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> UNIVERSITY OF DURHAM - M.ENG RESEARCH PAPER APRIL 2015 < 1

The Electric Gearbox ProjectJoshua Horton supervised by George Carter

Abstract—This report explores a method of reducing the highweight and cost of current hybrid electric buses (hybrid buses)and improving their overall efficiency. Using an adaptation ofa 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, clearlypresents the EGBS as a single controllable unit and a probabledesign for future development.

Index Terms—Hybrid vehicles, Power electronics, Urban buses.

ABBREVIATIONS

BEV Battery Electric VehicleEGBS Electric Gearbox SystemEngine Internal Combustion EngineHybrid Hybrid Electric VehicleHybrid Bus Hybrid Electric BusIM Induction MachineMachine Electric MachineNon−Hybrid Non-Hybrid Internal Combustion

Engine VehiclePCB Printed Circuit BoardPM Permanent MagnetPMM Permanent Magnet MachinePWM Pulse-Width ModulationSRM Switched Reluctance Machine

I. INTRODUCTION

THE general requirement to improve motor vehicle fuelefficiency 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 bytwo or more sources of propulsion, one of which is electrical.This is commonly an internal combustion engine (engine) andan electric machine (machine).

This report explores current hybrid powertrains, and throughthe design of a simple experimental demonstrator, seeks toprovide further improvements by means of the electric gearboxsystem (EGBS). The EGBS is specifically designed to improvethe urban hybrid electric buses' (hybrid buses) powertrain.

II. DRIVE SYSTEMS

A. Electric Machines

Developments in semiconductor technologies during the1980's and 1990's led to high-power, high-frequency electronicswitches that allow voltage-frequency control [1]; allowingmachines to be designed to have near ideal power, torque andefficiency profiles. However, at high speeds the efficiency ofthese machines fall.

Fig. 1: Battery electric vehicle powertrain.

Fig. 2: Non-hybrid internal combustion engine vehicle power-train.

Voltage-frequency control encouraged the advancement ofhybrids 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 byeither a petrol or diesel engine; the powertrain for such vehi-cles is displayed in Figure 2. Both engines use the explosivecombustion of high energy density hydrocarbons to drive thevehicle.

Diesel engines have higher fuel efficiency as they are capa-ble of a higher compression ratio due to the direct injectionof the diesel into the cylinders. However, petrol engines havefewer limitations on how fast the spark plug can fire comparedto the fuel injectors of a diesel engine.

Engines have a power, torque and efficiency curve plottedagainst speed; all of which have a peak, and either side ofwhich performance significantly deteriorates. This has resultedin the conventional clutch, multi-ratio gearbox powertrain


Fig. 3: Series hybrid vehicle Powertrain.

which rationalises the torque requirement but exacerbates theproblem of fuel efficiency.

The hybrid powertrain seeks to use the engine at bestefficiency, eliminating the need for a clutch and a complexmulti-ratio gearbox resulting in overall better performance.Depending on the duty cycle of the vehicle, hybrid drives arealready claiming fuel savings of 40% [5].

III. POWERTRAIN CONFIGURATIONS

A. Series Hybrid

A series hybrid connects the engine to a generator, whichin turn provides electricity that is used to charge the battery,and in turn power a motor; as presented in Figure 3. Giventhat machines are more efficient at low speeds, the seriespowertrain is best suited to urban driving, whilst at high speedit becomes less efficient.

There is no direct mechanical link between the engine andthe drive wheels in this setup, allowing the engine to runat a constant speed at its rated power output for optimalfuel efficiency [2]. The engine is mechanically linked to thegenerator only, providing greater flexibility of its placementwithin the vehicle.

The battery acts as a buffer between the motor and theengine, therefore allowing the maximum power output of theengine to be less than that of the motor.

B. Parallel Hybrid

A parallel hybrid mechanically connects both the machineand the engine directly to the drive wheels via a gearbox, asoutlined in Figure 4. This configuration is more efficient athigh speeds compared to the series hybrid; however it is alsoless efficient at lower speeds. Therefore the parallel hybrid isbetter suited to driving at a constant high speed with minimalstarting and stopping.

