ABSTRACTElectric vehicle development is at a crossroads. Consumerswant vehicles that offer the same size, performance, range,reliability and cost as their current vehicles. OEMs mustmake a profit, and the government requires compliance withemissions standards. The result - low volume, compromisedvehicles that consumers don't want, with questionablelongevity and minimal profitability.

In-wheel motor technology offers a solution to theseproblems; providing power equivalent to ICE alternatives in apackage that does not invade chassis, passenger and cargospace. At the same time in-wheel motors can reduce vehiclepart count, complexity and cost, feature integrated powerelectronics, give complete design freedom and the potentialfor increased regenerative braking (reducing battery size andcost, or increasing range). Together, these advantages createthe tipping point for OEM acceptance of in-wheel-motortechnology, offering them an immediate opportunity to buildlarger electric and hybrid vehicles, including full-size sedansand SUVs -vehicles that consumers want and are profitable tomanufacture.

INTRODUCTIONOEMs are currently modifying the architecture and design oftheir entire range to better respond to customer demand forgreener more efficient vehicles and the regulatory actionscurbing greenhouse gas emissions. Accordingly,manufacturers are planning to rapidly expand theimplementation of advanced vehicle, powertrain and enginetechnologies. In addition to the implementation of newpowertrain technologies, the reduction in mass and size of thevehicle systems, sub-systems and components has also shownvery promising opportunities for decreasing total vehicleemissions and running costs. Overriding this, consumers want

vehicles that offer the same size, performance, range,reliability and cost as their current vehicles, but OEMs mustmake a profit, and the government requires compliance withemissions standards. How can the advanced vehicletechnology and diverse and often conflicting requirementscome together to create the new fleet of desirable andeconomically viable vehicles?

This paper will explore in detail the technology of in-wheelmotors (IWMs), the challenges of their integration intovehicles and how they can make a real difference to theeconomic viability of vehicles in a changing consumer andregulatory framework. We aim to show the reader both theopportunities and challenges surrounding IWMs; the benefitsaround packaging, performance and economics, and how thetechnical challenges of unsprung mass, brake integration andcost are being addressed in a manner suitable for the eventualadoption by automotive OEM's.

Most vehicles on our roads share a fairly similar basic layout;that of a single engine driving through a gearbox driveshaft,differential, then half shafts to the wheels. Variations of thisarrangement have been used for over 100 years with very fewchanges to the basic layout. In recent years, electric andhybrid vehicle development has gained prominence amongmajor OEMs and specialist companies. The vast majority ofthese electric/ hybrid vehicles still use variations of this samelayout; a large, centrally mounted motor, driving through agearbox, driveshaft and differential.

An alternative arrangement does exist for these electric/hybrid vehicles, namely through the use of in-wheel motors;where the electric motors are housed inside the wheelsthemselves. This allows a far greater level of vehicle designflexibility than is possible with traditional centrally-mountedmotors. It also frees more space inside the vehicle's body forbatteries and allows each driven wheel to be controlled

The Technology and Economics of In-Wheel Motors 2010-01-2307Published

10/19/2010

Andy Watts, Andrew Vallance, Andrew Whitehead, Chris Hilton and Al FraserProtean Electric

Copyright © 2010 SAE International

entirely independently. This fully independent wheel controlallows better traction control, anti-lock braking and antiskidcapability than are possible with any other propulsion system.Infinitely variable torque can be applied during bothacceleration or braking to each wheel instead of thecomparatively crude ‘on or off’ system used by aconventional mechanically-braked ABS system.

Most electric motors of a sufficient power to propel apassenger or commercial vehicle require a large externalinverter/ power electronics unit to control the motor. For avehicle that used in-wheel motors, each motor would requireits own inverter, thus taking up space in the vehicle andreducing one of the key benefits of in-wheel motors; theincrease in usable volume inside the vehicle body. A solutionto this would be for the power electronics to be integratedinside the in-wheel motors. This mounting arrangementwould also substantially reduce the length of cableconnecting the motors to the inverters, thus reducing theheating (I2 × R) loss in this cable; considering that an electricvehicle drivetrain would normally use hundreds of Amps togenerate the required torque, the losses associated with thiscould be substantial.

Another significant source of losses in a drivetrain lies in theuse of gearing in order to reduce the speed of the propulsionsystem (either an ICE or electric motor) to a suitable wheelspeed. If the in-wheel motor was designed to operate in adirect drive configuration, the need for a gearbox would beeliminated, resulting in a lighter weight, compact andefficient drivetrain. If a direct drive design was not utilized,then each high speed electric in-wheel motor would requireits own gearing, potentially resulting in a greater part countwith more wearing components in a heavier drivetrain thanfor a single centrally-mounted electric motor.

When an in-wheel motor is referred to in this document it isassumed to be one having the optimum characteristics ofbeing of a direct drive design with the power electronicsmounted inside the motors themselves. An example of thistype of motor is the Protean Drive™ from Protean Electric.

This Paper will go on to discuss the following aspects ofElectric and Hybrid Vehicles, and In Wheel Motors:1. The History of In Wheel Motors2. The Electric and Hybrid Vehicle situation2.1. The Market and Motivation to produce Electric andHybrid Vehicles2.2. The Role of Mass and Footprint in Electric and Hybridvehicle Development2.3. The ‘Ideal’ Vehicle Platform for Electric and HybridVehicles3. Do IWM's provide the ‘tipping point’ for Electric andHybrid Vehicles?

3.1. Economics

3.2. Vehicle Design and Packaging

3.3. Active Safety and Torque Vectoring

3.4. Regenerative Braking

3.5. Overcoming the Technical Challenges of IWMs

4. Conclusions

1. THE HISTORY OF IN WHEELMOTORSThe concept of having an individual in-wheel motor drivingeach wheel of a vehicle is not a new idea. In 1900 at theWorld's Fair in Paris the ‘System Lohner-Porsche’ wasdebuted (reference 1). This vehicle was designed byFerdinand Porsche; the man who gives his name to thePorsche car company, at his first job in the automotive fieldworking with Jacob Lohner. The ‘System Lohner-Porsche’was an electric vehicle driven by two in-wheel motors; thisvehicle was capable of over 35mph and set several Austrianspeed records.

