TRANSMISSION FOR ELECTRICALLY DRIVEN TESTING VEHICLE

The goal of the project InDrive was to develop a driving simulator, which is able to simulate, test and experience innovative vehicle and drive concepts. The InDrive simulator gives a realistic impression of the dynamic and the driving experience of a future car model at an early development stage. The major development effort within the project was the design of a new differential at the TU Berlin.

AUTHORS

DIPL.-ING. KARSTEN KÜHLKAMP was Research Assistant

until September 2011 at the Department of

Machinery System Design at the Technical University

of Berlin (Germany).

DIPL.-ING. LARS KLIMENTEW

is Research Assistant at the Department of

Machinery System Design at the Technical University

of Berlin (Germany).

PROF. DR.-ING. HENNING J. MEYER

is Head of the Department of Machinery System

Design at the Technical University of Berlin

(Germany).

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1 MOTIVATION

The InDrive simulator is a tool applicable in early phases of the development process. Future drive concepts can be evaluated in the realistic environment of regular traffic. Results from offline vehicle simulations, pre-calculating the exact driving characteris-tics, are converted through a real-time computer via drive-by-wire into a qualitatively experienceable drive feeling. This way future drive concepts can be tested in traffic at an early stage of the development process as opposed to prototypes, which can only be tested under realistic circumstances at late stages of the develop-ment process. At late stages possibly necessary changes in the design result in high costs. Parallel comparisons of different ver-sions of drive concepts require the replacement of diverse compo-nents or the assembly of further prototypes. The InDrive simulator therefore is a tool shortening the development time as well as reducing the development costs.

Four partners, each with their own particular development focus, are involved in the project: : IAV GmbH: project management, development of simulation mod-

els, development of the energy storage systems, development and construction of the electric vehicle, safety

: Braunschweig University of Technology, IMAB: electric drive (motor and converter), drive test benches

: Braunschweig University of Technology, IfR: development of the electric-motor control system, development of the simulation sys-tem (hardware and software), interconnecting and controlling the electric vehicle’s drive

: Technische Universität (TU) Berlin, FG KM: development and assembly of the transmission, drive test benches.

In the course of the project two InDrive simulators were developed and assembled – one based on a vehicle powered by a combustion engine and the other powered by electric motors. The basis of the electric InDrive simulator is a Volkswagen T5. The joint research project had a duration of two years (October 2009 till September 2011) and was funded by the Federal Ministry of Economics and Technology and assisted by TÜV Rheinland Consulting GmbH [1].

2 PERFORMANCE AND REQUIREMENTS

The test vehicle is only able to simulate drive concepts whose power to inertia ratio is less than its own. In order to represent a wide range

of contemporary and future vehicles the InDrive simulator possesses a maximum drive power of 450 kW. Three asynchronous machines propel the simulator in short-time duty S2 with 150 kW each. The motors are electronically controlled. They generate a maximum torque of 370 Nm up to an engine speed of 3500 rpm. The maxi-mum rotational speed is 10,000 rpm; the continuous duty power is 61 kW. To generate the necessary driving torque for the required acceleration of the vehicle a transmission is needed with a trans-mission ratio larger than 9:1. The result is a theoretical driving torque of 3350 Nm per motor. Reduced by the transmission effi-ciency the vehicle is powered by a driving torque of 3100 Nm. Due to the high rotational speeds of the asynchronous machines the simulator is able to reach a top speed of 150 km/h.

Not only technical requirements and wishes were considered dur-ing the development process. The focus of design was a maximum of flexibility and modularity, allowing the transmission to be easily adapted to future redesigns and new drive concepts. Examples therefore are the realisation of a single wheel drive while maintain-ing the transmission ratio. The differential transmission can be removed with minor effort and the driving power of one motor can be transmitted towards a single wheel. Another wish was the pos-sibility of adding a second transmission and performing changes of transmission. Therefore the transmission was designed with this purpose in mind and if a second transmission should be installed many original parts can be re-used.

These requirements entail an all wheel drive for the simulator with one drive train composed of one or two motors and one transmis-sion per axle. Both transmissions are designed as axle drives and feature a differential transmission whose cage adds the drive power of both electric motors, 1.

3 CONCEPTION AND TRANSMISSION STRUCTURE

Since the dimensions of the chosen motor do not allow an axially parallel placing a bevel wheel stage was technically necessary. As a result from methodological design layout and sizing calculations of different transmission configurations a design with three trans-mission stages fulfilling every requirement was chosen. Solely the placing of the bevel wheel stage directly after the electric motors allows an overall design fulfilling all the requirements. At first the driving torque of the electric motors is transmitted through the bevel stage. Afterwards the driving power is distributed through the dif-ferential to both sides of the axle. Directly before the wheels the

1 MOTIVATION

2 PERFORMANCE AND REQUIREMENTS

3 CONCEPTION AND TRANSMISSION STRUCTURE

4 TECHNICAL CHALLENGES

5 TEST BENCH AND TESTING

6 ADVANCED TRANSMISSION DESIGN

7 CONCLUSION

1 InDrive simulator mounting position of the electric drive axles

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driving torque is transmitted a second time through one planetary transmission per wheel, 2.

