Automobile Engineering

As the story goes, Tesla tried to develop a two-speed ratio transmission for its Roadster around 2007. But after a series of failures that delayed production and almost tanked the company, a decision was made to use a single-speed-ratio transmission. Tesla’s ensued success with single-speed-ratio drivetrains, also adopted by other electric car manufacturers, leads to the question: Is there a need for multispeed drivetrains in electric vehicles? If so, for what class of vehicles? Is it economical? Our research has shown that the electrification of vehicles larger than class 3 is where multispeed transmissions may be considered, but perhaps not currently for smaller vehicles, apart for high-performance race cars or GTs. The cost question, while crucial in the automotive industry, is very difficult to settle for a new electric powertrain design because the production volumes are still very small. Given a set of performance specs for the vehicle, and assuming peak power is not the main constraint, is it better to choose a larger motor and power inverter for a single-speed-ratio architecture, or to select a smaller inverter and motor coupled to a two-speed transmission? To help resolve this question, we studied the cost-reliability-performance of EV powertrains to come up with formulas that could be used in a design optimization trading off the cost of the powertrain versus its performance and reliability. On the performance side, we designed a two-speed clutch-less automated manual transmission (AMT) and tested it on a compact electric car for dynamometer and road testing. Then, we studied friction in the synchromesh of an AMT to design a feedback controller for the synchronizer’s cone clutch. This was done to control friction at a favourable location in the mixed lubrication regime on the Stribeck curve that maximizes torque transfer while minimizing wear during gear shifts. A new type of two-speed transmission was also designed to alleviate the torque dip of the AMT while avoiding the use of a clutch. The resulting patented Dual Brake Transmission (DBT) has shown promise for high efficiency, high torque and smooth operation of EVs.