phases further.
A three phase supply is roughly 150% more efficient than
a single phase supply. Three phase is seen as the industry
standard for optimising power whilst not over complicating
the machine design [15]. The EGBS will thus adopt a three
phase system. All phases operate at the same frequency and
are 120° out of phase with one another.
F. Machine Poles
Irrespective of the number of phases, if there is one winding
per phase, a set of rotating North-South electromagnetic poles
is created. Increasing the number of windings per phase
increases the number of North-South poles created. The more
poles created in a PMM the slower it rotates for a set input
frequency. The speed (in rpm) of a PMM can be calculated
using Equation 1.
Nsync =
120fs
p
rpm (1)
Where Nsync is the EM's synchronous speed; fs is the
supply frequency; and p the number of poles.

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By increasing the number of poles the potential power trans-
fer increases. For example: a circuit contains three generating
windings connected in series. The terminal voltage is three
times that of the voltage across each winding, whereas the
terminal current is the same as that through one winding. If this
circuit is placed in parallel with a further two identical circuits,
then the terminal voltage remains the same, whereas the termi-
nal current triples. Power(W) = V oltage(V )×Current(A);
thus increase the number of windings in each phase and there
is an increase in overall power. The circuit described is that
of a three phase circuit, with each phase containing three
windings.
G. Star and Delta Connections
There are two methods of connecting three phase windings:
star and delta; called so because of their appearance shown in
Figure 11.
As Figure 11 presents, the voltage and current of the circuit
can be measured across the windings - phases, and across the
inputs/outputs - lines.
Equations 2a and 2b show how line and phase voltages and
currents relate for a delta connected circuit; and Equations 3a
and 3b show the same for a star connected circuit.
Delta connection:
Vline = Vphase (2a)
Iline = 3
Iphase (2b)
Star connection:
Vline = 3
Vphase (3a)
Iline = Iphase (3b)
The speed of a machine is proportional to voltage, thus
for the same input signal, a star connected motor will rotate
slower than its delta connected counterpart. The star connected
motor, however, benefits from less current and therefore less
heat loss. If a winding or phase fails in a delta connected
machine, the other lines and phases are unaffected; however
if this happens in a star connected machine there is a resultant
change in the voltage and current which can cause damage to
the machine. Because of these attributes the delta connection
is used primarily in industry and the star connection preferred
for power transmission. Thus the EGBS adopts the delta
connection.
Fig. 11: Three phase star (left) and delta (right) connections
[16].
Fig. 12: Two-disc demonstrator design.
Fig. 13: Three-disc demonstrator design; (a) winding-magnet-
winding (b) magnet-winding-magnet.

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Fig. 14: Electric gearbox system final CAD design.
VIII. ELECTRIC GEARBOX DESIGN CONCEPTS
An axial, three phase, PMM, using discrete windings is
adopted for the EGBS demonstrator. These design conditions
result in three concepts; one two-disc design and two three-
disc designs that are presented in Figure 12 and Figure 13.
The two-disc design, Figure 12, weighs less than the three-
disc design and consists of one PMM. One disc holds the
PMs and the other the exciter windings. Unlike a conventional
PMM, where one disc would be kept stationary, this design
allows both discs to rotate. Each disc is connected to the engine
and drive wheels respectively. When the drive wheels need to
motor or generate, a mechanical break is applied to the diesel
engine disc to oppose the reacting torque; and when the engine
requires starting or generating a mechanical break is applied to
the drive wheel disc. The complication arises when the EGBS
requires both discs to rotate. It is because of this complexity
that the two disc concept, despite its weight reduction, is not
incorporated in the EGBS demonstrator.
The two three-disc designs are only subtly different. They
are both essentially two PMMs back-to-back, where the centre
disc is utilised by both PMMs. The difference between the two
designs in Figure 13 is the placement of the PMs and windings.
By placing the PMs on the centre disc, design (a), the two
PMMs can share the same PM. Design (a) allows the centre
disc to be compact, however it requires the windings to be on
a rotating disc, thus requiring slip rings which cause electrical
noise and heat. Contactless slip rings, using either fibre optics
or EMF, are available and are also a growing market for use in
wind technologies; however like most of the new technologies
they are still in their infancy.
Placing the exciter windings on the centre disc, design (b),
eliminates the need of slip rings. However, to ensure the flux
from the windings in the separate PMMs do not interfere with
one another, the centre disc must be insulated and thus must be
less compact, as can be seen in Figure 13. The objective of the
EGBS demonstrator is control, and the manufacture process to
be as simple as possible, therefore design (b) is implemented
so as to not create reasons for failure by using slip rings.
IX. ELECTRIC GEARBOX ARCHITECTURAL DESIGN
Figure 14 displays the final CAD design of the EGBS
demonstrator in SolidWorks. The design is symmetrical; two
45W DC machines are located on either end to represent the
diesel engine and drive wheels. Two plastic struts ensure the
alignment of the axis and present the EGBS as a clear unit.
The green discs in Figure 14 are the rotators and hold the
PMs, whereas the central blue disc holds the windings shown
in orange.
The demonstrator is a simple tool to illustrate the idea
of the EGBS, and consequently portability of the unit is an
objective. The final EGBS demonstrator has a disc size of
190mm diameter, and the unit as a whole has dimensions
in width, length and height of: 240mm, 600mm and 240mm
respectively.
The rare earth PMs employed by the EGBS have a length,
width and depth of 22mm, 15mm and 5mm respectively. At a
distance of 25mm apart the attraction/repulsion force increases
significantly; thus to prevent fatigue of the PM discs and allow
the windings time to change polarity and high speeds, the
average distance between PMs is 30mm.
Using an estimate for the required voltage and expected
current flow through each winding, the windings are manufac-
tured using a coated 19 SWG (Standard Wire Gauge) copper
wire; 1.016mm in diameter and a maximum current carrying
capacity of 9A (a short 12V wire) [17]. To ensure the flux
density at the base of each winding remains high, the windings
are designed to be small in length, 20mm; and thus a diameter
of each windings is also 20mm to ensure a high number of
turns per winding.
The angle between windings in three phase is θwinding =
120° and/or 240°. The angle between the poles generated

