Thanks to the technological challenges associated with the storage of large amounts of electrical energy being overcome in recent years, electric vehicles have become increasingly popular.
Not only are they cheaper and more environmentally friendly but they also have increased performance characteristics due to an easily implemented independent four-wheel drive system. That is when compared to a four-wheel drive system in a combustion engine car which requires numerous driveshafts and complex differentials.
With the costs of electric powertrains decreasing, universities began developing electric Formula Student cars. The University of Stuttgart started the trend, debuting its Electric Vehicle in 2010. Not to get left behind, Queen’s Formula Racing decided last year that instead of upgrading its Yamaha R6 motorcycle engine to a more powerful and efficient combustion engine, it would instead start development of an EV. This began with three final-year mechanical engineering students doing projects to set the foundations for future years. They were:
- Gavin White – Electric drive design
- Connor McShane – Accumulator (battery) design
- Darryl Doyle – Vehicle modelling and control
Gavin White’s project aim was “to propose a detailed solution for the motor and drive system for a single seater race car”.
“This was purposefully vague as there was no experience within the university on the best way to approach it,” said Gavin. “For example, do we buy a full motor package complete with controllers or should we build our own and self-develop the control system.”
“Each had their advantages, the off-the-shelf system would be quicker to get up and running, probably be higher spec but would be extremely expensive.
“Alternatively, the self-developed system may take longer to get working but would probably be cheaper and would be much easier to upgrade in future years.
After a lot of investigation into electric motor design, it became clear that a self-developed motor would be extremely complex if it were to be competitive. A whole new suite of software and three years of testing and analysis would be required before having something worth putting onto a car. The best value-for-money system was sought and narrowed down to the best two packages available on the market.
From comparing the options, the AMK kit was a clear winner due to its impressive power to weight/size ratio as well as its reasonable price.
“The decision was a little more complex than that,” said Gavin. “There were more factors which played into the decision making such as the AC/DC power choice, but that’s a whole article in itself.”
Another key aspect of choosing a motor package is the vehicle layout and the distribution of motors and power. Four concepts were presented and alongside motor selection, Concept 4 was chosen as the most desirable thanks in part to the low mass, small size and high torque of the AMK motors.
With a motor selected, it was time to focus on the transmission, i.e. how the power is taken from the motor and transmitted to the wheel. This would have to include a reduction gear ratio to increase the torque of the motor from 20 Nm to around 300 Nm. The exact gear ratio selected took weeks of analysis, a trade-off between top speed and acceleration.
“From Concept 4 it can be seen that each of the four motors is placed within each wheel rim, therefore, four transmissions would be required – all packaged alongside the brakes, hub and upright,” explained Gavin. “This was technically challenging, but many other teams have been choosing this layout for several years now.”
A Microsoft Excel sheet was developed over many months to calculate the transmission strength and geometry. This included a full analysis of the shafts, bearings, keys and keyways, and fixings inside the transmission to ensure it would not fail. After many concepts were proposed, the final design was chosen and refined in specialist industry-standard software provided by Ricardo.
The resulting design was a two-stage transmission featuring an initial reduction followed by an epicyclical/planetary stage. The advantage of the two-stage reduction was that a high gear ratio of 15:1 could be achieved while having reduced stress in the gear teeth thanks to the planetary stage having three gears to distribute the higher torque.
The transmission features:
- Off-the-shelf gears which reduced the manufacturing cost of the gears from over £4,000 to under £1,000 per transmission.
- A polygon shaft connection to the hub, transmitting the rotational motion while allowing for axial movement between the hub and transmission, reducing deflections from the wheel.
- Increased gear tooth life by supporting the planet gear shaft on both sides.
- Increased bearing life from the balancing of the gear loads
- Proposed design for the hub and upright to favour manufacturability and assembly.
To verify that the transmission could be assembled and that all the tolerances on the components were correctly specified, a prototype was commissioned using exact replicas of the gears chosen but in a polymer material to reduce cost. All the correct bearings, circlips, and keys were used to ensure that there were no clashes during operation of the transmission.
“As I am graduating this year, I won’t get to be part of the team which brings this design to life in the prototype electric car,” said Gavin.
“But I wish the team the best on what I believe is the most challenging and exciting project I have ever worked on!”