C. Series-Parallel Hybrid

A series-parallel hybrid, shown in Figure 5, enables thevehicle to utilise the advantages of both the series and parallelpowertrains. The series-parallel hybrid is capable of runningas a series hybrid during urban, low speed driving; beforeswitching 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 developmentsin their series-parallel designs. Toyota adopts a planetarygearbox design in the Toyota Prius, whereas Honda has its newIntelligent Multi-Mode Drive (i-MMD) system incorporatedinto the Honda Accord [3].

IV. HYBRID ELECTRIC VEHCILE TECHNOLOGIES

A. Regenerative braking

Developments in power electronics have had a further effectof allowing four quadrant drive electronics to come to fruition -a single motor can now operate as both a motor and generatorbi-directionally. Hence a single machine can drive a hybridforwards, backwards, and also provide regenerative braking -


transferring the car's kinetic energy into electrical energy usingthe machine (generating).

B. Integrated Starter Motors

Integrated starter motors have also developed from fourquadrant drive motors. A machine connected to an engine isused to generate electricity; however, this machine can also beoperated as the engine's starter motor; reducing the weight ofthe hybrid, as no separate starter motor is now required.

C. Wheel-Hub Machines

A machine can either connect to a differential (in lieu ofthe gearbox in a non-hybrid), or be placed inside the wheelhubs achieving fewer mechanical losses. Wheel-hub machinesare expensive [2]; this is because the machine must endureup to 20G within the wheel-hub compared to the 5G of amachine in the conventional setup [4]. The wheel-hub motormust allow steering, braking and suspension to fit into theconfined volume. Hence safety is also an issue; if the machineis damaged it can cause further damage to the steering andbraking systems. Wheel-hub motors are in their infancy andrequire further research before they can compete with otherdesigns [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 buspowertrain providers as of 2013 [2].

The most recent addition to the UK's hybrid transportnetwork is the new London Routemaster (otherwise knownas the Boris Bus) which made its first appearance in London,in February 2012. Its red iconic design has been retained, ascan be seen in Figure 6, and the new hybrid bus is said to be40% more fuel efficient compared to its predecessor. During2011 London buses traveled 486 million kilometres [5]. A40% saving in 2011 would have saved over £88 million infuel costs [6].

The powertrain of the new Routemaster is the SiemensELFA, as displayed in Table I; and the diesel engine isa Cummins 4.5 Litre ISBe turbodiesel engine [5]. The 4.5Litre 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 generatingelectricity, the diesel engine is used to compress air andsupport the hydraulic system. Due to the demand on the engineto open and close the doors; raise and lower the bus; assist thesteering and provide electronics to the buses' accessories; thediesel engine runs the majority of the time. Thus the operationof the 4.5 Litre engine isn't entirely symbolic of the intendedpower-on-demand application within series hybrids. On theother hand, Cummins later suggested that in hindsight, thenew London Routemaster's power demands could have beenrealised with a smaller 2.7 Litre turbodiesel [7].

B. Advantages and Disadvantages

The high capital costs and weight of hybrid buses aretheir primary drawback. The new London Routemaster costs£330,000 and weighs almost 18,000 kg [5]. Compare this withthe previous non-hybrid Routemaster: costing £190,000 [9]and weighing 7,500 kg [10].

The cost savings during the operation of hybrid buses areexpected to recuperate their high capital costs. Hybrid busesshow increased fuel economy; increased brake life - due toregenerative braking; little, if any, gearbox servicing; lessmechanical parts to service; less engine wear - due to reducedload on the engine; and a less expensive engine - due tothe small power output required [2]. Due to these factors,hybrid buses have a longer life expectancy compared to aconventional non-hybrid bus, and are less prone to unexpecteddowntime.

The only exception to the low operational costs is the batteryreplacement. Batteries are currently underdeveloped comparedto engine technologies, and rarely last the lifetime of an urbanbus. An urban bus is expected to operate for a minimum of 12years; during this time the battery in a hybrid bus may haveto be replaced several times [2].

Recent studies are stating that improved battery storageand electrically-driven accessories are required to ensure thewidespread uptake of hybrid buses [2]. If battery storage isimproved, batteries will provide power to the machine forlonger; reduce the amount of time the engine needs to chargethe battery; and reduce the size and weight of the battery.


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 bothallow four quadrant operation. The unit converts a constantspeed input into a variable torque and speed output; thus thename, EGBS. Figure 7 displays the schematic of the concept;and when compared to Figure 3 the reduction of componentduplication can be observed.