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is θmag = 180°. Therefore the ratios of θwinding to θmag is
displayed in Equations 4a and 4b.
θmag = 1.5 θwinding (4a)
OR
θmag = 0.75 θwinding (4b)
The 30mm restriction between PMs and the 20mm diameter
of each winding creates restrictions on the number of PMs and
windings that can fit on the 190mm disc. The circumference
of the discs allow for more PMs compared to windings, and
thus the ratio of PMs to windings is that of Equation 4b. As
the EGBS is a three phase system, the number of windings is
a multiple of three. The result is 12 PMs on the rotator discs
and 9 windings on the stator discs, where θwinding = 40° and
θmag = 30°.
X. ELECTRIC GEARBOX CIRCUIT DESIGN
Figure 15 is the circuit schematic for one of the EGBS's
PMMs. Figure 15 is duplicated for both PMMs of the EGBS.
Pins; P1, P2, P3 and P4 are inputs, and P5 and P6 are outputs.
When the EGBS is generating from the engine or regenerative
braking (depending on the side of the EGBS) the switches
are in the up position as shown in Figure 15. P1 and P2 are
fed a DC signal to drive the DC motor. A sinusoidal signal
is induced in the windings L1-L9 which is then rectified to
produce a DC output at P5 and P6.
Figure 16 presents the output signal of the three phase rec-
tifier without the 220µF smoothing capacitor C1 connected.
The output in Figure 16 has an RMS output of 2V.
When the EGBS is either starting the engine, or driving
the drive wheels, the switches S1, S2 and S3 are switched
to the down position to connect the electric speed controller
(ESC) to the windings. The ESC receives a DC input from P3
and P4 and outputs a three phase AC signal using pulse-width
modulation (PWM). The servo attached to the left of the ESC
Fig. 15: Electric gearbox system demonstrator's circuit
schematic for one, of two, permanent magnet machines.
Fig. 16: Three phase rectified sinusoidal signal - no smoothing
capacitor.
Fig. 17: Sinusoidal signals from two individual windings (out
of phase).
Fig. 18: Sinusoidal signals from a single winding (blue) and
a complete phase (yellow) (out of phase).

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Fig. 19: Electronic gearbox system demonstrator.
in Figure 15 controls the frequency of the PWM output, and
thus the speed of the PMM.
XI. CONSTRUCTION
To ensure the success of the EGBS demonstrator the signals
generated in each winding were carefully monitored. Figure
17 presents two sinusoidal signals from two individual out
of phase windings. As can be seen, the two signals are
exactly 120° out of phase and roughly generate 0.5V. Once all
windings are connected in their respective phases the voltage
difference between phases is minimal. This quality control
measure was completed for every winding.
Figure 18 compares signals from a singular winding (blue)
and a phase with all three windings connected in series
(yellow). This measurement verifies that the terminal voltage
of a phase (roughly 1.5V) is three times that of the voltage
over a singular winding (roughly 0.5V).
Both Figures 17 and 18 display a double peak at the
maximum amplitudes of the sinusoids. Figure 9 demonstrates
how the flux densities around the solenoid windings vary. At
the centre of the winding the flux density is less than that at its
extremities. Therefore the PM energises the winding initially;
the EMF then drops with the flux density as the PMs pass over
the centre of the windings; and then energises the winding
further as the flux density again increases.
This effect can be reduced by introducing an iron or steel
core at the centre of the windings. The EGBS currently adopts
nylon screws as the winding's core; this is due to the DC
motors not being able to overcome the attraction/repulsion
forces that a steel core would create.
XII. RESULTS
Figure 19 presents the completed EGBS demonstrator, and
Tables II and III contain its performance data. During gener-
ation mode, at a speed of 1800rpm, the voltage and current
across a 10Ω resistor is 2.32V and 2.54A respectively for the
diesel engine generator, and 3.08V and 2.42A for regenerative
braking mode.
TABLE II: Speed limitations of electric gearbox system
demonstrator (no load).
Demonstrator Mode Speed (rpm)
Maximum generation speed (Diesel engine generator) 2200
Maximum generation speed (Regenerative braking) 1900
Maximum motoring speed (Integrated starter) 134
Maximum motoring speed (Drive wheels) 2500+
TABLE III: Specification of electric gearbox system demon-
strator's connections.
Parameter P1, P2 P3, P4 P5, P6
Socket type 4mm Socket 4mm Socket 4mm Socket
Input/output Input Input Output
DC/AC DC DC DC
Max. current (A) 1.3 16 N/A
Max. voltage (V) 50 14 N/A
XIII. MANUFACTURE
Figure 20 demonstrates how the EGBS axial demonstrator
could be altered into a radial machine. Whilst an axial machine
is preferred for the demonstrator, a radial machine may prove
preferable for application in a hybrid bus.
The rotor connected to the diesel engine is merely expected
to provide a constant low power input or a short, high power
output to start the diesel engine; these demands are easily
provided by a low power machine. The drive wheels however
require a constant high power input or output to ensure
immediate acceleration or regenerative braking. Therefore the
drive wheels require a high power machine.
The design in Figure 20 is capable of fulfilling these
requirements. The diesel engine connects to the central, low