Following the success of this vehicle, Ferdinand Porsche thenutilized Daimler's and Panhard's internal combustion enginesconnected to generators to provide power for the in-wheelmotors; thus creating the world's first series hybrid vehicle(SHEV), the “System Mixt”. Porsche's in-wheel motor -driven vehicles in both electric and series hybridconfigurations continued to claim more speed records in bothtwo and four wheel drive configurations, eventually resultingin Ferdinand Porsche winning the 1905 Poetting Prize asAustria's outstanding automotive designer.

Figure 1. An early 1900s ‘System Lohner-Porsche’,propelled by in-wheel motors (reference 2)

2. THE ELECTRIC AND HYBRIDVEHICLE SITUATION2.1. THE CURRENT MARKET &MOTIVATIONThe US has a unique problem and opportunity whenconsidering the emerging vehicle marketplace and emissionsroadmap. As recently reported in the Wall Street Journal, July2010 (reference 3), the US passenger vehicle market hasLight Duty Trucks, SUVs and Crossover vehicles combiningto be around 60% of passenger vehicle sales so far in 2010.Cars make up the other 40%, and within that segment, 66%are midsize or larger. In the overall US market, only 13% ofvehicles are classed as being in the Small Car Segment andtherefore 87% are classed as being ‘larger’. Encouragingly,the industry is showing signs of recovery with all segmentshaving sales growth on a year to year basis, the onlyexception being the Small SUV segment, which has had a15% reduction.

In order to maximize the available market size andenvironmental savings in the US, electric vehicles and theirtechnologies need to address the requirements of buyerswanting larger vehicles; there should be no loss ofperformance, space, comfort, and many would argue mostimportantly price and running costs.

This paper does not intend to compare overall characteristicsof the US, European and Far East Markets; it will concentrateon analyzing popular US vehicles and how they can be bestelectrified. However, for further motivation it is well worthconsidering the findings of the International Council on CleanTransportation and their Passenger Vehicle Greenhouse Gasand Fuel Economy Standards report, published 2007 andupdated 2010 (reference 4).

“This new study compares passenger vehicle greenhouse gasand fuel economy standards from eight major countries,states and regions. It finds that Japan and Europe are closelytied in the “race to the top” for the world's most efficient newpassenger vehicle fleet, while the United States lags behindthese two regions by a large margin.

Figure 2. International Council on Clean Transportation and their Passenger Vehicle Greenhouse Gas and Fuel EconomyStandards report, published 2007 and updated 2010. (reference 4)

Figure 3. International Council on Clean Transportation and their Passenger Vehicle Greenhouse Gas and Fuel EconomyStandards report, published 2007 and updated 2010. (reference 4)

So why is the US being portrayed as ‘lagging’ behind in suchreports. Again, the ICCT provides some insight into this bylooking at the average vehicle mass and engine size for newcar sales

Figure 4. International Council on Clean Transportation and their Passenger Vehicle Greenhouse Gas and Fuel EconomyStandards report, published 2007 and updated 2010. (reference 4)

Some people may find these figures hard to accept, but,further on in this paper we look at the masses and power forthe top 5 selling vehicles in each market, and the figures areremarkably consistent.

Markets are difficult to compare, but the simple figures arethat, on average, the US consumer buys cars which areapproximately 50% heavier, have 25% more footprint and are>100% more powerful than either the Europeans or Japaneseconsumers. It is worth noting that the representative EU carabove, is a petrol Golf. VW's recently launched awardwinning Golf Bluemotion 4dr has a 1.6 diesel producing 104bhp with 99g/km carbon emissions and an official 74.3 mpgcombined fuel economy and will seat 4 people in comfort.The representative US vehicle, the Chrysler 300C has a 3.5LV6 with 250bhp and 250 ft lbs torque, returns 25 mpg on thehighway and 20 mpg on the combined cycle and will seat 4people in comfort.

Again, it is not the role of this paper to present an argumentto force the US vehicle market into a shape that does not meetits consumers' needs or desires. One must approach theproblem from both sides, firstly, government programs andenvironmental education will lead the market towards moreefficient cars, but secondly, technology must supply moreefficient and environmentally friendly drivetrains suitable forthe larger vehicles that the majority of the US market wantsto buy.

The message is thus clear; the US market is and will continueto be, dominated by larger vehicles, with high levels ofcomfort, space and performance. To be successful in the US,drivetrain technology suppliers must be capable of deliveringsolutions for vehicles that typically have a GVW of 4800 lbs(small SUV / Crossover) through to Midsize 5750 lbs andonto Large SUVs with a GVW of around 7000 lbs. Lookingat typical engine sizes and power gives a similar picture with2.5L V6's (161 hp) through to 5.3L V8's (320 hp). Thesevehicles frequently have some off road / towing capability,and can therefore deliver large amounts of torque to the roadat low speeds.

2.2. THE ROLE OF MASS &FOOTPRINTHistorically over the last 40 years cars have become heavierand bigger as safety and features have increased. If one looksat the ‘compact’ car class; the typical mass in 1970 wasbetween 650kg and 800kg, whereas today it's between1200kg and 1350kg, an increase of around 75%. Consideringa specific example, such as a VW Golf, it's interesting to seethat a Mk1 Golf GTi 1600 had a curb weight of 966 kgs, aGVW of 1332 kgs, a 0-60 of 9 seconds and achieved 27 mpg.

Today, a Mk 6 Golf 1.4 TSi GT has a curb weight of 1280kgs, GVW of 1840 kgs, a 0-60 of 8 seconds and achieves 45mpg on the combined cycle. So despite a large increase insize and weight, engine and drivetrain technologyimprovements have outpaced the increase in mass.

So, going forward we could conclude that the increases invehicle mass and size are not a problem from an emissions, orrunning costs perspective. This would be incorrect. It isroughly estimated that reducing the mass of a ICE vehicle by70kgs will result in a 1mpg improvement in economy.Likewise, internal work within Protean Electric shows thatreducing the mass of a Battery Electric Vehicle (BEV) by20% will improve the range on an urban cycle by a useful14%. As fuel prices increase, the consumer is becoming verysensitive to increases in running costs; witness the increase inhybrid vehicle sales the last time fuel hit $4 per gallon.Compelling as these reasons may be, it is clear thatregulations will also play an important role. As the reductionin CO2 emissions become more important, governments arenow using weight and size (footprint) to classify vehicleswithin a regulatory framework:

Figure 5. International Council on Clean Transportation and their Passenger Vehicle Greenhouse Gas and Fuel EconomyStandards report, published 2007 and updated 2010. (reference 4)

2.3. THE IDEAL VEHICLEREQUIREMENTSIn order for in-wheel motors to become a viable option forelectric vehicle propulsion, it is necessary to understand therequirements of the vehicle's end users. These requirements(and customer expectations) vary substantially betweendifferent markets, thus meaning that the same drivetrainarchitecture may not necessarily be suitable for everyapplication or every market. A clear illustration of thediffering vehicle requirements for different parts of the worldcan be seen in the three figures below; showing the fivehighest selling vehicles in Europe, U.S.A and Japan for 2009.

Figure 6. The most popular vehicles sold in Europe

Figure 7. The most popular vehicles sold in the U.S.A

Figure 6. The most popular vehicles sold in Europe)

Figure 6. The most popular vehicles sold in EuropeFigure 6. The most popular vehicles sold in Europe in 2009 (reference 5 in 2009 (reference 5

in 2009 (reference 6)

Figure 8. The most popular vehicles sold in Japan in 2009 (reference 7)

From the above figures it can clearly be seen that vehicles inthe USA tend to be substantially larger and more powerfulthan those in Europe or Japan. This presents a problem forelectric vehicle adoption in the U.S.A as customers seem lesswilling to dramatically change their expectations of vehiclesize and performance. Small compact electric vehicles andcity cars would find a far more receptive audience in Japanand Europe than in the U.S.A, as higher population densities,smaller roads and lower typical commuting distances makesmall electric/ hybrid vehicles less of a departure from whatcustomers are familiar with. For example the least powerfulcar (in the top five vehicles) sold in the U.S.A is the samepower as the most powerful cars (in the top five vehicles)sold in Japan or Europe.

Another point to consider is that in general, Japan appears farmore accepting towards new and unfamiliar technology thanthe U.S.A, illustrated by the fact that the bestselling vehiclein Japan in 2009 was the Toyota Prius hybrid and the 5thbestselling vehicle was the Honda Insight hybrid, whereas inthe U.S.A two out of the top five most popular vehicles werefull-size pick-up trucks; the Ford F Series and the ChevroletSilverado.

A traditional centralized electric drivetrain capable ofdelivering 150 - 200kW (or more) to propel a large sedan orfull-size pick-up truck would be of similar size to the ICE andgearbox it replaces, and consist of an electric motor, gearboxand power electronics/ inverter. This powerful drivetrainwould take up a substantial amount of space inside the bodyof a vehicle, making it challenging to package. This vehicleintegration/ packaging challenge is compounded by the needto have a correspondingly large battery pack to provide theenergy needed for the electric drivetrain. In larger vehicleapplications such as this, multiple in-wheel motors couldprovide the required power/ torque while easing vehicleintegration and leaving the entirety of the space inside thevehicle body available for battery packs.

An example vehicle

Traditional vehicles' ICE's are sized around a combination ofcustomer requirements, performance targets, fuel economy,and parts bin choices (e.g. engine and gearbox ranges). SomeOEMs will differentiate on performance, others runningcosts, etc. A key component in all this is the drivingexperience given by an ICE and gearbox, Manual vsAutomatic, 4 speed to 8 speed etc.

A direct drive in-wheel motor drivetrain has different drivingcharacteristics, especially in the way torque is delivered to thewheels. The highest torque is available from standstill, andchanges in torque demand are far faster (potentially a coupleof msec vs 150msec) than a conventional drivetrain with allthe compliance / lag inherent therein. Torque delivery is verysmooth, and acceleration is rapid over the vehicle's speedrange. Top speed is typically limited by battery SoC / Voltagerather than either torque or power. In order to specify anelectric drive we need to establish the torque and powerrequirements over the whole speed range and ensure thesegive adequate vehicle performance.

Protean studied the various performance metrics, e.g. 0-60,50-70, top speed, gradeability, regenerative braking force andconcluded that for our direct drive PM motors, the PD18, thelimiting cases were peak gradability from standstill andcontinuous gradeability at around 10mph. Couplinggradeability together with a high efficiency requirement fornormal drive cycles, it was found all other metrics ‘took careof themselves’. That is to say, the consequences of themachine design to meet high torque and high efficiencyresulted in a motor which also gave very good performance inall other metrics. The detail won't be delved into here, butProtean concluded that requirements for a 30% peak gradientfrom standstill and 22% continuous gradient at 10mphenabled us to analyze and match the PD18 performance tovarious vehicles. We also compared the torque and powerrequirements for the more aggressive / normal driving drivecycles including Hyzem Urban, Road and Motorway, NYCC,US06 and FTP75 City. Together the torque requirements areshown on Figure 9.

Figure 9. Torque Requirements Vs Market Segments and Drive Cycles.

Figure 9 shows that a drivetrain of 4 × PD18 motors willprovide a good performing drivetrain for vehicles up toapproximately 7100 lbs GVW, a figure limited by the PD18'scontinuous rating which is a thermal limit rather than peakpower.

As an illustration let us consider a new model Ford Explorerwhich has been “converted” analytically to a BEV. The keyassumptions and parameters for the stock vehicle andsimulation are:

Original vehicle = 4450lbs (2021kgs) curb weight, 60101bs(2730kgs) GVW. The mass of components removed inelectric conversion = 480 kg (Based on Protean's F150project). Aerodynamic Cd.A= 1 m2, Coefficient of rollingresistance = 0.011 and Rolling radius 0.381 m, Tires 235/65R18.

Average ancillary load 2 kW, Install an electric drivetrainpowered by 4 × PD18 motors each weighing 30Kgs / 68lbsand giving 1000 Nm peak torque (for 20s) and 715 Nmcontinuous torque, 83 kW peak power at 400 V supply and1400 rpm maximum speed.

We considered 2 battery cases: Battery (40kWh and 70kWhoptions). We assumed Lithium Ion chemistry from A123: 97Wh/kg pack weight, an effective pack resistance 1.5ohm.kWh (e.g. 0.15 ohm for 10 kWh pack) and SoC 0% to100% usable.

The stock vehicle has a 4.0L V6 producing 157kW and344Nm. The vehicle tractive limit with fully loaded rear axleswas calculated to be 4250Nm, and the engine/gearbox wouldgive 3950Nm plus whatever the torque converter gives,which is usually somewhat variable between 1 and 2 times.

The PD18 drivetrain will give 4000Nm peak and 2860Nmcontinuous, with a maximum power of 332kW available,though 216kW continuous.