B. Benefits

The systems integrated into the EGBS are intuitive incomparison to current series hybrids. The power-on-demandfunctionality is driven heavily by a hybrid buses' duty cycle,rather than the current state of the battery. Urban buses operateon the same bus routes continuously. The EGBS is linked toGPS and traffic data feeds to understand when best to activatethe engine; when significant regenerative braking energy willbe received; and allow the battery capacity, and thereforeweight, 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 conceptinto one electrical machine.

• Minimising the necessary battery capacity to match theexpected duty cycle.

• Reducing the maximum power output of the engine toequal the average power demand rather than the maxi-mum power demand, thus reducing the size of the engineand saving weight.

Efficiency -• Designing the associated engine to only run at a high

and efficient speed, unrestricted by road or gearboxlimitations; further reducing weight for a required poweroutput.

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, canbe designed.

Pollutants -• It is possible that running the engine at one speed and one

torque, and the addition of an advanced power-on-demandfunction will allow the engine to be better designed toreduce gaseous pollution.

The EGBS does place added limitations on the placementof the engine compared to the conventional series hybridpowertrain shown in Figure 3. Similar to that of the gearboxin a non-hybrid, the EGBS must be connected directly ontothe drive axle, and thus the engine is also placed in closeproximity 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 forindustrial applications.

The manufacturing costs of the PMM are greatest due tothe rare-earth materials required for the permanent magnets(PMs) and the difficulty in fitting them. The PMM is alsothe largest of the three motors and is unable to free-wheelwithout cutting flux paths due to the PMs; whereas IMs andSRMs are capable of coasting due to their controllable flux.The PMM does however present the highest absolute efficiencyat maximum torque. Conversely the low speed, freewheeling,or light/heavy loading duty cycle of a hybrid bus, may presentits 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 thatit suffers from high copper losses at low frequencies (heatlosses) and high iron losses at high frequencies (hysteresislosses). Nonetheless IMs and SRMs are more robust comparedto the PMM.

The proposal specifies control as the objective for theEGBS. IMs offer limited control at lower speeds comparedto the PMM and SRM, which are known for their precisecontrol across their speed ranges. Therefore the IM cannotbe considered for the demonstrator. The relatively immaturedata surrounding SRMs and its need for precision manufacturemeans the hand-built demonstrator will adopt a PMM.

B. Overexcitation and Underexcitation

Figure 8 shows an open circuit, per-phase equivalent circuitof 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


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 relativeto the terminal voltage Vt defined by |E|ejδ . During generationthe EMF is larger than the terminal voltage, and is defined tobe overexcited; whereas when the terminal voltage is largerthan the EMF, the PMM is motoring and is defined to beunderexcited [12].

C. Axial and Radial Machines

There are two designs of machine: axial and radial flux. Theradial flux machine is cylindrical and has a diameter smallerthan the length of the machine, the flux lines flow radially tothe rotating axis. An axial machine is again cylindrical, but itsdiameter is larger than the length of the machine and the fluxflows axially. It comprises of two discs that are placed normalto one another.

Axial machines are not a new design. They were previouslyoverlooked due to the small air gap required by inductionmachines, which proved hard to mass manufacture. Onlyrecently have axial machines become more developed. Thedevelopment of more advanced PMMs allows axial machinesto reappear in machine design. Larger air gaps are nowpossible 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 tomanufacture windings;however in industry,continuous windingsare commonly used. Acontinuous wire is wrappedaround the machine toproduce a singular phase.The windings can bemanufactured in a varietyof designs.

Fig. 10: Flux path through demonstrator's windings, permanentmagnets and steel core.

The use of printed circuit boards (PCBs) to create windingsis an area of current research. Using the etched copper to actas the windings allows accurate and cheap manufacture, andin 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 EGBSuses discrete windings to excite the PMM. The continuouswindings are harder to accurately manufacture by hand andrequire lower tolerances; they are also less modular and thusfailure is hard to locate and replace. PCBs are not considered;whilst they offer easy alterations and quick replacement, theyare 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 resemblethat of a conventional North-South PM.

Figure 10 presents a diagram that shows how the magneticflux 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 thepower delivery. For example, if only one phase is adopted,the instantaneous power equals zero twice a cycle (sinusoidalwave); however when this is increased to three phases andthere is no instantaneous power that equals zero. The sameprincipal applies when increasing the number of phases further.A three phase supply is roughly 150% more efficient thana single phase supply. Three phase is seen as the industrystandard for optimising power whilst not over complicatingthe machine design [15]. The EGBS will thus adopt a threephase system. All phases operate at the same frequency andare 120° out of phase with one another.

F. Machine Poles

Irrespective of the number of phases, if there is one windingper phase, a set of rotating North-South electromagnetic polesis created. Increasing the number of windings per phaseincreases the number of North-South poles created. The morepoles created in a PMM the slower it rotates for a set inputfrequency. The speed (in rpm) of a PMM can be calculatedusing Equation 1.

Nsync =120fsp

rpm (1)

Where Nsync is the EM's synchronous speed; fs is thesupply frequency; and p the number of poles.


By increasing the number of poles the potential power trans-fer increases. For example: a circuit contains three generatingwindings connected in series. The terminal voltage is threetimes that of the voltage across each winding, whereas theterminal current is the same as that through one winding. If thiscircuit 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 thereis an increase in overall power. The circuit described is thatof a three phase circuit, with each phase containing threewindings.

G. Star and Delta Connections

There are two methods of connecting three phase windings:star and delta; called so because of their appearance shown inFigure 11.

As Figure 11 presents, the voltage and current of the circuitcan be measured across the windings - phases, and across theinputs/outputs - lines.

Equations 2a and 2b show how line and phase voltages andcurrents relate for a delta connected circuit; and Equations 3aand 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, thusfor the same input signal, a star connected motor will rotateslower than its delta connected counterpart. The star connectedmotor, however, benefits from less current and therefore lessheat loss. If a winding or phase fails in a delta connectedmachine, the other lines and phases are unaffected; howeverif this happens in a star connected machine there is a resultantchange in the voltage and current which can cause damage tothe machine. Because of these attributes the delta connectionis used primarily in industry and the star connection preferredfor power transmission. Thus the EGBS adopts the deltaconnection.

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.


Fig. 14: Electric gearbox system final CAD design.

VIII. ELECTRIC GEARBOX DESIGN CONCEPTS

An axial, three phase, PMM, using discrete windings isadopted for the EGBS demonstrator. These design conditionsresult 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 thePMs and the other the exciter windings. Unlike a conventionalPMM, where one disc would be kept stationary, this designallows both discs to rotate. Each disc is connected to the engineand drive wheels respectively. When the drive wheels need tomotor or generate, a mechanical break is applied to the dieselengine disc to oppose the reacting torque; and when the enginerequires starting or generating a mechanical break is applied tothe drive wheel disc. The complication arises when the EGBSrequires both discs to rotate. It is because of this complexitythat the two disc concept, despite its weight reduction, is notincorporated in the EGBS demonstrator.

The two three-disc designs are only subtly different. Theyare both essentially two PMMs back-to-back, where the centredisc is utilised by both PMMs. The difference between the twodesigns in Figure 13 is the placement of the PMs and windings.By placing the PMs on the centre disc, design (a), the twoPMMs can share the same PM. Design (a) allows the centredisc to be compact, however it requires the windings to be ona rotating disc, thus requiring slip rings which cause electricalnoise and heat. Contactless slip rings, using either fibre opticsor EMF, are available and are also a growing market for use inwind technologies; however like most of the new technologiesthey 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 fluxfrom the windings in the separate PMMs do not interfere withone another, the centre disc must be insulated and thus must beless compact, as can be seen in Figure 13. The objective of theEGBS demonstrator is control, and the manufacture process to

be as simple as possible, therefore design (b) is implementedso 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 EGBSdemonstrator in SolidWorks. The design is symmetrical; two45W DC machines are located on either end to represent thediesel engine and drive wheels. Two plastic struts ensure thealignment of the axis and present the EGBS as a clear unit.The green discs in Figure 14 are the rotators and hold thePMs, whereas the central blue disc holds the windings shownin orange.

The demonstrator is a simple tool to illustrate the ideaof the EGBS, and consequently portability of the unit is anobjective. The final EGBS demonstrator has a disc size of190mm diameter, and the unit as a whole has dimensionsin width, length and height of: 240mm, 600mm and 240mmrespectively.

The rare earth PMs employed by the EGBS have a length,width and depth of 22mm, 15mm and 5mm respectively. At adistance of 25mm apart the attraction/repulsion force increasessignificantly; thus to prevent fatigue of the PM discs and allowthe windings time to change polarity and high speeds, theaverage distance between PMs is 30mm.

