Ev11 Design Presentation

Ev11 Design presentation

AERODYNAMICS Main challenges

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he aerodynamics team is responsible for the aerodynamic performance of the vehicle. Aerodynamics affect the handling characteristics of the car and its ability 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.

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.

Workflow Research This 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 competition rules to identify where you can gain the most performance, and what you can afford to sacrifice.

Development Most 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 optimised and the wings are dimensioned and positioned to maximise downforce and achieve the desired weight balance.

Manufacturing The 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.

AERODYNAMICS This year’s design

Aerodynamics package 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 thoroughly from the bottom. The initial design is based entirely on external research and everything in the design has been simulated thoroughly.

Rear wing

Undertray

Front wing

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

CHASSIS & ERGONOMICS Main challenges

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uspension, 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 minimum 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 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 packaging satisfying? Does it fit with the suspension design?

Focus groups The different groups are gathered every week with the chassis designer to complain with their packaging requirement.

Any of their change to last year’s design has to be told immediately to the chassis group in order to arrange a working packaging.

CAD Design Autodesk 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.

Manufacturing Finally comes the time to manufacture our parts. 99% of the chassis parts are made in-house. The group has to use a variety of materials (steel, aluminum, carbon/glass-fiber and even wood for the molds) and various manufacturing technologies (from water-cutting to welding, CNC, or carbon cooking under vacuum).

CHASSIS & ERGONOMICS This year’s design

Focus on the packaging The direction we choose this year was to focus on the packaging for the frame, allowing us to add a bit of weight if required. The goal was to simplify the work for the other 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.

Chassis

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 endurance race requires a minimum comfort for amateur drivers to perform well (Formula 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.

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.

Pedal box

DRIVETRAIN Main challenges

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ow 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 Analysis What are the advantages and disadvantages of the drivetrain types used in FSAE? Cost, weight, drivetrain efficiency, tolerances, 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.

Optimization The design created is analyzed using FEM software (Altair Hyperworks 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.

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

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

DRIVETRAIN This year’s design

Engine system Specifications

A fast glance at the system reviles the difference of the system from last year. The use “offthe-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.

Spring and Dampers

Gear ratio ≈2 (16 teeth in front, 33 teeth in rear) Chain 520 Motorcycle chain Bearings Needle bearing in front, Double row angular contact bearing in rear Material(for most parts) ALUMEC89 Total weight ≈ 33kg (motors weigh 24.4kg)

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 together in the swirl pot from both inverters and motors, giving the system a uniform cooling. To ensure the safety of the driver in case of a chain failure, an scatter shield made out of 3mm steel is attached over the chain.

Radiator

Swirl pot

Motors

Driveshaft

Motor controller

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

DRIVETRAIN This year’s design

Drivetrain package The drivetrain packaged differs a lot from last year. With the purchase of new and different motors, a new package had to be designed. The main goals behind this package where cost, weight, ease of manufacture and assembly. 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.

Test rig Created to withstand the loads produced by testing 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!

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.

SUSPENSION Main challenges

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orsepower, 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 vehicle handling characteristic and hence enhance vehicle’s performance.

Since vehicle suspension can be designed to deliver different characteristic- providing a comfortable ride which is a priority for passenger vehicle to providing the maximum grip from the tires et cetera. For our car the focus is maximum tire grip, maximal lateral acceleration and maximum cornering stability.

Workflow 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 vehicle’s behaviour in real environment. A KTH Formula Student database was created which includes the models of the former 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.

SUSPENSION This year’s design

Front suspension

Re-design of rear suspension

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

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 rotational axis of the bellcrank. This resulted in better packaging, weight distribution 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.

Front suspension

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.

Double wishbone ( Upper Control Arm)

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

Spring and Dampers

Bellcrank Push Rod

Upright Ackermann

Reverse Akermann

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

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

∂A

Rear suspension Ackermann angle L ∂A R

POWERTRAIN & ELECTRONICS Main challenges

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

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 Tuning Once 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.

POWERTRAIN & ELECTRONICS This year’s design

Model-based design

Specifications

Model-based design is a process that enables faster, more cost-effective development of automotive systems, and the powertrain & electronics 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.

Battery box

Single battery cell

Electromagnetic compliance (EMC)

Another key approach to this year’s design is to take into account of electromagnetic compliance (EMC) of all the powertrain and electronic systems. Poor cable management and placement 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).

POWERTRAIN & ELECTRONICS Specifications

Electrified Powertrain EMRAX 228 from Enstroj Continuous motor power Continuous motor current Continuous motor torque Maximum rotation speed Nominal motor efficiency

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

BAMOCAR D3 from UNITEK Maximum DC-­link voltage Continuous current Peak current Switching frequency Weight

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

LP9759156 from Melasta Chemistry LiPo Nominal voltage Typical capacity Con. dis. current Max. dis. current Con. ch. current Max. ch. current Cell config.

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

Battery Management System In-­house designed and manufactured Compact design for constrained space Every parallel cell voltage measurement 30% cell temperature monitoring State-­of-­Charge estimation Decentralized architecture SPI and CAN

Electronic Control Unit (ECU) Driver Controls Unit Torque encoder (drive-­by-­wire) Brake pressure sensor (regenerative braking) Analogue signal conditioning; implausibility check CAN message packaging No Contact Hall Effect Rotary Position Sensor Dual redundant output Operation in extreme conditions Brake Pressure Sensor Resistant to pressure peaks Shockproof and vibration-­proof Vehicle Monitoring Unit (Front) Front wheel speed sensors Steering wheel angle sensor TBA: Suspension dampers, tyre temperature Vehicle Monitoring Unit (Rear) Rear wheel speed sensors Gyroscope, accelerometer TBA: Suspension dampers, tyre temperature Torque Vectoring Unit dSPACE MicroAutoBox II Model-­based design Rapid prototyping Two CAN bus

Safety Interlock System Safety Interlock Circuit Main junction point Control of contactors BMS, IMD, BSPC