The bevel stage is composed of a bevel pinion and a bevel wheel with a gear ratio of 2.1. The pinion features internal teeth congru-ent with the splined shaft of the electric motors transmitting the torque towards the bevel wheel stage, 3. The big bevel transmits the torque through a screw joint towards the differential cage, which summarises the driving power of both electric machines. The complete backsides of the bevel wheels are steadied against the differential cage avoiding an unallowable warpage of the bevel wheel. The differential consists of four bevel wheels; compensat-ing and driving bevel have a different number of teeth, 4. The bevel wheels of the differential have to transmit a very high torque as rigid coupling and are thus equally dimensioned as usually in commercial vehicles. The differential cage has to support the driv-ing torque of both engines and has to additionally withstand the added load caused by the centrifugal force. It was crafted in the

department workshop and meets the high requirements concern-ing precision and rotational symmetry.

The driving bevel are positively locked with shafts transmitting the driving power to the sun wheel of the subsequent helical cut planetary transmissions. The sun wheel is likewise positively locked with the driving power transmitting shaft out of the differential trans-mission, 5. Inside the planetary transmission the driving power is partitioned in three equal parts that are transmitted via the planetary wheels. The planetary wheels roll off the ring transmission wheel, which is screwed to the housing and transmit the driving power to the planet transmission carrier, which is the joint towards the drive shaft of the vehicle. It features internal teeth corresponding to the splined drive shaft and represents the output shaft of the transmission.

The transmission prototypes were housed by processed precision flat aluminium assembled exclusively with screw joints that allowed monitoring of the transmission at all times. Along with the housing a framework, positioning the electric machines towards the trans-mission box as well as the whole drive train towards the vehicle was developed. The framework is mounted to the chassis at three posi-tions – frontal in the middle of the transmission box and on both sides of the framework next to the electric motors – absorbing the resulting forces and the moment loading. Pre-stressed elastomer supports absorb most of the oscillations resulting from the drive train, preventing the chassis to resonate.

4 TECHNICAL CHALLENGES

As the simulator was designed as a developing tool there is no detailed knowledge of possibly resulting stresses and strains or future operating cycles. A high operating reliability was agreed on for the complete drive train to endure all operating states and pos-sible dysfunctions safely and therefore offer a reliable development platform. To avoid damage resulting from the high rotational speed and the resulting circumferential velocity spiral bevel wheels with high surface qualities were chosen. In addition the bevel wheels were grinded in pairs to ensure a silent run and a longer operating life. The circumferential velocity and high drive power of the electric motors necessitate the systematic insertion of lubricating oil into the tooth engagement of the bevel wheels to enable a controlled heat removal and to assure a sufficient oil film between the engag-ing teeth. Hence an active lubrication circuit was designed. In addi-tion to the bevel wheel stage all of the bearings are supplied with lubricating oil through customised injectors. Afterwards the oil is drawn in by externally controlled electric pumps.

Because the resulting axial and radial forces of the tooth engage-ment of the bevel wheel stage are too high to be absorbed by the internal bearings of the electric motors, another way of deflecting had to be designed without injuring the existing available space. Therefore a hollow shaft slid over the drive shaft of the electric motor was used which deflects the forces by the use of additional bearings into the housing. The driving torque of the electric motors is trans-ferred directly to the bevel pinion wheel via the splined shaft of the motors. The resulting design transmits the driving torque from the motors to the bevel wheel stage but deflects the resulting forces and protects the bearings inside the electric motors. To achieve a small diameter of the planetary transmission despite of the high torque, the planetary axles are each supported by two needle roller bear-ings, which direct the radial forces into both sides of the planet transmission carrier.

2 Transmission layout

3 Pinion wheel of the bevel wheel stage

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5 TEST BENCH AND TESTING

The manufactured parts of the transmission were assembled at TU Berlin and tested at the InDrive transmission test bench. In coop-eration with all partners a complete drive train including power electronics, sensor system and controller was assembled. The design of the test bench was based on an existent flywheel test bench at TU Berlin, 6 (above). It is composed of a controlled direct current machine and interchangeable flywheels of different abso-lute values of rotary inertia mounted on the shaft of the test bench. The transmission and the electric motors were fastened in a frame matching the chassis of the InDrive simulator. Adaptors with spline shafts on both sides were a rigid coupling between drive train and test bench and transmitted the driving power. In order to test the performance and driving power flow of all transmission stages – including the differential and the mechanical strength of its cage – the mechanical output had to be realised at both transmission output shafts. The output driving power was transmitted via two chain drive systems and summarised at the test bench shaft. The direct current machine was utilised to simulate the driving resist-ance of the vehicle at constant velocities and furthermore for the towing of the transmission during the initial start-up, ⑥ (bottom left). The driving power and the maximum torque of the load

machine are not sufficient to test the maximum driving power of both electric motors and their transformated driving torque of 2 × 3100 Nm. That is why the rotary inertia of the flywheel was required as an additional load, ⑥ (bottom right).