Using an estimate for the required voltage and expectedcurrent flow through each winding, the windings are manufac-tured using a coated 19 SWG (Standard Wire Gauge) copperwire; 1.016mm in diameter and a maximum current carryingcapacity of 9A (a short 12V wire) [17]. To ensure the fluxdensity at the base of each winding remains high, the windingsare designed to be small in length, 20mm; and thus a diameterof each windings is also 20mm to ensure a high number ofturns per winding.

The angle between windings in three phase is θwinding =120° and/or 240°. The angle between the poles generated


is θmag = 180°. Therefore the ratios of θwinding to θmag isdisplayed in Equations 4a and 4b.

θmag = 1.5 θwinding (4a)

OR

θmag = 0.75 θwinding (4b)

The 30mm restriction between PMs and the 20mm diameterof each winding creates restrictions on the number of PMs andwindings that can fit on the 190mm disc. The circumferenceof the discs allow for more PMs compared to windings, andthus the ratio of PMs to windings is that of Equation 4b. Asthe EGBS is a three phase system, the number of windings isa multiple of three. The result is 12 PMs on the rotator discsand 9 windings on the stator discs, where θwinding = 40° andθmag = 30°.

X. ELECTRIC GEARBOX CIRCUIT DESIGN

Figure 15 is the circuit schematic for one of the EGBS'sPMMs. 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 regenerativebraking (depending on the side of the EGBS) the switchesare in the up position as shown in Figure 15. P1 and P2 arefed a DC signal to drive the DC motor. A sinusoidal signalis induced in the windings L1-L9 which is then rectified toproduce 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 drivingthe drive wheels, the switches S1, S2 and S3 are switchedto the down position to connect the electric speed controller(ESC) to the windings. The ESC receives a DC input from P3and P4 and outputs a three phase AC signal using pulse-widthmodulation (PWM). The servo attached to the left of the ESC

Fig. 15: Electric gearbox system demonstrator's circuitschematic for one, of two, permanent magnet machines.

Fig. 16: Three phase rectified sinusoidal signal - no smoothingcapacitor.

Fig. 17: Sinusoidal signals from two individual windings (outof phase).

Fig. 18: Sinusoidal signals from a single winding (blue) anda complete phase (yellow) (out of phase).


Fig. 19: Electronic gearbox system demonstrator.

in Figure 15 controls the frequency of the PWM output, andthus the speed of the PMM.

XI. CONSTRUCTION

To ensure the success of the EGBS demonstrator the signalsgenerated in each winding were carefully monitored. Figure17 presents two sinusoidal signals from two individual outof phase windings. As can be seen, the two signals areexactly 120° out of phase and roughly generate 0.5V. Once allwindings are connected in their respective phases the voltagedifference between phases is minimal. This quality controlmeasure 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 voltageof a phase (roughly 1.5V) is three times that of the voltageover a singular winding (roughly 0.5V).

Both Figures 17 and 18 display a double peak at themaximum amplitudes of the sinusoids. Figure 9 demonstrateshow the flux densities around the solenoid windings vary. Atthe centre of the winding the flux density is less than that at itsextremities. Therefore the PM energises the winding initially;the EMF then drops with the flux density as the PMs pass overthe centre of the windings; and then energises the windingfurther as the flux density again increases.

This effect can be reduced by introducing an iron or steelcore at the centre of the windings. The EGBS currently adoptsnylon screws as the winding's core; this is due to the DCmotors not being able to overcome the attraction/repulsionforces that a steel core would create.

XII. RESULTS

Figure 19 presents the completed EGBS demonstrator, andTables II and III contain its performance data. During gener-ation mode, at a speed of 1800rpm, the voltage and currentacross a 10Ω resistor is 2.32V and 2.54A respectively for the

diesel engine generator, and 3.08V and 2.42A for regenerativebraking mode.

TABLE II: Speed limitations of electric gearbox systemdemonstrator (no load).

Demonstrator Mode Speed (rpm)

Maximum generation speed (Diesel engine generator) 2200Maximum generation speed (Regenerative braking) 1900Maximum motoring speed (Integrated starter) 134Maximum 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 SocketInput/output Input Input Output

DC/AC DC DC DCMax. current (A) 1.3 16 N/AMax. voltage (V) 50 14 N/A

XIII. MANUFACTURE

Figure 20 demonstrates how the EGBS axial demonstratorcould be altered into a radial machine. Whilst an axial machineis preferred for the demonstrator, a radial machine may provepreferable for application in a hybrid bus.