With the smaller battery, the BEV Explorer will reach107mph and 0-60 in 5.9sec, and with the larger battery,109mph and 6.7sec. Both these 0-60 figures beat the stockICE vehicle's time of 9sec.

In terms of gradeability, both the BEV's would match thestock vehicles peak pullaway gradient of 45 degrees.

So, the above shows that if such a Explorer BEV we're built itwould provide the same space and comfort as the stockvehicle, and would exceed the on-road performance in allareas except low speed towing.

3. DO IN WHEEL MOTORS PROVIDEA TIPPING POINT?The previous section describes a market situation where boththe OEMs and consumers are being driven down a path toreduce CO2 emissions by electrifying the drivetrain. Howeverthe current state of play means that consumers cannot buy‘green’ cars they actually want to drive, and the OEMscannot truly profit in the electric and hybrid vehicle market.

But can in-wheel motors assist the Industry in providing atipping point for electric and hybrid vehicles? This sectiongoes on to discuss the issues relating to the application ofIWMs to mass market vehicles and whether they can help theindustry in providing a tipping point.

3.1. ECONOMICSIn the short term electric vehicles are likely to remain moreexpensive than the equivalent IC engine vehicles, largely dueto a combination of the low volumes in which they willinitially be built and the cost of the battery packs.Fundamentally consumers do not want to pay extra for a carwith lower specifications than their existing vehicle. In arecent Deloitte survey conducted in the U.S.A respondentswere asked “what is the top factor that would prevent youfrom purchasing an EV?” The answers to this survey areshown below.

purchasing of an EV (reference 10)Figure 10. Most common factors preventing

The three most common answers were that EVs were tooexpensive, had a limited range and they didn't want a smallcar. In contrast to this is another 2009 Deloitte report whichstated that the additional features customers would be mostwilling to pay for would be ones such as “vehicle skidcontrol” (reference 11).

In-wheel motors have the capability to deal with both of theseissues, in that they have the capability to propel largervehicles while leaving space inside the vehicles to allow largeenough battery packs to give adequate range. At the sametime the inherent ability of in-wheel motors to deliver torqueto each driven wheel independently allows features such astraction control, launch control, ABS and torque vectoring tobe implemented in an extremely straightforward manner,without adding to the vehicle's construction cost.

This would allow OEMs to recoup some (or potentially all) ofthe additional battery costs, without alienating customers. Inturn end users would get a vehicle with greater capability inthe areas they want, without having to compromise on theperformance parameters they take for granted. The precedenthas already been set for customers paying a premium formore fuel efficient vehicles, as long as they have similarcapabilities to their existing vehicles. One example of this canbe seen with the Toyota Prius, a vehicle that has an MSRPstarting at $22,800 (reference 12) compared to a vehicle ofsimilar size like the Ford Focus, a vehicle that has an MSRPstarting at $16,290 (reference 13).

Another aspect to the economic advantages offered by in-wheel motors lies in the parts that can be eliminated from thevehicle during its construction, namely the engine, fuel tank,gearbox driveshaft, differential, and other ancillarycomponents. The result would be a vehicle that issubstantially easier to construct, with fewer wearingcomponents in a vehicle that would give designers far greaterflexibility with different vehicle layouts. An example of thenumber of components (along with their weights) that can beremoved from a vehicle fitted with in-wheel motors is shownbelow, this example uses the data from a Ford F150retrofitted with four in-wheel motors and shown at the 2008SEMA Show.

Figure 11. Components removed from a Ford F150 retrofitted with four in-wheel motors (Protean Electric data)

The overall increase in drivetrain efficiency resulting fromthe removal of these components can be shown in figure 12,where the drivetrain losses (expressed in Wh/ mile) for a rearwheel drive electric sports propelled by a conventional high-speed, centrally mounted motor (driving through a gearbox)are compared to the drivetrain losses for the equivalentvehicle fitted with two direct-drive in-wheel motors (originalvehicle drivetrain losses can be found in reference 15). It canclearly be seen that the vehicle fitted with the in-wheelmotors loses far fewer Wh/ mile through its drivetrain thanthe centrally mounted motor and that this discrepancyincreases as the vehicle speed rises.

Figure 12. Drivetrain losses for a rear wheel drive sports car fitted with a conventional in-board mounted motor (and gearbox)in comparison to the same vehicle propelled by direct-drive in-wheel motors (data from reference 15 combined with company

modeling)

The example above assumes that other losses in the vehicle(such as ones caused by aerodynamic drag and rollingresistance) will remain constant between the two differentdrivetrain architectures. In this particular example the vehiclefitted with in-wheel motors would be able to have a travelapproximately 14% further (when travelling at a constantspeed) for a given battery size. This increase in range is dueto a combination of the reduced drivetrain mass (resulting ina 3% range improvement) and the increase in drivetrainefficiency (resulting in an 11% range improvement).

If it is assumed that the same Wh/ mile losses are presentduring regenerative braking as the ones shown above formotoring, then the amount of energy returned to the batteryduring regenerative braking would be greater for the vehicleequipped with in-wheel motors than for the one with an in-board motor. In the vehicle equipped with in-wheel motors,less energy would be wasted in overcoming the drivetrainlosses. The increase in range offered by the greater drivetrainefficiency of the in-wheel architecture would be highlydependent on the particular vehicle, battery size and drivecycle, thus it would be extremely difficult to give a ‘rule ofthumb’ value for this improvement. Multiple companiesround the world are currently investigating the integration ofin-wheel motors into vehicles and more precise data will beavailable once these tests have been completed.

The other economic advantage offered by in-wheel motors istheir ability to be easily retrofitted to existing vehicleswithout an entirely new vehicle being constructed. A goodexample of this is a through-the-road hybrid (TTRH) retrofitof a light-medium commercial vehicle. In this configurationelectric motors are added to the non-powered axle of a two-wheel drive commercial van along with a suitable battery

pack. This allows either the vehicle's original drivetrain, orthe electric motors, or a combination of both to propel thevehicle. As a TTRH retrofit keeps the original drivetrainintact, there is less available space in the vehicle to packagemotors and batteries than there would be if the drivetrain wasremoved, this constraint makes the highly compact design ofdirect drive in-wheel motors ideal for this application.

