Ev11 Design presentation

AERODYNAMICSThe aerodynamics team is responsible for the aerodynamic performance of the vehicle. Aerodynamics affect the handling characteristics of the car and its abili-

ty to accelerate. The main concern of the aerodynamics group is to produce load on the tyres without the corresponding addition of mass. This gives more grip which allows for better acceleration and faster corners. The downside is that the addition o f wings to the car induces more drag, which has to be carefully taken into account.

Workflow

Main challenges

ResearchThis is were our pre-study of vehicle aerodynamics and computational fluid dynamics is done. Books and technical papers are read though to get a good understanding of how one can utilize aerodynamic devices to improve lap times and vehicle handling. It is also important to consider the com-petition rules to identify where you can gain the most performance, and what you can afford to sacrifice.

DevelopmentMost of the time spent when designing the aerodynamics package goes into the development phase. This phase relates closely to the research phase as much of what is learned during research is applied here directly. A multi-element airfoil is developed, the undertray and diffuser is opti-mised and the wings are dimensioned and positioned to maximise downforce and achieve the desired weight balance.

ManufacturingThe manufacturing stage is the final sprint of the project year and where the aerodynamics package is taken from the drawing board into the real world. We make moulds of MDF at the school of architecture at KTH and then use these for carbon fiber layup. Carbon fiber is used for its high strength to weight ratio, which is important when building for speed. We want constructions that can widthstand the physical tests of a race track, yet still being as lightweight as possible.

Our work is strongly research driven and a big part of the design phase is dedicated to that. We need to identify which tests and procedures should be carried out and for that we turn to literature and other sources of information. Based on the results of our research and the goals we wish to accomplish in the competition we use everything from hand calculations to computational fluid dynamics for development. The workflow is highly iterative and certain simulations take a considerable amount of time.

This year’s design

Aerodynamics package

Front wing

AERODYNAMICS

The aerodynamics package this year is an entirely new design. We felt that we needed to build a design base for the team and the best way to gain knowledge is by doing everything thor-oughly from the bottom. The initial design is based entirely on external research and every-thing in the design has been simulated thoroughly.

“The main concern of the aerodynamics group is to produce load on the tyres without the corresponding addition of mass”

Undertray

Rear wing

CHASSIS & ERGONOMICSSuspension, motors, Driver’s cell, electronics… There needs to be someone to carry them all. That is the first role of the chassis. Thus, it is mostly about packaging issues.

Then, what happens in case the driver loses the control and hits a wall? What if he gets hit from the back by another car? There we touch the second requirement the chassis has to fulfill: protecting the driver. This security issue is driven by lots of rules imposed by the FSAE – Formula Society of Automotive Engineers – which impose mini-mum requirements for the structure to be able to survive.

Moreover, the suspension is design to give maximum drive-ability with a certain stiffness for the chassis. But who believes that 4 meters of steel don’t bend under loads? Here comes the third issue the chassis deals with: rigidity requirements. The suspension group describes the loads that will be applied to the chassis, at which points, and how much deformation is acceptable. Then, the design must cope with these limits. Finally, as our car is not autonomously driven yet, the chassis design includes creating a nice area for the driver. Designing the chassis thus includes designing the seat, the pedal box, and more generally the entire driver’s cell to allow him/her to give his best. And as different drivers share the same car in FSAE, the cell must be adaptable.

Workflow

Main challenges

Analysis It is the basis of everything: starts from what you have if it works. A simple analysis performed on last year’s chassis and its drivers allowed a good collect of information about all the requirements: was it comfortable for the driver? Was the pack-aging satisfying? Does it fit with the suspension design?

CAD DesignAutodesk Inventor comes in handy for us to start elaborating with the new design. It gives a good creation tool, as well as fast FEM results which show if the design is going in the right direction. This year, we also performed physical testing on the actual frame from last year, in order to measure the error our FEM calculations create and take it into account.

Focus groupsThe different groups are gathered every week with the chas-sis designer to complain with their packaging requirement. Any of their change to last year’s design has to be told im-mediately to the chassis group in order to arrange a working packaging.

ManufacturingFinally comes the time to manufacture our parts. 99% of the chassis parts are made in-house. The group has to use a va-riety of materials (steel, aluminum, carbon/glass-fiber and even wood for the molds) and various manufacturing tech-nologies (from water-cutting to welding, CNC, or carbon cook-ing under vacuum).

