Build of the IIT Bombay Racing’s Electric Racecar - EVoLV

Introduction

IIT Bombay Racing has been a contender in the Formula Student competition since 2008. Originally focused on combustion vehicles, we made a pivot to electric technology in 2012. This move was fueled by electric motors' high-torque capabilities at low RPMs, the potential for advanced controls, energy regeneration benefits, and a vision for a greener future. Our ultimate aim since then is to revolutionize electric mobility in India through sustainable technologies. Participating in Formula Student allows us not only to stay ahead of the curve but also to prepare future engineers for impactful careers.

Design Objectives

The objectives for FSUK 2023 that drove our engineering efforts are as follows. Foremost was the overhaul of the electrical systems, aimed at achieving reasonable reliability. We tackled issues like low integration and frequent repairs by rigorously testing circuitry and utilizing Computer-aided electrical system layouts. This approach was especially crucial given our new accumulator design featuring cylindrical cells.

Aside from the electrical aspects, weight reduction was a pivotal target. We managed to shave off 6 kg from the previous design by adopting a Carbon-Fiber Monocoque chassis and a new, lighter accumulator package. Improved vehicle handling also made the list, as we capitalized on the monocoque's high torsional rigidity and optimized mechanical assemblies for tunability.

Subsystem Wise Design Summaries

Let’s first look at how our Electrical Systems Roll, followed by other parts of our Formula Student Vehicle.

The Vehicle Dynamics Model

Our Vehicle Dynamics Model (VD) is the bedrock of performance tuning. Utilizing a transient full-car VD model, we ran acceleration simulations to cherry-pick motors, nail down a 4.55:1 gear ratio, and calibrate braking torques for both front and rear. The goal was clear: neutralize steer geometry and crank up cornering velocity based on prior performance data and driver feedback.

We leaned on steady-state bicycle model simulations to sketch out Yaw Moment Diagrams (YMDs), guiding us to an optimal 52% rear axle weight bias. The precision didn't stop there. Further simulations helped us distribute lateral load transfer and aerodynamic force efficiently across axles, maxing out lateral accelerations to 1.75g and achieving a desirable neutral steer geometry.

The Pacejka 2002 tire model was our choice to portray tire behaviour accurately, factoring in steering-induced load transfer for a more authentic two-track model. Through meticulous parameter sensitivity analysis, we tuned the balance, control, and stability of our vehicle, making it not just a machine, but a finely calibrated extension of our engineering prowess.

Controller Area Network (CAN) Systems - The Nervous System of the Car

In the web of our car's electrical systems, the Controller Area Network (CAN) serves as the essential communication highway. Designed for resilience against harsh conditions and electromagnetic noise, our CAN setup consists of two separate buses: Data CAN and Control CAN.

The Data CAN, operating at 1 Mbps, serves as the conduit for an array of crucial real-time sensor data—ranging from cell temperature to yaw and rpm. This data is not just relayed but also displayed on a Nextion dashboard through a self-developed graphical interface. On the other hand, the Control CAN, with its 0.5 Mbps speed, deals with mission-critical signals like APPS, BPS, and STEER, channelling them directly to the vehicle’s ECU.

What sets our CAN system apart is its robustness. High-speed configurations necessitated the use of twisted wire pairs with a termination resistance of 120 Ohms, effectively minimizing data reflection. The heart of this system is the Teensy 4.0 and 4.1 microcontrollers (ARM Cortex M7 based), around which our custom-designed PCBs, or CAN nodes, function.

Data acquisition is threefold: wirelessly via LoRa modules, through a Vector Data Logger (GL1000), and potentially through an in-built SD card in TEENSY 4.1. The data we collect feeds into Python GUIs built with Asammdf (Open source API for automotive), helping us derive actionable insights.

