December 6, 2020

2Speeds

Elon Musk’s original Tesla Roadster used a two-speed transmission. It proved problematic, so he dropped the extra ratio and still delivered cars and SUVs capable of both brutal acceleration and license-endangering top speeds. So, who needs multiple gears? Gearing an electric motor to deliver strong launch torque and then spinning it fast enough for autobahn speeds presents serious cooling challenges and reduces efficiency. Hence today’s fastest EVs—like the Porsche Taycan, the Pininfarina Battista, and Rimac C_Two—use a two-speed transmission.

Why a two-speed transmission when 10-speeds are readily available? Electric motors have a vastly broader “sweet spot” of efficient torque delivery than combustion engines, so they don’t need frequent shifting to remain efficient. A low range with roughly double the ratio of high range generally suffices for passenger vehicles. Adding a second ratio should boost overall efficiency unless the transmission adds parasitic losses like that original Tesla transmission, Porsche’s twin-clutch, and Rimac’s planetary transmissions do. That’s where this novel Ingear transmission design from Toronto-based supplier Inmotive comes in.

Chief Technical Officer Anthony Wong started out trying to improve upon a bicycle’s derailleur gears with a design that didn’t require the chain to move laterally. With the chain remaining in one plane, he reckoned, he could use a super-strong chain from a transfer case or engine valvetrain.

His brainstorm was to “assemble” the larger (lower-ratio) gear by sliding segments of it into position in the open triangle that exists between the input (pedal crank), the output (axle), and the chain tensioner (derailleur) while maintaining chain contact with the smaller (high-ratio) gear until the other gear was almost fully “assembled.” This maintains continuous torque transmission while motor-torque management ensures shift smoothness.

When shifting from high to low, the first “key” segment that slides into place includes a curved kneelike feature that guides the chain onto the larger-radius gear without kinking it. As everything rotates, four other segments slide into place. The entire shift occurs within one rotation of the output shaft, typically below 50 mph. When shifting from low to high gear, the final large-gear segment includes another kneelike guide to ease the chain down onto the smaller gear without kinking, and a simple spring tensioner controls chain slack.

The five interlocking gear segments slide on pins that transfer torque to the output shaft, and a patent-pending finger deflector engages roller pins that move each segment into or out of contact with the chain. This works like some variable-valve-lift cam-lobe-shifting devices and requires no closed-loop control to account for wear, etc.

Here’s where the Ingear concept trumps the established two-speed EV trannies: There are no losses from multiple gearsets being paired simultaneously and no rolling contact losses. Because no torque gets transmitted while the segments are moving, none is required to keep them in either position, and because the whole works is splash-lubricated, there is no need for a hydraulic pump.

Straight teeth on the gears and chains don’t generate the side loads and friction

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