EXAGON FURTIVE-EGT SPORTS GRAND TOURER WITH ELECTRIC DRIVEWith its Furtive-eGT, the race car manufacturer Exagon Motors has developed a battery-driven high-performance

sports car. Siemens is supplying the electric drive for this vehicle with start of production scheduled for spring

2014. The special features of this sports grand tourer include the lightweight structural design of the carbon

body work and the connection between the aluminium chassis and the drivetrain. The greatest innovations are

hidden within the two independently controllable electric motors and a three-stage manual transmission.

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COVER STORY SPORTS CAR ENGINEERING

POSITIONING

The four- seater Exagon Furtive-eGT sports coupé is a purely battery-driven electric vehicle (BEV). Luc Marchetti, the Exagon boss, set out to combine the fea-tures of a sports car with those of a com-fortable grand tourer. To achieve this, proven technologies in automotive con-struction have been combined with new and unique technologies: : the latest generation of electric motors

from Siemens AG, inside e-Car Divi-sion (two permanent-magnet synchro-nous motors)

: a three-stage gearbox with no inter-ruption of traction

: a high-performance lithium-ion bat-tery from Saft

: the chassis and the wheel suspension system – a joint development together with Michelin

: the wheel suspension comprising double wishbones from Saint Jean Industries in the front and in the rear

: the “eMSC Exclusive e-Motion Concept” (eMSC) audio system from Focal

: a new multimedia interface. On the one hand, the Furtive-eGT goes to market as state-of-the-art, well-equipped automobile, however, it also has all of the traditional hallmarks of a sports car. Mar-ket analyses and discussions with poten-tial customers indicate that especially high price premium and niche vehicles have an excellent market potential in the up-and-coming electric vehicle market. The start of a new era, naturally involving more cost intensive high tech, can be best marketed in the form of technologically advanced, attractive vehicles capable of portraying an up- market image. From this particular market niche, which will continue to play a leading role for techni-cal innovations, more favourably-priced components and systems will be able to be developed for vehicles subsequently produced in high volumes.

The sports car accelerates from 0 to 100 km/h in under 3.5 seconds. The max-imum speed is electronically limited to 250 km/h. Deceleration rates of 1.2 g can be achieved with a combination of electric (recuperation) and mechanical braking.

TRACTION BATTERY

The traction motors are complemented by the performance of the batteries from Saft, one of the world’s leading manufac-turers of batteries and battery cells that have built up production capacities to address the demand. The battery is part of the vehicle structure, weighs 450 kg and has a capacity of 53 kWh. This means that the system weight of the bat-tery lies significantly below 10 kg/kWh. This is a very good ratio considering the high power rating of up to 300 kW. With a cycle capability of up to 3000 full cycles at a residual capacity of 80 %, it reaches very good values in terms of energy den-sity, power density and service life. The Furtive e-GT has a range of 360 km in a city cycle (ECE15 standard) and 310 km in a mixed cycle (NEDC standard). Addi-tional performance data is listed in ❶.

CARBON FIBRE BODY

The monocoque construction, ❷, weighs 124 kg, and is made completely from car-bon-fibre materials in a honeycomb struc-ture – a technology that has been derived from formula 1 racing cars. The battery is

AUTHORS

LUC MARCHETTIis CEO of Exagon Motors in

Magny-Cours (France).

DR. TILO MOSERis Senior Key Expert Consultant at

Siemens AG, Corporate Technology in Munich (Germany).

DR. FRANZ X. WAGNERis “Drives” Project Manager at

Siemens AG in Erlangen (Germany).

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integrated in the floor pan as a structural element. This supports the optimal com-promise between optimum lightweight design and a high degree of stiffness. Fur-ther, the design sets itself apart as a result of its extremely low centre of gravity for a vehicle of this class, coupled with excel-lent roll behaviour.

CHASSIS AND WHEEL SUSPENSION

The chassis was a joint development of Exagon Motors and Michelin. After

extensive tests, the result is an excellent compromise between sporty road hold-ing with optimum power transmission and comfort. The wheel suspension com-prises double wishbones, ❸, front and rear, manufactured by Saint Jean Indus-tries, utilising what is known as Cobap-ress technology, an innovative manufac-turing process that combines forging and die-casting concepts.

The wheel suspension elements are attached directly in front of the carbon fibre body and at the rear on the alumin-

ium fork structure. This means that the electric power is transmitted directly to the road with minimal losses. The tyres have been developed specifically for the Furtive-eGT, to ensure optimum adhesion and low rolling resistance. The vehicle is equipped with Beringer brake calipers – six at the front and four at the rear, with four brake cylinders acting on the brake disks. The brake disks have diameters of 380 mm and 360 mm respectively.