Recent studies undertaken in this area by a prestigiousaftermarket US vehicle integrator have shown that the highvolume cost of converting light commercial vehicles intoTTRH vehicles using two direct-drive in-wheel motors wouldadd around $19,000 to the base price of the vehicle (with a10kWh battery pack). Not only can the TTRH configurationbe applied to existing fleet vehicles but it is substantiallycheaper than the all-electric version of the same vehicle,which is projected to sell for “between $50,000 and $70,000”(reference 10). The precise reduction in fuel costs facilitatedby the TTRH configuration would heavily depend on a widevariety of variables including the mass of the vehicle, theparticular drive cycle, the daily driving distance and the exactway the electric motors are utilized (torque assist, low speeddriving, etc). What is certain is that the valuable PR benefitsof ‘being seen to be green’ could be achieved at a much lowercost than the full electric alternative, while still providingsignificant fuel savings.

3.2. VEHICLE DESIGN ANDPACKAGINGA prominent example of what can be achieved whendesigners start from scratch with in-wheel motors is theEDAG light car concept (reference 8). As well as usinginnovative materials and display systems, the vehicle's drive

drivetrain features Protean Electric PD18 motors withintegrated control and power electronics. Highly efficientmotors and clever battery placement have providedconsiderable creative scope for designers both internally andexternally:

concept is based around a pure electric in-wheel drive with arange of 150Km, making it suitable for everyday use. The

Figure 13. EDAG light car concept

One only needs to study the Lightcar's interior spacecompared to overall length to see the packaging benefits. TheLight Car's 4.0m length is less than a VW Golf 5 door, in factit's about the same as the VW Polo (3970mm). However it'swheelbase of 2.9m is more than the Mercedes E Class Saloonand about the same as the M-Class (2915mm).

In-wheel motors have played a major part in creating a carwith a sub-compact class footprint and mass, but with luxurycar class levels of wheelbase and interior passenger space. Inthe longer term, in-wheel motors will also allow multiplesystems such as steering, brakes, drive and suspension to beintegrated into a single package to give yet more vehiclepackaging advantages.

3.3. VEHICLE PERFORMANCE ANDACTIVE SAFETYMost people with an interest in vehicle dynamics will befamiliar with the often conflicting quest for a balancebetween ride comfort, response, stability and fuel economy(to name but a few factors). This part of the paper intends totouch on the advantages in-wheel motors provide that reducethe compromise between these often opposed factors.

LONGITUDINAL - Driveline Compliance andTractive EffortLongitudinally, the potential for in-wheel motors to improveperceived performance is high. In a typical ICE driveline, theresponse to a change in throttle demand is slurred first byinlet manifold transit time, then by the need to wind up the

engine on its elastic mounts and finally by the need to windup the driveshafts before the torque is delivered to the wheel.A final, minor source of lag is the delay while the wheelchanges speed to deliver a different slip ratio at the contactpatch, but this is normally short. Typical road car drivelinescan take up to around 300 msec to respond, particularly withautomatic gearboxes. In contrast, in-wheel motor responsetimes are typically a few milliseconds to a change in torquedemand. It doesn't sound like much, but when driving amotorsport car (with stiffer sideshafts and non-compliantengine mounts), the immediacy of performance feel and thesense of improved response is acute. Similarly with in-wheelmotors, any throttle response delays in the system areessentially imperceptible as in-wheel motors are directlycoupled to the point at which torque is required, without theneed for compliant driveline components or ICE mounts.

In terms of increasing absolute longitudinal performance, inwheel motors offer further advantages. Due to theelectromagnetic design opportunities of a radial flux in-wheelelectric motor, the tractive effort that can be applied to theroad is subtly different to the conventional centralized motorvehicle. In-wheel motors allow a large flat-torque region,which although does not achieve similar magnitude in peaktractive effort when compared to an ICE driven vehicle,extends to much higher speeds (typically 50-70mph). Figure14 illustrates the advantage. A typical 30-50mph accelerationevent is seamless and significantly shorter with in-wheelelectric motors, reducing the important “time exposed todanger” during overtaking maneuvers and making joiningfast moving traffic easier. Note that although the IWMdrivetrain in this example does not reach the first gear effortof the ICE engine, real-world comparisons between the carswould reveal very little difference in low speed accelerationperformance. The tractive effort of the ICE car is heavilyaffected by driver launch (clutch) skill, time taken to changeup to 2nd gear and the compliances mentioned above,whereas the IWM tractive effort delivery is effectively instantand seamless.

Figure 14. Typical tractive effort curves showing theaccelerative advantage of an IWM driven car versus a

traditional ICE drivetrain car.

Note also that in wheel motors provide a simply implementedfour wheel drive system, which increases mobility of alltraditionally front or rear-wheel drive vehicles over low-gripsurfaces. Also in many cases vehicles fitted with in-wheelmotors will offer better mobility than equivalent four wheeldrive cars due to the lack of mechanical coupling of thewheels - each tire can use all the grip it has available withouteffects from the grip of other wheels and without expensiveactive or locking differentials.

LATERAL - Active Safety and Torque vectoringThe addition of separate, highly controllable motors indistributed locations in the vehicle offers a substantialopportunity for improved vehicle dynamics, primarilythrough the manipulation of longitudinal wheel slip and itsconsequent impact on lateral force capacity at individualwheels. For agility, fidelity of behavior and high speed yawdamping, such techniques have an excellent potential tostrongly manipulate vehicle behavior [Harty, D, “Brand-by-Wire” - A Possibility, Aachener Colloqium 2002, pp.1009-1024].

Torque vectoring has been used for many years in the form ofESP/ESC etc. by modulating brakes in order to impart yawmoments to a vehicle, however torque vectoring through theapplication of both drive and brake torque is an emergingtechnique. This is well suited to electric vehicles withindependent wheel torque control, as the cost and controlfidelity of active differentials and brakes presents a sizeableproblem for widespread implementation with a centralizeddrivetrain, which is also only able to apply positive torquewhen the driver is giving an accelerator demand. Torquevectoring has the potential to reduce the conflict betweenstability and response while enhancing other attributes.