After the successful initiation of the test bench and the transmis-sion the following parameters were tested to verify the functioning of the transmission: : maximum driving speed : maximum driving torque with one and two motors : temperature curve of the lubricating oil.At first only tests with slow driving speed and low torque were per-formed to monitor the teeth engagement and ensure its evenness. Subsequently the driving speed was increased gradually till the maxi-mum rotational speed of 10,000 rpm. After the transmission was operated successfully at maximum speed, the driving torque was increased gradually as well.

The driving torque of one electric motor at a time was gradually increased up to its maximum. Afterwards both electric motors were operated synchronously and the combined driving torque was gradu-ally increased until the maximum driving torque of the drive train reached its maximum. A run-up of the electric motors under full load was only possible because the rotary inertia of the fly wheel was used as an extra load. The durability of the transmission was the proved result of those tests.

Trials lasting up to three hours with different test cycles were exe-cuted to analyse the lubricating oil temperature changes. One test cycle was composed of the repeated and uninterrupted acceleration and deceleration of the fly wheel via the electric motors (recupera-tion); a second test cycle was composed of different but constant levels of driving torque and speed. That way maximum and average energy inputs were simulated. The evaluation of the test results dis-played the stabilisation of the lubricating oil temperature on a level below the critical temperature of the oil. After the successful test-ing of the two prototypes for front and rear axle transmission the two configurations of the drive train were passed on to the project partners to be installed in the vehicle.

4 Bevel wheel differential

6 Test bench and load configurations

5 Planetary transmission stage

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6 ADVANCED TRANSMISSION DESIGN

After the handover of the prototypes for the tests with the experi-mental vehicle an optimised version of the transmission was designed and produced. This current version was re-designed with focus on weight reduction and housing optimisation of the trans-mission. The transmission stages and the bearings were not modi-fied. To reduce the transmission weight a valuation method was applied comparing all of the components of the transmission in consideration of their potential for weight reduction. Additional criteria were the necessary time, cost and complexity to put the changes into effect.

The prototype housing with its highly simplified design and its extra monitoring possibilities of the transmission stages in the first phases of testing was replaced with a cast aluminium housing with horizontal partition, 7. The connections with the likewise weight optimised framework are positioned directly at the housing provid-ing the possibility of an uncomplicated adaption of the transmission to other vehicles. Besides the housing the differential was identified as a component with great potential for weight reduction. Hence the bevel wheel differential was substituted by a spur differential. Its weight is reduced by 43 % in comparison to the bevel transmission differential with a slightly reduced reliability, 8. Identical connect-ing dimensions and the modular design allow a quick and simple changing of the differentials without having to alter or replace other components of the transmission. The mass of the planet transmis-sion carriers was reduced by 45 % through an enhanced production technique because it was possible to design their contour more appropriate to the arising stresses.

7 CONCLUSION

Within a project duration of two years a transmission for the electric simulator vehicle of the project InDrive – funded by the German Fed-eral Ministry of Economics and Technology – was designed, assem-bled and tested at the Department of Machinery System Design of the TU Berlin. The transmission transmits a maximum driving torque of 2 × 370 Nm up to an engine output power of 2 × 150 kW and transformates it with a transmission ratio of 9.1:1. The maximum driving input speed is 10,000 rpm. The transmission can be powered

by one motor as well as two motors and is used to power the front and rear axle of the InDrive simulator. Its design is modular and flex-ible regarding future utilisations as a transmission for a single wheel drive, torque vectoring, shift ability, use of different electric motors and application in different vehicles. The lubrication of the drive train is ensured by an active lubrication circuit. The transmission was tested successfully to its maximum performance characteris-tics. A subsequent revision reduced the weight by 22 % while reduc-ing the noise vibration harshness and simplifying the assembling process. The assembled and tested transmissions are currently pow-ering the electric InDrive simulator.

REFERENCES[1] IAV customer magazine “automotion”. July 2010, p. 7[2] Cornelsen, K.; Form, T.; Jänsch, D.; Nietschke, W.; Wolter, T.-M.: Neue Methodik zur realitätsnahen Auslegung hybrider Antriebskonzepte. 29th International Vienna Motor Symposium 2008[3] www.indrive-simulator.com[4] Naunheimer, H.; Bertsche, B.; Lechner, G.: Fahrzeuggetriebe. Berlin, Heidelberg: Springer-Verlag, 2007[5] Kirchner, E.: Leistungsübertragung in Fahrzeuggetrieben. Berlin, Heidelberg: Springer-Verlag, 2007

8 Assembled spur differential and section of the spur differential

7 Current state of development of the transmission

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