DRIVE SYSTEM

The drive system of the Furtive-eGT con-sists of two interior permanent-magnet synchronous motors (IPM motor), ❹, and two three-phase inverters, all water-cooled. Each system is coupled with a two-stage gearbox, so that the motors can be separately switched. This allows for fast gear changes with hardly any loss of acceleration. The output shafts of these gearboxes are coupled with one another, and the torque is transmitted to a limited-slip differential. Of the four possible combinations, three gear stages are used (1-1, 2-1, 2-2). Based on this technique, high acceleration rates can be achieved as well as high speeds, without requiring a high motor weight to develop high shaft torques.

Normally, the drive systems would be operated with closed-loop torque control, however, when shifting gears, a switch is made to closed-loop speed control in order to adapt the motor speed to the fol-lowing gear stage. This demands precise and fast speed control of the motor, as the gear change does not involve a clutch. For example, it takes just 200 ms to adjust the speed from 8500 to 4250 rpm. Here, the inside e-Car business unit can tap into and leverage the expertise of Siemens’ long-standing and traditional involvement in industrial automation. This is because industrial applications generally require higher dynamic perfor-mance than automobiles.

A manual gearbox allows not only for high acceleration rates and high top speeds, it also improves overall efficiency and allows the motors to be operated within their optimum speed range. A graphic example indicates the significance: if the efficiency were improved by 1 %, then the battery weight could be reduced by 5 kg, while still maintaining the same range when operated purely electrically. As a consequence, it is extremely impor-

TECHNICAL DATA

POWERTRAIN

Motors Two permanent-magnet, liquid cooled synchronous motors (Siemens)

Maximum power (296 kW)

Maximum torque (516 Nm)

Transmission Three-stage semi-automatic gearbox with continuous torque

transmission, developed by Exagon

Traktion battery Lithium-ion technology (manufacturer Saft)

Capacity (53 kWh)

Service life (residual capacity > 80 % after 3000 cycles)

Weight (450 kg)

CHASSIS Rear wheel drive with limited slip differential

Wheel suspension Aluminium double wishbone suspension front and back, manufactured

using the Cobapress technique

VEHICLE BODY

Type Passenger cabin manufactured from carbon fibre composites in a

honeycomb structure (124 kg), with integrated traction

BRAKING SYSTEM

Front wheel brakes Ventilated disks (380 mm) – six-cylinder brake calipers (Beringer)

Rear wheel brakes Ventilated disks (360 mm) – four- cylinder brake calipers (Beringer)

Braking Up to 1.2 g

WHEELS/TYRES

Front wheels 8.5" x 20", Michelin Pilot Super Sport 245/35 ZR20

Rear wheels 11" x 20", Michelin Pilot Super Sport 305/30 ZR20

PERFORMANCE DATA

Top speed Limited to 250 km/h

Acceleration 0 - 100 km/h in 3.5 s

Power/weight ratio 5.5 kg/kW

Range 360 km (city driving cycle according to the ECE15 standard)

310 km (mixed driving cycle according to the NEDC standard)

310 km (combined driving cycle according to the NEDC standard)

VEHICLE DIMENSIONS

Length 4465 mm

Width 2160 mm (with wing mirror)

Height 1292 mm

Wheelbase 2780 mm

Weight 1640 kg

Trunk 325 l

❶ Performance data

COVER STORY SPORTS CAR ENGINEERING

6

tant to find a perfect compromise between efficiency and the other chal-lenges that need to be addressed – such as power density, mounting space and costs.

This performance data was achieved by selecting the optimum rotor and stator sheet steel laminations, the magnetic materials, the shape of the magnets and the design of the rotor laminated core. The power electronics supplies a continu-ous phase current of 300 A – and 500 A for a minimum of 30 s. In order to ensure the highest possible power and service life, the development engineers selected a relatively high coolant flow rate of 12 l/min. The motor was designed to guaran-tee a low pressure drop at this flow rate. The thermal behaviour of inverter and

motor are so well coordinated that there are no significant differences between the thermal behaviour of the two elements.

Further details on the Siemens electric motor concept can be found in MTZ 10/2013. Here, the opportunity is taken to explain why Siemens and Exagon selected this particular motor version and configuration.

SYSTEM INTEGRATION AND FUNCTIONAL SAFETY

An additional challenge when developing and integrating drive systems for electric vehicles is the functional safety in com-pliance with ISO 26262. The most impor-tant requirements include preventing

uncontrolled acceleration or braking of the vehicle – while moving and when stationary – and guaranteeing the safe behaviour of the drive system and in turn the entire vehicle.

To achieve this, the drive system is equipped with an additional torque mon-itoring function. This system monitors the mechanical torque by using redun-dant electronics to calculate the electric air gap torque of the motor. In order to achieve the specified ASIL C safety level (Automotive Safety Integrity Level C) this is split up into two independent monitor-ing systems, with safety levels ASIL B(C) and ASIL A(C).