A brief example application of torque vectoring is in activeyaw damping. Following a steering input, particularly at highspeed, after an initial delay period (100's of msec), vehicleyaw rate can overshoot and oscillate before settling to asteady-state. In extreme examples this can cause a loss ofcontrol. Even at lower speeds this can make the car feelunstable and the driver may find they have to make multiplesteering adjustments to follow the intended path. This is oftenaddressed through suspension kinematic and static changeswhich make the car feel more stable, but often at the expenseof response “feel”, dulling the car. Figure 15 illustrates howtorque vectoring allows the suspension setting to retain betterfeel and/or fuel efficiency settings, by implementing activeyaw damping which addresses the yaw overshoot andoscillation problem with a wholly software based solution:

Figure 15. Illustration of yaw damping through torquevectoring

An alternative approach is to retain a stable/dull base vehiclecharacteristic and use the yaw authority of the independentIWM to increase yaw response gain during the turn-intransient, illustrated in Figure 16:

Figure 16. Illustration of increasing yaw gain/authoritythrough torque vectoring

The possibilities for modifying vehicle behavior throughapplication of drive and brake torque at each wheel representsa very exciting opportunity for vehicle dynamicists the worldover, who up until now have been limited by comparativelyslowly responding, low fidelity control of hydraulicallyactuated brakes, compliant centralized drivetrains and, ifthey're lucky, complex and expensive active differentials.

3.4. REGENERATIVE BRAKINGRegenerative braking in electric vehicles offers theopportunity to extend the range of the vehicle for a givenbattery capacity or to reduce the battery size (and cost).Typical values are shown in Figure 17 and are derived fromsimulation of a Ford Explorer BEV at GVW (6000 lbs)powered by four in-wheel electric motors. Losses in themotors and battery are accounted for in these figures.

Figure 17. Range extension from regenerative braking ina Ford Explorer BEV

The ability to recapture the vehicle's kinetic energy usingregenerative braking relies on the motors functioningefficiently as generators at torque-speed points typical ofbraking events in normal driving. Permanent magnet motorssuffer losses through ohmic heating of the coils of theelectromagnets, hysteresis and eddy currents in the iron,bearing and seal friction and switching in the inverter.Considering the energy equation when the motor is acting asa generator (where mechanical power → electrical power +loss) and where mechanical power is provided to the motorfrom the reducing kinetic energy of the vehicle and theelectrical power is provided to the battery, the efficiency ofregeneration (determined electrical power/ mechanicalpower) can even be negative.

This arises when the losses are greater than the mechanicalpower input and is visible in the efficiency map forregenerative braking for an in-wheel electric motor, shown inFigure 18. In this region the motor is consuming poweralthough it is providing braking torque. In a stationary vehicleon a slope the mechanical power is zero and the electricalpower merely services losses in the motors and battery.

Figure 18. Regenerative braking efficiency (%) for aPD18 in-wheel electric motor. The region between theaxes and the 0% efficiency contour represents negative

efficiency, i.e. power consumption.

The vehicle's range is optimized by

i). Ensuring that the negative efficiency region of the motoris not coincident with torque-speed regions where significantregenerative braking energy is available in normal driving

ii). Using an alternative braking technology where theefficiency of the motor is negative

iii). Using an alternative braking mechanism, a park brake,when the vehicle is stationary.

iv). Points ii and iii should preferably be realizedautomatically and seamlessly as part of a blended brakingstrategy, mixing regenerative braking, hydraulic or electro-mechanical braking and a park brake.

Figure 19 shows where, on a torque-speed map, energy isavailable for regeneration averaged over a number of drivecycles ranging from urban to highway and tame toaggressive. The scale is normalized. This relates to aparticular application: a Ford Explorer BEV at GVWpowered by four in-wheel motors, and the torque values referto a single motor. This allows a comparison with Figure 18which is presented on the same scale. In this case it is clearthat the amount of regenerative energy available in the regionof negative efficiency is small and that the torque-speedregions where the most energy is available coincide withregions of motor efficiency in excess of about 85%. Thismeans that the motor is well matched to this particularapplication in terms of gaining as much range advantagethrough regenerative braking as possible.

Figure 19. Normalized power density per motor as afunction of torque and speed in braking events averaged

over a selection of drive cycles.

For comparison with the braking efficiency, Figure 20 showsthe efficiency map for the same motor when motoring,defined as mechanical power/ electrical power. In this casethere is no region in which the efficiency becomes negativeas by definition, the mechanical output power is positive in

motoring. Figure 21 shows where, on a torque-speed map,most mechanical power is produced during an aggregate of aselection of drive cycles. As above, the vehicle is a FordExplorer BEV at GVW powered by four in-wheel electricmotors. As with braking the high efficiency operating regionof the motor coincides well with the torque -speed region inwhich the motors operate most of the time.

Figure 20. Motoring efficiency (%) for an in-wheelelectric motor.

Figure 21. Normalized power density per motor as afunction of torque and speed in motoring averaged over

a selection of drive cycles.

It can be seen that an in-wheel motor solution usingpermanent magnet motors is well-placed to maximize therange of the vehicle by recapturing braking energy in thebattery. Permanent magnet motors are more efficient thanother types of electric motor and the high efficiency operatingregion of the motor is well matched to the operating pointsduring normal vehicle braking events. Furthermore the in-wheel solution has the advantage over a central electric motorof avoiding drive-line losses

3.5. OVERCOMING THE TECHNICALCHALLENGES OF IN WHEEL MOTORSIn the short term, as with any leading edge technology thereare barriers to market entry and overall acceptance. In termsof vehicle packaging, for in-wheel motors there are two maintechnical concerns to overcome; the introduction of extraunsprung mass and mechanical brake integration.

Unsprung MassReference 9 is a jointly authored paper (Anderson/ Harty)describing the effects of increased unsprung mass. The broadinvestigations behind this paper involved two completelyindependent approaches to the perceived problem, which wasleveled squarely at in-wheel motor drivetrains and includedsubjective assessment, objective measurements and predictiveanalysis.

A common challenge to the packaging advantage of in-wheelmotors is the trade off required by moving the drivetrain massfrom the sprung to the unsprung mass. This increasedunsprung mass is often challenged with weakly supportedevidence that there is an unsprung mass/sprung mass ratio“threshold”, which, if you exceed, will result in dangerous,uncomfortable vehicles which will never sell. This part of thepaper will summarize the results found during those studies,which show that the perceived problems with unsprung massare actually blown out of all proportion, and top-of-the-segment performance can be achieved with already defineddevelopment techniques.

According to Anderson/Harty, ground vehicle performancecan be broadly split into:

Ride - The ability of the vehicle to absorb disturbances

Refinement - The ability of the vehicle to attenuate noise andvibration

Active safety - The ability of the vehicle to stop and steer inemergency situations (in a passive/grip sense, i.e. notimplying the presence of “active” safety systems such asESP)

Driveability - The response of the vehicle to the driversinputs (handwheel, brake and accelerator pedals) in normalsituations.