LV & HV Systems - The Car’s Lifeblood

Two pivotal systems come to the fore -: the Low Voltage (LV) and High Voltage (HV) systems. Here's how they work in harmony to ensure optimal performance and safety -

• The LV System at its core lies an Emergency Shutdown Circuit that instantaneously isolates the accumulator from the inverters in case of an error. What catches the eye is the use of an Isolated DC-DC Converter for low voltage supply. Why? It's lighter, greener, and offers a constant output voltage. This converter boasts an impressive 92.4% efficiency, delivering 400W at 12V, while an LC filter network handles noise attenuation.
• The HV System is where power gets transferred from the accumulator to the motors. The linchpin? Accumulator Isolation Relays (AIRs) that control the state of power flow. To bridge the gap between high and low-voltage systems, we use Optocouplers to scale down high voltages, maintaining galvanic isolation.
• Harness and Enclosures - Our electrical package harness employs Automotive-Grade Connectors, and all connections are both vibration-proof and waterproof. Similar standards are maintained for the electrical compartments, ensuring they too are waterproof.

Accumulator Management System (AMS)

This is probably one of the coolest features that sets IITB Racing apart from most other FSAE Teams, owing to our in-house-built AMS, as opposed to off the shelf approach taken by most other teams. Ensuring the car's accumulator runs like clockwork is our custom-designed Accumulator Management System (AMS).

• At its heart, the AMS features Seven Cell Monitor Boards (LTC6813) and a Battery Pack Monitor (LTC2949), all orchestrated by a Teensy 4.1 microcontroller.
• The system employs Iso-SPI protocol, facilitated by LTC6820, transforming the SPI communication from Teensy. These boards are connected in a Daisy Chain Configuration, ensuring uninterrupted communication even if a wire breaks.
• The Cell Monitor Boards make lightning-fast measurements at a 7 kHz ADC conversion frequency, with a maximal error margin of just 2.2 mV. They monitor the voltages and temperatures of 96 cell modules, utilizing NTC thermistors (103JT-075) for temperature tracking.
• Energy Management - The Battery Pack Monitor not only measures the complete stack voltage but also gauges the High-Voltage Current, feeding this data to the Master Board. This assists in calculating utilized energy and predicting the State of Charge (SOC).
• Reliability and Flexibility is ensured, as a PCB-based cell-connection technique provides a neat harness and enhanced cell cooling. The AMS performs passive cell balancing during charging and switches to slow mode to reinitiate communication if an error occurs.

From safeguarding energy storage to optimizing performance, our AMS is a silent yet critical player in our electric vehicle's success story.

Electrical Powertrain - The Heart of the Beast

Our E13 boasts a rear-wheel independent drive system, a choice that cuts unsprung mass, simplifies design, and trims costs—all without sacrificing acceleration. We opted for Permanent Magnet Synchronous Motors over their DC counterparts for their higher power density and efficiency. The end result? A substantial 22% weight reduction (9.9 kg)compared to our previous model, E11.

When it came to the gear ratio, we crunched numbers on multiple fronts: lap time, acceleration, energy use, and even weight. We landed on a torque-up gear ratio of 4.55, striking a sweet balance. To further our weight-reduction goal, we integrated the planetary carrier and tripod housing into a single lightweight aluminium part, slashing another 18.5% off the weight and simplifying assembly.

Controller-wise, we employ two BAMOCAR-D3-700-160 Motor Controllers. But the real kicker is in the mounting—the gearbox mount integrates into the motor mount, no longer relying on the chassis. This move not only fulfils the need for a scatter shield but also aligns the gearbox and motor axes, effectively nullifying axial loads.

The Accumulator - Our Power Bank on Wheels

Safety and efficiency drive the design of our Accumulator, the energy reservoir of our electric marvel. We opted for 576 Li-ion cells in a 96S6P configuration, crafting an 8.7 kWh battery pack. Each cell is part of a module enclosed in 3D-printed FR ABS material, grouped into seven stacks. This layout was strategically chosen based on endurance requirements, power limits, and voltage ratings, culminating in an assembly that's as compact as it is robust.

Cooling isn't an afterthought—it's integral. Our cooling system, designed through rigorous simulations, employs 24 axial fans to maintain cell temperatures below 56°C. This ensures that even at a 70A current, our battery pack remains within safe thermal limits. Moreover, a mini wind tunnel test validated our cooling system's efficacy.

As for assembly and maintenance, A busbar PCB eliminates wiring needs for the AMS, speeding up assembly by 36%. The accumulator container is CFRP sandwiched between Kevlar, adhering to UL-94-V0 fire-retardant standards. Retractable wheels facilitate quick and easy removal, making servicing a breeze.