This year’s design

Focus on the packaging

CHASSIS & ERGONOMICS

The direction we choose this year was to focus on the packaging for the frame, allow-ing us to add a bit of weight if required. The goal was to simplify the work for the oth-er groups: easy cooling for the powertrain, soft body-shape for the aerodynamic and lower Center of Gravity to help the suspension group. Some small improvements have been made as well to lower the costs and save weight where it was possible, but that was not the priority of this year.

Seat

Finally, the pedal assembly was widely modified to increase the comfort and the dapability first, and to reduce costs on the other hand. We chose to have 2 sliding assemblies – one for the foot rest, and one for the pedals – which can be set separately to fit with the size & preferences of each driver.There is not much weight to save on such an assembly, but last year’s design was quite expensive and FSAE widely judges the quality of a design on its cost. We thus left carbon-fiber to come back to a more classical, simpler and cheaper aluminum solution.

The ergonomic goal was to deliver more elbow room to the drivers. The R10 was too narrow for them to have a comfortable driving position, yet the length of the en-durance race requires a minimum comfort for amateur drivers to perform well (For-mula one drivers cope with terrible driving positions, but they train for this everyday). Besides, the weight and cost bars stood the same as last year for the ergonomics.

Pedal box

Chassis

DRIVETRAINDRIVETRAIN

How do you get the power from the electrical motors to the rear wheels and how do you keep the motors at the right temperature?

These are the main questions answered by the Drivetrain team. The team is an extension of the Powertrain & Electronics team and focus on the mechanical part the powertrain. The four members of the team tackle the problems with test rig construction, cooling of motors and controllers as well as power transfer.

Workflow

Main challenges

AnalysisWhat are the advantages and disadvantages of the drivetrain types used in FSAE? Cost, weight, drivetrain efficiency, toler-ances, heat dissipation, ease of manufacturing and ease of assembly are some of the major areas that are closely looked at to find a drivetrain that works this year.

Problem solving through visual designThe use of Autodesk Inventor gives us the advantage of visual problem solving. Parameters such as drivetrain type and sys-tem strength are taken in account and a detailed CAD model is produced.

ManufacturingOne of the last stages towards the final goal; Most parts are manufactured in-house, meaning that they are done in man-ual lathes and mills. Specific parts are machined in 3/5 axis CNC machines and water cut.

OptimizationThe design created is analyzed using FEM software (Altair Hy-perworks and Autodesk Inventor) to ensure that the system will hold during load. The system is also integrated together with the rest of the car so that it fits and works well.

This year’s design

Spring and Dampers

Engine system

Motor controller

DRIVETRAIN

A fast glance at the system reviles the difference of the system from last year. The use “off-the-shelf” motor controllers and motors makes the system look bigger compared to last year, although it weighs less. The drivetrain weighs 17kgs less than last year, which is a substantial weight reduction.

With the motor controllers being placed on the side of the car, a longer and more complex cooling system had to be made. The cooling system is a parallel system with two radiators which are cooling one inverter (motor controller) and motor each. The water is mixed togeth-er in the swirl pot from both inverters and motors, giving the system a uniform cooling. To en-sure the safety of the driver in case of a chain failure, an scatter shield made out of 3mm steel is attached over the chain.

Gear ratio≈2 (16 teeth in front, 33 teeth in rear)Chain520 Motorcycle chainBearingsNeedle bearing in front,Double row angular contact bearing in rearMaterial(for most parts)ALUMEC89Total weight≈ 33kg (motors weigh 24.4kg)

“Overall the drivetrain system of this year is bigger but lighter and easier to work on”

Radiator Swirl pot

Driveshaft

Motors

Specifications

DRIVETRAIN

How do you get the power from the electrical motors to the rear wheels and how do you keep the motors at the right temperature?

These are the main questions answered by the Drivetrain team. The team is an extension of the Powertrain & Electronics team and focus on the mechanical part the powertrain. The four members of the team tackle the problems with test rig construction, cooling of motors and controllers as well as power transfer.

Workflow

Main challenges

AnalysisWhat are the advantages and disadvantages of the drivetrain types used in FSAE? Cost, weight, drivetrain efficiency, toler-ances, heat dissipation, ease of manufacturing and ease of assembly are some of the major areas that are closely looked at to find a drivetrain that works this year.