It is with respect to these aspects of vehicle character that theinvestigations were targeted.

Subjective AnalysisThe subjective analysis was conducted on a European 2007MY Ford Focus hatchback, which was fitted with a range ofadditional hub masses, including a 30kg addition which is

typical of a wheelmotor that fits within an 18″ wheel(reference 16). The assessment was carried out byprofessional vehicle evaluators and was rated on the “VehicleEvaluation Rating” (VER) scale through a series of well-proven standard tests at a well renowned vehicle developmentcompany.

The result of the subjective analysis in the Anderson/ Hartypaper (reference 9) was that even with no remedial work, and30kg of additional unsprung mass, the vehicle was in linewith “what might be expected in the middle of a normaldevelopment program and gives no particular cause fordisquiet.” The largest deficit observed was actually in theweight of the steering feel. The Ford Focus base is noted as acar that occupies the top of the class it resides in, in terms ofride and handling. The addition of 30kg of unsprung massmerely moves this car, subjectively at least, to the middle ofthe class, and no one factor, or combination of factors,showed any degradation of performance which would not berecoverable back to the standard (unmodified) car throughusual development techniques.

Objective AnalysisThe objective measurements were also carried out on thesame 2007 MY Ford Focus, again with no remedial work,through well proven tests at the same vehicle developmentestablishment.

In line with the subjective tests it was found that “overisolated bumps the modified vehicle gives measurably poorerbehavior”, and that “some difference in steering characterhas been wrought”. However, the overall conclusion from theobjective analysis was that “none of the differences arebeyond normal deviations from target in a typical vehicledevelopment program”.

Numerical AnalysisThe numerical analysis was approached with a ‘simplemodels, smartly used’ mindset, and was implemented in theMATLAB/ Simulink environment. It involved one or twodegree of freedom numerical models based on assumedparameters of the rear axle of a light commercial vehicle.This vehicle was chosen mainly as the variation in payload islarge and gives a wide variety of relevant unsprung/sprungmass ratios (the unsprung was varied from 5% to 25% of thesprung mass). The basic parameters and the resultingcharacteristics from the study are shown in figure 22 andfigure 23.

Figure 22. Vehicle parameters used in numerical study

Figure 23. Resulting axle characteristics of numericalstudy.

The study used a variety of “key performance indicators”,which are derived from the results of simulations usingsimple numerical models such as the vertical “quarter car”model shown in figure 24.

Figure 24. quarter car vertical response model

The model shown in figure 24 was used to obtain KPI valuesfor ride and active safety. Other models and techniques wereused to develop an understanding of the refinement anddriveability effects of increasing unsprung mass.

Quoting directly from the Anderson/ Harty paper (reference9), the numerical analysis concluded that:

Ride overall: difference in road roughness results in verylarge differences in scores compared to influence of unsprungmass

Primary ride: no discernible difference on smooth roads,slight degradation in rough road performance

Secondary ride: slight degradation in both rough and smoothroad performance may require detail changes to seat orsuspension components

Refinement: some change in suspension component detailmay be required to recover small loss in refinement behavior

Active safety: noticeable but not severe loss in smooth andrough road grip levels; slight increase in damping levels maybe required to optimize performance

Drivability: slight changes to suspension components may berequired to restore agility

A more generalized conclusion from the Anderson/ Hartypaper (reference 9) was that:

“While perceptible differences emerge with increasedunsprung mass, on the whole they are small and unlikely tobe apparent to an average driver. The nature and magnitudeof the changes appears to be nothing that cannot beovercome by the application of normal engineering processeswithin a product development cycle..”

Unsprung Mass ConclusionThe work performed in the making of Anderson/ Harty paper(reference 9) shows that unsprung mass, while it shouldn't notbe considered a good thing, is by absolutely no means a‘show stopper’ for wheelmotor drivetrains. This report alsoconcluded that “the promise of individual wheel motorcontrol shows good potential for substantial improvements invehicle behavior”. The change in vehicle “performance” (asdefined above), while measurable, can be recovered throughthe use of a normal ‘engineering toolbox’ of developmentactivities which all vehicles must go through at present beforebeing signed off for production. Even retrofitting vehicleswithout any rectification work does not cause any majorcause for concern. Wheelmotor drivetrains do not cross some“threshold” of unsprung mass. Other work, such as thatperformed by Schalkwyk et al (reference 17) reinforces thisconclusion.

Mechanical Brake IntegrationWhen fitting an electric motor ‘in-wheel’ allowance needs tobe made for foundation friction brakes to provide the additionretardation force the vehicle requires. To ensure the viabilityof in-wheel motors (IWM), the foundation braking solutionmust cohabit the ‘in-wheel’ real estate with the motor, andreplicate the performance of the OEM system.

The key engineering challenges centered around packaging,torque transfer and heat. Packaging - simply fitting afoundation brake with an IWM is not a trivial task. Modernautomobiles have compact, intricate suspension systems thatallow little room for extra components. Torque transfer - In a

conventional vehicle tractive effort is applied to the tire via acentral shaft and the brake actuates on the interface betweenwheel rim and bearing. When fitting an IWM and a brake,two significant torque actuating devices now exist in-wheel.Heat - Electric motors are sensitive to heat and theirperformance is directly related to the temperature of themotors' coils and power electronics. During retardation of thevehicle a friction brake generates significant heat from whichthe electric motor needs protection.

The preferred solution developed by Protean Electricinvolved attaching a foundation brake to the IWM rotor via afloating bobbin interface, and using twin, radially opposed,in-side-out calipers to brake the disc (see figure 25). The twincalipers are positioned to avoid suspension componentsallowing for the packaging of the foundation brake and IWMout board. The brake components are sized to replicate OEMbraking performance both in terms of maximum decelerationforce available (1G) and adequate thermal mass to carry outrepeated Vmax stops. The torque is transferred from thefoundation brake disc, through the IWM rotor to the wheelbearing interface as with the torque of the IWM. Twin,radially opposed calipers are used to balance axial forces andguarantee pure torque through the rotor to ensure no closingup the IWM air gap. The floating bobbing disc/rotor interfaceis used to minimize conductive heat transfer into the IWM.The final cooling loop of the liquid cooled IWM passes nextto the brake components to remove heat from the system.