Keeping It Cool, with Motor and Controller Cooling

Optimal performance demands optimal cooling—that's a given. To meet the high flow rate needs of our new motors and controllers, we switched gears from a self-priming diaphragm pump to a centrifugal pump. Lighter by 1.3 kgs and more cost-effective, this pump is designed to keep things cool under the hood.

To minimize pressure losses and improve reliability, we went big - literally. Our cooling system employs larger pipe diameters and avoids reducers by using different inlet and outlet sizes. We also upgraded to aluminium pressure relief caps, cutting both weight and cost while enhancing reliability.

The radiator design also got a facelift. It's wider but shorter, reducing drag and fitting into our side pod design. With an average airflow of 5m/s, these radiators dissipate heat at a rate of 1.71 kW. And let's not forget the self-priming setup, cutting down priming time to a fraction.

Setting the Pace with our Electronic Differential

Our Electronic Differential is a game-changer in longitudinal and lateral control. Leveraging RTOS and interrupt-based programming, the system optimizes slip ratios for peak acceleration. But it's not just about speed; it's about finesse. Using data from various sensors and CAN, the differential employs torque vectoring algorithms to correct understeer or oversteer, optimizing yaw rate during cornering.

Steering, Brakes, and Aerodynamics - Our Symphony of Control

The synergy between steering, braking, and aerodynamics plays a pivotal role in the car's performance. Our steering system, employing a rack and pinion gear system, ensures precise linear motion transmission from the driver's actions down to the wheels. With a steering wheel rotation optimized to 105° on each side and an Ackerman geometry of 92%, the design enhances driver feedback and car manoeuvrability.

On the braking front, a re-designed pedal now withstands a force of 2000N while shedding 25-30% of its weight from the previous design. The brake and throttle pedals, connected to push-type master cylinders, provide a firm pedal feel, enabling full brake lock-up at a pedal force of 500N. The meticulous design extends to brake discs manufactured from SS420, hardened to endure temperatures up to 580°C.

Aerodynamics doesn't take a back seat either. A meticulously designed aerodynamic package, featuring 3-element wings and side diffusers, promises a downforce of 324N at 80km/h, while aiding in tuning understeer and oversteer. Although a DRS system was considered, it was foregone as our VD models showed no significant gains for the added weight and complexities. However, the design flexibility allows for changing the angle of attack to tune driver feedback for different events. The wings, crafted from carbon fibre laid on CNC-milled polystyrene foam, ensure a high strength-to-weight ratio, contributing to both cooling and additional downforce.

The extensive use of composites extends to the bodywork, where CFRP - Aluminium Honeycomb sandwich monocoque prototypes were developed to optimize strength, weight, and cost. Engineering and manufacturing prototyping stages helped finalize the ply configurations and cure processes, leading to an optimized, robust, and aesthetically pleasing body that encapsulates the essence of our engineering endeavour.

Our Future Plans

The journey towards perfection is perpetual. Our eyes are set on elevating the Carbon-Fibre Monocoque chassis manufacturing by varying panel thicknesses across the chassis, fine-tuning adhesive volumes for load-bearing inserts, and exploring different prepreg types. Alongside, validation of simulation results like torsional stiffness through practical test setups is on the cards.

The suspension system too has planned refinements for enhanced jounce and rebound characteristics, previously constrained by node locations in our spaceframe design. Aiming for a leaner brake disk weight via a robust simulation model, and venturing into the design and implementation of an undertray and rear-diffuser assembly, we anticipate a surge of 30% in downforce according to current CFD simulations. The continuous interplay between theory, simulation, and real-world testing drives our narrative of incessant advancement.

On the Electrical side of things, We plan to incorporate a SoC (State of Charge) and SoH (State of Health) monitoring capability onto our AMS, thereby enabling a much simpler testing and tuning process for our car.

And Finally, Thanks RS!

We extend our sincere gratitude towards RS Components for their unwavering support throughout our project. Their contributions played a crucial role in actualizing our concepts into a fully functional electric race car, making this venture a rewarding experience for every member of the IIT Bombay Racing team. Our journey in the Formula Student UK 2023 has not only honed our engineering skills but also fostered a culture of innovation and teamwork among us. Through continuous learning and improvement, we look forward to making significant strides in electric mobility in our future endeavours.