Problem solving through visual designThe use of Autodesk Inventor gives us the advantage of visual problem solving. Parameters such as drivetrain type and sys-tem strength are taken in account and a detailed CAD model is produced.

ManufacturingOne of the last stages towards the final goal; Most parts are manufactured in-house, meaning that they are done in man-ual lathes and mills. Specific parts are machined in 3/5 axis CNC machines and water cut.

OptimizationThe design created is analyzed using FEM software (Altair Hy-perworks and Autodesk Inventor) to ensure that the system will hold during load. The system is also integrated together with the rest of the car so that it fits and works well.

This year’s design

Drivetrain packageThe drivetrain packaged differs a lot from last year. With the purchase of new and different motors, a new pack-age had to be designed. The main goals behind this package where cost, weight, ease of manufacture and as-sembly. The new drivetrain type unveiled new problems that had to be solved. A chain has to be tensioned for it to deliver good performance. This creates a problem because the bearings inside the motors should not take any radial load. The problem was solved by adding a supporting structure which takes the load instead of the bearings.

Created to withstand the loads produced by test-ing the motors on a bench. The construction is made built up by water-cut aluminum and steel plates.The main goal was for it to be robust due to the fact that it has to withstand the load created when two motors are tested against each other (one acts as a generator), especially when one motor produces 240Nm of torque!

Test rig

The use of two motors gives us a so-called “electrical differential”. Therefore individual tensioning for each chain was required. The use of a rail system on the supporting axle solved the challenge of individual tensioning. The gear ratio chosen this year was around 2 which gives us a maximum of 480 Nm of torque on each wheel.

DRIVETRAIN

SUSPENSION

Horsepower, nought-to-sixty acceleration, torque- sounds familiar? But what would all that power be if the driver couldn’t utilise it.

Hence, the focus now turns to suspension system, to optimize the vehi-cle handling characteristic and hence enhance vehicle’s performance. Since vehicle suspension can be designed to deliver different character-istic- providing a comfortable ride which is a priority for passenger vehi-cle to providing the maximum grip from the tires et cetera. For our car the focus is maximum tire grip, maximal lateral acceleration and maxi-mum cornering stability.

Workflow

Main challenges

Simple physically testing of the R10E was done in the KTH Formula Student Garage and feedback from engineers and drivers for the previous car was used in the analysis

ADAMS MSC SOFTWARE enables the group to model, validate and analyse the different effects of parameters and vehicle design via simulation results and hence understand the vehi-cle’s behaviour in real environment. A KTH Formula Student database was created which includes the models of the for-mer car, R10E and the prospective car, eV11.

The manufacturing & testing -phase is when the group’s ideas come to life. Some parts are purchased but most are actually made by the team. When the car is completed the group finalizes their work by making smaller adjustments to the suspension after input from the driver and data collected during the testing.

SUSPENSIONThis year’s design

R

L

∂A

∂A

Ackermann angle

Ackermann• Better high speed• High load transfer turns • High steering input

Reverse Akermann• Better for low speed • Low load transfer turns • Low steering input

Re-design of rear suspension

Bellcrank

Spring and DampersDouble wishbone ( Upper Control Arm)

Push Rod

Rear suspension

Rear suspension was re-designed to fit the eV11’s packaging in order to accommodate new motors, inverters, sensors and battery pack. The most noticeable change in rear suspension is the location and rotation-al axis of the bellcrank. This resulted in better packaging, weight distri-bution and aerodynamic flow. Further Full Vehicle Simulations were carried out to optimize the overall geometry. This will further be tuned during the real life testing phase for optimal performance.

Upright

Front Suspension Analysis and physical test-ing was done to analyse and evaluate the behaviour/dynamics of R10E’s geometry in order to optimize it.

The term racing is linked with high speed, thus a Reverse-Ackermann (RA) is more standard setting as it is better for high speed and high low transfer turns.

However, for a Formula Student competi-tion, the endurance test track has tight cor-ners and requires higher steering input. A neutral steer to Ackermann (A) setup was used for steering placement for low speed, low load transfer turns.

