Formula Racing at UC Davis (FSAE Electric)
What is Formula Racing SAE Electric?
Formula SAE Electric is a design and engineering challenge for university students designed and run by the Society of Automotive Engineers. The goal of the competition is to design and build an open-wheel, single-seat, electric racecar conforming to a stringent set of rules, which emphasize drivetrain innovation, safety, and energy efficiency in a high-performance application. Our team’s cost, design, and strategic positioning decisions are made to take into account the interests and requirements of a weekend autocross racer. These decisions are then presented to a panel of industry professionals in the business presentation.
Lead Drivetrain Engineer (2022-Present)
Role:
As the lead drivetrain engineer, my responsibility is to design, manufacture and integrate a new high-voltage drivetrain architecture that maximizes powertrain efficiency and performance. Drivetrain consists of packaging the motor, motor controller, and differential. I constantly apply concepts from my previous courses like statics, mechanics of materials, properties of materials, and manufacturing processes.
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Below is the drivetrain assembly being manufactured this quarter (winter 2023)
The 2022-2023 season marks the first season our team has incorporated a new drivetrain architecture. This year we are using the EMRAX 228 HV LC motor and CM200DZ Inverter, which produce 40% more torque and power over the previous architecture. Our differential is the Drexler adjustable limited slip differential which allows for our torque to be split and allocated to the wheel with highest traction.
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Having selected the components that power the drivetrain, the first step was to determine the gearing of the vehicle. We use a roller chain drive design for our vehicle due to it's simplicity, high efficiency and reliability. When selecting a Final Drive Ratio there are many factors to consider. FSAE tracks are shorter in length with multiple tight turns, which means your vehicle will not have the ability to reach high speeds. In this case, higher torque is needed to more effectively go out of tight corners, but is it something your vehicle can handle? The wheels of the car can only handle so much torque before the tractive limit is surpassed.
In our case, I selected a final drive ratio that surpassed the tractive limit but ensured that are motor would be running at the highest efficiency. Motors have efficiencies that range throughout different RPMs. You want your vehicle to run at the highest motor efficiency which is the greatest factor for determining your gearing.
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My next step is to determine a packaging solution for the motor and differential. There are two different approaches: package each component separately or package both components together. In our case, we determined that having a separate mounting solution did not produce the required stiffness causing large displacement in the system. With the help of Finite Element Analysis at an assembly level, a single mounting solution between the motor and differential was made.
Packaging Solution:
Gearing:
Differential Sprocket:
Unlike previous years, the differential sprocket was mounted directly onto the Drexler differential and is therefore on the inside of the planes of the support brackets The differential sprocket is held by 12 low profile M6 SHCS at a calculated torque of 10Nm per fastener. This simplified the manufacturing & packaging process, alongside reducing weight by removing the extra adapter used to mount the sprocket to the differential.
Motor Shaft:
The EMRAX 228 provides a motor shaft that is not ideal for our loading application. A custom motor shaft had to be designed for our loading application. Before I could get to the final design of a motor shaft, I had to
understand the type of motor in hand and loading the motor would undergo. The EMRAX 228 is
an axial flux motor in which the entire casing of the motor rotates around the stator. The motor
will power our wheels through a chain drive system which will put a shear load on the shaft. I
calculated the load applied by the chain drive system using a MATLAB script. With an
understanding of the loading and setup, I began the design of the motor shaft. The challenge
was to incorporate a sprocket and extra bearing on the shaft for support. The shaft was created
using a key that would allow a sprocket with a key slot to attach to the shaft. The advantage of
using a key and key slot compared to splines or an incorporated sprocket is the complexity level
of manufacturing being lower. A machined sprocket can be purchased heat treated at a very
low cost while the production of splines is more complex and high in price. The motor is not
designed for the type of loading seen on a chain drive system. I ran multiple assembly level
static FEA simulations which confirmed the need for extra support due to the high displacement
in the system. How could this be limited, well I decided to add a bearing at the end of the shaft
to attach a support bracket. To reduce the displacement in the system more, I reduced the
length of the shaft which will reduce the moment being created. As a result, the final motor shaft
consists of a design that incorporates a method to transfer torque in the system through the
sprocket and has the appropriate support needed to prevent failure in the part.
Drivetrain Engineer (2020-2022)
Tripod Housing:
GD&T:
