New and Improved
Following the lessons learned from the E-Model 1, our team completely redesigned our system architecture with three main factors in mind: Reliability, Sustainability, and Manufacturability. The result is the E-Model 2 optimized for the dynamic events of Formula SAE Electric.
The high voltage battery (a.k.a. the accumulator module) in the E-Model 2 is constructed of Electric Power 6.3 Ah LiCoO2 cells. Key system design decisions were determined through our MatLAB simulator, which compares the effect of power limit, capacity, and mass on lap times in all Formula SAE dynamic events. Upon review, the cells have been connected in a 72s4p configuration making up a 300V pack with an energy density of roughly 6.7 kWh. Each cell is monitored for voltage and temperature through a custom designed battery management system ensuring safe operation throughout the charge/discharge cycle. Key improvements in the E-Model 2 battery from our previous model include a compact container design and a simplified forced-air cooling strategy.
The E-Model 2 is a dual rear-wheel drive consisting of two Emrax 208 axial-flux permanent-magnet AC motors capable of an 80 kW peak power output. To convert power from the battery to the motors, the E-Model 2 has two Sevcon Gen5-S9 controllers capable of torque requests over CAN for improved safety. Power is transmitted from the motors to the rear tires through a chain drive system with an endurance efficiency of 290 Wh/km. Overall, light, store-bought motors was the major goal for the E-Model 2 versus the custom in-hub motors of the E-Model 1. This ensures reliable performance throughout the competition season.
Our firmware system controls the vehicles multiple PCBs, performing key functionality such as battery management, motor control, and general safety monitoring. Our firmware is built using industry standard techniques, such as real time operating systems and CAN J1939 messaging. The major improvement for this car was to build safety into the backbone of our firmware. This was accomplished through leveraging our distributed architecture to ensure that all boards are constantly monitoring each other to verify correct behaviors. As well, a state machine-based approach was taken that has specific error handling for various fault events depending on the current state.
The chassis is redesigned from the ground up to integrate the new accumulator and powertrain. Ergonomics and mass distribution studies were completed to improve driver comfort while passing regulations. The space frame is built using AISI 4130 "Chromoly" steel tubing. Suspension and steering design incorporated extensive tire data analysis to achieve an optimal configuration and reach vehicle dynamics targets.