The rotor connected to the diesel engine is merely expectedto provide a constant low power input or a short, high poweroutput to start the diesel engine; these demands are easilyprovided by a low power machine. The drive wheels howeverrequire a constant high power input or output to ensureimmediate acceleration or regenerative braking. Therefore thedrive wheels require a high power machine.

The design in Figure 20 is capable of fulfilling theserequirements. The diesel engine connects to the central, low


Fig. 20: Radial flux electronic gearbox system design.

Fig. 21: PWM-driven (controlled) machine drive.

power (small diameter) rotor; and the drive wheels to theexterior, high power (large diameter) rotor.

Whilst the design is more complex and requires lowertolerances in comparison to the axial design, it prevents thePMM connected to the diesel engine being needlessly large.

XIV. FUTURE DEVELOPMENT

To improve the efficiency of the current demonstrator designthe steel core needs to be laminated. The current designincorporates a steel ring as its core and thus suffers fromhigh steel losses (eddy currents). The open nature of theEGBS demonstrator also contributes to the large flux leakageexperienced by the demonstrator. An improved housing andsteel core design would improve the flux linkage.

Currently the demonstrator treats both PMMs as separateidentities. To progress the development of the EGBS, thetwo PMMs ought to operate together, and be successfullyconnected to a battery. Figure 21 is a potential circuit toconnect the two PMMs.

On the far left and right of Figure 21 are the two three phasePMMs. Connected to the PMMs is a circuit that resembles athree phase rectifier; D1-D12 are diodes, and work identicallyto those in Figure 15 to rectify the three phase input intoa DC output. The alteration in this circuit is the additionof the voltage sourced converters, V1-V12, which are gateturn-off devices that allow a DC signal to be converted intoa three phase PWM signal. In fundamental terms, Figure

21 is a circuit in which the ESC is superimposed upon thethree phase rectifier. Central to the circuit is the battery; asmoothing capacitor C1; and a dynamic braking resistor-switchcombination. The dynamic braking resistor Rb and switch Sbare for the situation whereby the voltage across the batteryexceeds its intended maximum terminal voltage. At whichpoint Sb allows a current to flow through Rb and the voltageacross the battery terminals reduces.

XV. CONCLUSION

Current hybrid buses suffer from high cost and weight incomparison to their non-hybrid counterparts. Series hybridpowertrains are understood to be well suited to urban driving.The EGBS makes simple alternations to the series hybridschematic to half the number of components required by thepowertrain.

The EGBS thus improves weight and cost. The design alsosuggests a more intuitive approach to the power-on-demandfunctionality; proposing it be driven more heavily on a buses'duty cycle and less so on the current state of the battery; furtherimproving fuel efficiency and pollution reduction.

A demonstrator built to support these claims, establishes theEGBS concept to be a valid and compact design. The demon-strator successfully fulfills the design criteria: controllable andclear 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 andadopting elements of automation is the next step in validatingthe concept further. The two-disc concept and/or the radial fluxmachine 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 thetransport industry. Primarily a locomotive is used to haul atrain; however, manufacturers are beginning to develop trainswhere the machine-diesel engine combination is situated inthe chassis of each carriage. It is believed that several lowpower machines and diesel engines are more efficient and alsoprovide operators with further flexibility due to their modulardesign. In this circumstance, a light weight and effectivemethod of connecting the diesel engine to the machine iscrucial; and would be perfectly suited to the EGBS.

The aerospace industry is another sector in which weightreduction is of high importance. Hybrid technologies arebecoming a focus of aerospace; with this in mind, the EGBSwould again be well suited to such an environment.

ACKNOWLEDGMENT

The author thanks Phillip and Collin for their efforts towardsthe construction of the EGBS demonstrator; Ian, Jim andGeorge's advice and guidance on electric machines; and thesupport from friends and family. Thank you also to the womanat the engineering coffee bar, the conversation, beef salad andcoffee also was a great contributor to the end goal.


REFERENCES

[1] I. Husain, Electric and Hybrid Vehicles: Design Fundamentals, SecondEdition. CRC Press, 2011.

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