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Fig. 20: Radial flux electronic gearbox system design.
Fig. 21: PWM-driven (controlled) machine drive.
power (small diameter) rotor; and the drive wheels to the
exterior, high power (large diameter) rotor.
Whilst the design is more complex and requires lower
tolerances in comparison to the axial design, it prevents the
PMM connected to the diesel engine being needlessly large.
XIV. FUTURE DEVELOPMENT
To improve the efficiency of the current demonstrator design
the steel core needs to be laminated. The current design
incorporates a steel ring as its core and thus suffers from
high steel losses (eddy currents). The open nature of the
EGBS demonstrator also contributes to the large flux leakage
experienced by the demonstrator. An improved housing and
steel core design would improve the flux linkage.
Currently the demonstrator treats both PMMs as separate
identities. To progress the development of the EGBS, the
two PMMs ought to operate together, and be successfully
connected to a battery. Figure 21 is a potential circuit to
connect the two PMMs.
On the far left and right of Figure 21 are the two three phase
PMMs. Connected to the PMMs is a circuit that resembles a
three phase rectifier; D1-D12 are diodes, and work identically
to those in Figure 15 to rectify the three phase input into
a DC output. The alteration in this circuit is the addition
of the voltage sourced converters, V1-V12, which are gate
turn-off devices that allow a DC signal to be converted into
a three phase PWM signal. In fundamental terms, Figure
21 is a circuit in which the ESC is superimposed upon the
three phase rectifier. Central to the circuit is the battery; a
smoothing capacitor C1; and a dynamic braking resistor-switch
combination. The dynamic braking resistor Rb and switch Sb
are for the situation whereby the voltage across the battery
exceeds its intended maximum terminal voltage. At which
point Sb allows a current to flow through Rb and the voltage
across the battery terminals reduces.
XV. CONCLUSION
Current hybrid buses suffer from high cost and weight in
comparison to their non-hybrid counterparts. Series hybrid
powertrains are understood to be well suited to urban driving.
The EGBS makes simple alternations to the series hybrid
schematic to half the number of components required by the
powertrain.
The EGBS thus improves weight and cost. The design also
suggests a more intuitive approach to the power-on-demand
functionality; proposing it be driven more heavily on a buses'
duty cycle and less so on the current state of the battery; further
improving fuel efficiency and pollution reduction.
A demonstrator built to support these claims, establishes the
EGBS concept to be a valid and compact design. The demon-
strator successfully fulfills the design criteria: controllable and
clear demonstration of the EGBS principal.
In conclusion this report suggests the EGBS to be an appli-
cable contender for the urban hybrid bus powertrain market.
Reducing the flux leakage of the current demonstrator and
adopting elements of automation is the next step in validating
the concept further. The two-disc concept and/or the radial flux
machine should be considered for the future of the EGBS.
XVI. FURTHER APPLICATION
Whilst the EGBS was intended for urban hybrid buses,
it offers many applications; especially in other areas of the
transport industry. Primarily a locomotive is used to haul a
train; however, manufacturers are beginning to develop trains
where the machine-diesel engine combination is situated in
the chassis of each carriage. It is believed that several low
power machines and diesel engines are more efficient and also
provide operators with further flexibility due to their modular
design. In this circumstance, a light weight and effective
method of connecting the diesel engine to the machine is
crucial; and would be perfectly suited to the EGBS.
The aerospace industry is another sector in which weight
reduction is of high importance. Hybrid technologies are
becoming a focus of aerospace; with this in mind, the EGBS
would again be well suited to such an environment.
ACKNOWLEDGMENT
The author thanks Phillip and Collin for their efforts towards
the construction of the EGBS demonstrator; Ian, Jim and
George's advice and guidance on electric machines; and the
support from friends and family. Thank you also to the woman
at the engineering coffee bar, the conversation, beef salad and
coffee also was a great contributor to the end goal.

11. > UNIVERSITY OF DURHAM - M.ENG RESEARCH PAPER APRIL 2015 < 11
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