Several system sensors are used for both monitoring functions, for instance the inverter current measurement and the motor temperature sensors. To achieve the specified accuracy, which is ultimately defined by the requirements at the vehicle level, a precise tolerance anal-ysis was carried out over the complete monitoring chain. As a result, a range is obtained in which the mechanical torque is “safe”.

The question now arises, at what point will the deviation of the actual torque from the required torque result in an “impermissible torque”; i.e. a torque that could lead to a critical driv-ing situation. For such a scenario, the drive system must be brought into a safe state, which in the ideal case involves a torque equal to zero. Before the details relating to a safe state are

❷ Passenger cell manufactured out of carbon fibre

❸ Double wishbone suspension

❹ Siemens motor for the Exagon Furtive-eGT

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discussed, the shutdown criterion should first be explained. An applica-tion with a high dynamic response is used as basis, which is additionally increased as a result of the gear change. After previous analyses, it became apparent that simply comparing the demanded torque with the mechanical torque is not sufficient in such applica-tions. Instead the difference between the actual and the required torque is integrated, and the safe state is only activated if the integrated difference falls below a specific limit value. This limit value was determined based on simulated driving situations and con-firmed in several, real measurements made with the actual vehicle. In addi-tion to the integrated torque difference function, there is a special motor torque monitoring function that is even more sensitive. For instance, this is used when the vehicle is stationary and a torque is not demanded. This avoids the vehicle inadvertently accelerating from standstill, for example at a traffic light.

In addition to the torque monitoring function, as already explained, it must be ensured that the drive system assumes a safe state with a remaining torque below a specific threshold. This is especially critical for permanent- magnet synchronous motors, where the induced voltage in conjunction with the battery can result in an uncontrolled regenerative braking torque as result of an uncontrolled energy flow in the bat-tery. For instance, this can occur if the inverter goes into an inactive state if the power supply voltage fails. In this par-ticular case, the regenerative braking torque could be avoided by opening the battery contacts. However, due to the complexity, it is not worthwhile to inte-grate the battery switch into the func-tional safety concept. Additional critical reasons also include the response times of the battery switch. This is the reason that generally an active short-circuit (ASC) is created, which then short-cir-cuits the motor phases when a fault develops. This allows generally accept-able residual torques to be achieved, ❺.

However, an ASC results in a complex architecture inside the inverter, and it requires considerable effort to verify that the ASC can be implemented with the adequate safety level, such as ASIL C. Siemens therefore selected an electric motor design, where the induced volt-

age is so low, that even for low battery voltages the braking torque is no more than 25 Nm for motor speeds less than 9000 rpm, ❻ and ❼. This is achieved using a high motor reluctance torque, which in turn places high demands on the inverter’s closed-loop phase current control. Many concepts from the area of

machine tools were employed in the phase current control, for instance, in some cases, the very low motor induct-ances are very quickly and precisely taken into account. Two innovative features are certainly worth mention-ing: One of these is the torque monitor-ing of the electric motor (with a low

❻ Induced voltage (counter EMF) as a function of the motor speed

❺ Active short-circuit (ASC) of the three motor phases in the case of a fault in order to achieve the lowest possible residual torques

COVER STORY SPORTS CAR ENGINEERING

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braking torque in the case of uncon-trolled regenerative feedback), which fulfills the software safety standard ASIL C. The other is the decoupled permanent-magnet synchronous motors that the development engineers inte-grated with a manual gearbox archi-tecture.

VEHICLE TEST STAND

The validation of the safe state, and in turn the safety concept, must be made in the vehicle itself, so that here the inter action between the battery and drive is decisive. A complete vehicle can be built up on the vehicle test stand.

The wheel hubs are directly mounted on the four dynamometers. This allows the torque to be precisely monitored and allows the load scenarios to be simulated with a high dynamic response. A low dynamic response would have been sufficient for validating the safe state, however, other validations, for example the ESP and ABS control algorithms, require an extremely high dynamic dynamometer response. This cannot be achieved using conventional roller test stands. As a result, this test stand opens up a completely new perspective when it comes to vehicle-in-the-loop tests and validations.

OUTLOOK

With the Furtive-eGT sports car, the French race car manufacturer Exagon and Siemens as the supplier of the vehi-cle’s drive technology are fully exploit-ing today’s technological possibilities. Both companies want to play a role in establishing a completely new industry segment.

Exagon is extending its portfolio to include a sports car that can be driven on public roads. In addition to other electric drives presently under develop-ment, Siemens is using this opportu-nity to create high-end solutions as well as systems for high-volume markets.

❼ Regenerative braking torque as a function of the motor speed

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