Figure 25. Brake mounting illustrations - CAD render

Figure 26. Brake mounting illustrations - Section

While the solution highlighted above has not yet beenoptimized for a volume application, it was designed todemonstrate the possibility of fitting foundation brakes alongwith IWMs. It is hoped that as IWMs become more prevalentthis braking solution too will be advanced and optimized.

CONCLUSIONSIt is clear that governments around the world are nowconvinced of the requirements to reduce CO2 output, and ifthese targets are to be met, a proportion of those reductionsmust come from the transport sector. It is therefore clear thatin order to meet these targets OEMs must reduce their fleetaverage CO2 emissions and that will require significantadoption of electric and hybrid vehicles in their ranges. Inconjunction with this, consumers want vehicles that offer thesame size, performance, range, reliability and cost as theircurrent vehicles and OEMs must make a profit. This papershows in-wheel motors are now well placed to take advantageof, and indeed catalyze this new and growing electric andhybrid vehicle market.

Existing electric vehicle technology is generally as large andcumbersome as the ICE it replaces, and there is limited scopefor the vehicle design changes required to fully exploit thebenefits of electric vehicles. In-wheel motor technologyoffers a solution to these problems, providing powerequivalent to ICE alternatives in a package that does notinvade chassis, passenger and cargo space. Direct-drive in-wheel motors with integrated power electronics are theoptimum form of this design as they reduce vehicle part countand complexity while increasing design freedom andincreasing the efficiency of the drivetrain through thereduction in drivetrain weight and a reduction in gearbox/differential losses.

There are a number of technical challenges that need to beovercome in order for in-wheel motors to be widely adoptedinto vehicles, in that the motors have to provide adequatepower and torque for the desired vehicle size, while notsubstantially increasing the unsprung mass. However studies

referred to in this report have shown that adding the latestgeneration of in-wheel motors can be added to vehicleswithout out any significant degradation in the ride orhandling performance due to the extra unsprung mass. It hasalso been shown that the latest in-wheel motor designs can beintegrated with a mechanical braking system while still onlyutilizing the space inside the wheel rim.

The inherent ability to control each wheel independently in avehicle equipped with in-wheel motors facilitates a far higherlevel of traction control, anti-skid control and torquevectoring than is possible with any other propulsion system.All these elements are ones that consumers have indicatedthey are the most willing to pay extra for, thus allowingOEMs to recoup some of the additional costs of the largebattery packs required for EVs, at very little additional cost.This principle of permitting consumers to drive vehicles ofthe same size, but with greater capabilities than their existingICE vehicles will accelerate the acceptance and adoption ofelectric/ hybrid vehicles in the market.

In addition, the inherently compact nature of direct drive in-wheel motors (with integrated power electronics) makes thistechnology ideal for retrofitting in applications such asthrough the road hybrid vehicles. In this case the IC engineand conventional drivetrain can be retained with the addedfeatures of pure electric range and/ or an electric torque assistwithout losing load space inside the vehicle. Thisarrangement allows the customer to enjoy fuel savings,reduced emissions and good green PR value at a substantiallylower cost than in if they had to purchase an entirely newvehicle.

In-wheel motors offer OEMs an immediate opportunity tobuild larger electric and hybrid vehicles, including full-sizesedans and SUVs. At the same time allowing these vehiclesto have greater capabilities in the areas that customers valueand are willing pay for, such as anti-skid control and torquevectoring, while simultaneously easing the market acceptanceof electric and hybrid vehicle technology through theirapplication in through the road hybrid vehicles. Together,these factors combine to create the tipping point for OEMacceptance of in-wheel-motor technology.

REFERENCES1. The Automotive Century: Most Influential People,Ferdinand Porsche, http://www.autohistory.org/feature_6.html, August 2000, accessed 25th July 2010

2. Lohner-Porsche Mixte Voiturette, http://www.ultimatecarpage.com/pic/3456/Lohner-Porsche-Mixte-Voiturette_2.html, November 2007, accessed 25th July 2010

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5. Data from Jato Research, displayed on http://www.european-traveler.com/uncategorized/list-and-photos-of-the-top-ten-best-selling-cars-in-europe-in-2009, January2010, accessed 20th July 2010

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9. Anderson, M, Harty, D, “Unsprung Mass with In-WheelMotors - Myths and Realities”, presented at AVEC 2010

10. 2009 Deloitte Touche Tohmatsu report “Gaining traction,A customer view of electric vehicle mass adoption in the U.S.automotive market”

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12. Toyota Prius manufacturer website, http://www.toyota.com/prius-hybrid/, accessed 26th July 2010

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14. $50,000 Electric Ford Coming in 2010, 170 MPG Hybridin 2012, http://www.foxnews.com/story/0,2933,512423,00.html, April 2009, accessed 26th July 2010

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17. Van Schalkwyk, D.J, Kamper, M.J, “Effect of HubMotor Mass on Stability and Comfort of Electric Vehicles”,September 2006.

CONTACT INFORMATIONProtean Electric Inc100. W. Big Beaver Rd., Ste. 200Troy, Mich. 48084U.S. Phone: 203-246-3457

Protean Electric LtdCoxbridge Business Park, Unit 10BAlton RoadFarnham GU10 5EHUnited KingdomUK Phone: 44 (0) 1252 741852

ABBREVIATIONSABS

Anti-Lock Braking System

BEVBattery Electric Vehicle

EVElectric Vehicle

GHGGreen House Gasses

OEMOriginal Equipment Manufacturer

GVWGross Vehicle Weight

ICCTInternational Council on Clean Transportation

ICEInternal Combustion Engine

IWMIn Wheel Motor

KPIKey Performance Indicators

MIRAMotor Industry Research Association

MSRPManufacturer's Suggested Retail Price

PMPermanent Magnet

SHEVSeries Hybrid Electric Vehicle

SUVSports Utility Vehicle

TTRHThrough The Road Hybrid

VERVehicle Evaluation Rating

The Engineering Meetings Board has approved this paper for publication. It hassuccessfully completed SAE's peer review process under the supervision of the sessionorganizer. This process requires a minimum of three (3) reviews by industry experts.

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ISSN 0148-7191

doi:10.4271/2010-01-2307

Positions and opinions advanced in this paper are those of the author(s) and notnecessarily those of SAE. The author is solely responsible for the content of the paper.

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