Front suspension

Front suspension

POWERTRAIN & ELECTRONICS

Electricity is what the new world brings horsepower to the modern and future cars. Electric vehicles have already gone beyond and above the performance of petroleum cars and the talk of V12 engines has long been old-fashioned. The operating nature of electric motors is completely different from that of internal combustion engines

(ICEs). Because of it, the control of the electric motors has different characteristics and parameters. What we do is to ensure the electric motors are properly controlled and their performance is maximized while maintaining the stability of the vehicle.

Workflow

Main challenges

Signals and SystemsWhat do we want our vehicle to do? How do we achieve our goals? Our team first starts by defining technical specifications for the vehi-cle that we are developing. We discuss what signals should be communicated between systems and how to go about controlling the sig-nals.

Electronic Control Unit (ECU) development and testing The signals are generated from multiple ECUs in the vehicle. The units communicate with each other via controller area network (CAN). Some ECUs are in-house designed and developed from schematics to printed-circuit board (PCB) layout.Some ECUs are off-the-shelf units such as dSPACE MicroAutoBox which integrate model-based design. Using both dSPACE ControlDesk and Vector CANoe, we can fully simulate the whole powertrain system and are also able to test individual ECUs and their integration.

Parameter Fine TuningOnce the integration of all the systems on the vehicle is done, it is now time to fine tune some vehicle parameters. This is the phase when we take the vehicle out for track testing and acquire lots of data to see how the vehicle behaves in real life. Simulated results are never accurate and final tuning of vehicle parameters is key in the development of an automobile.

This year’s design

Model-based design

POWERTRAIN & ELECTRONICSModel-based design is a process that enables faster, more cost-effective development of automotive systems, and the powertrain & electron-ics group has newly employed this approach in the development of the next generation KTH Formula Student electric racing car. This not only saves the development time but also reduces implementation errors, which means more time for testing and fine tuning of the vehicle.

Another key approach to this year’s design is to take into account of electromagnetic compli-ance (EMC) of all the powertrain and electronic systems. Poor cable management and place-ment of ECUs have been one of the bottlenecks in the previous project cycle. Thus, all the high voltage (HV) systems have been carefully placed with shortest possible HV cable travels. Low voltage (LV) PCBs have also been more carefully designed to reduce any electromagnetic interference/vulnerability (EMI/V).

Battery box

Specifications

Single battery cell

Electromagnetic compliance (EMC)

Specifications

Electrified Powertrain

POWERTRAIN & ELECTRONICS

EMRAX 228 from EnstrojContinuous motor powerContinuous motor currentContinuous motor torqueMaximum rotation speedNominal motor efficiency

BAMOCAR D3 from UNITEKMaximum DC- link voltageContinuous currentPeak currentSwitching frequencyWeight

LP9759156 from MelastaChemistry LiPoNominal voltageTypical capacityCon. dis. currentMax. dis. currentCon. ch. currentMax. ch. currentCell config.

Battery Management SystemIn- house designed and manufacturedCompact design for constrained spaceEvery parallel cell voltage measurement30% cell temperature monitoringState- of- Charge estimationDecentralized architecture SPI and CAN

Electronic Control Unit (ECU)

Driver Controls UnitTorque encoder (drive- by- wire)Brake pressure sensor (regenerative braking)Analogue signal conditioning; implausibility checkCAN message packaging

No Contact Hall Effect Rotary Position SensorDual redundant outputOperation in extreme conditions

Brake Pressure Sensor Resistant to pressure peaks Shockproof and vibration- proof

Vehicle Monitoring Unit (Front)Front wheel speed sensorsSteering wheel angle sensorTBA: Suspension dampers, tyre temperature

Vehicle Monitoring Unit (Rear)Rear wheel speed sensorsGyroscope, accelerometerTBA: Suspension dampers, tyre temperature

Torque Vectoring UnitdSPACE MicroAutoBox II Model- based design Rapid prototypingTwo CAN bus

50 [kW]240 [Arms]125 [Nm]5000 [RPM]95 [%]

400 [V]200 [ARMS]400 [ARMS]8-16 [kHz]6.8 [kg]

3.7 [V]8,0 [Ah]35 [C]40 [C]2 [C]4 [C]96s2p

Safety Interlock System

Safety Interlock CircuitMain junction point Control of contactors BMS, IMD, BSPC