TRANNY TALK

HOW COME electric vehicles don’t have transmissions whereas traditional vehicles need gears, seemingly the more, the better? The answer is surprisingly straightforward—it’s all a matter of torque.

Torque, or twisting force, from an electric motor or a conventional engine is what ultimately spins the drive wheels and propels the vehicle. See www.wp.me/p2ETap-Nx for the basics. What’s interesting is the way an engine or motor produces this torque.

Contrasting torque curves of an internal-combustion engine, left, and an electric motor, right.

These two graphs show the crucial difference. With an internal-combustion engine, its torque depends on engine speed. In particular, very little torque is produced at low rpm. Then torque increases until overcome by frictional losses and aspects of an engine’s breathing—its intake and exhaust.

This peak torque, measured in lb.-ft., is what’s reported along with the particular rpm at which it’s achieved. Variable-valve hardware, advanced injection, enhanced combustion and other technology can broaden this torque peak over a range of rpm.

But it’s accepted that a conventional engine’s torque isn’t much at low rpm. Therefore, this startup torque needs to be multiplied by use of a gear. And, for a full range of a vehicle’s road speeds, more than a single gear is required.

By contrast, once an electric motor is spinning at all, it’s producing peak torque. Its torque “curve” is as flat as Kansas. And thus an electric vehicle can run in direct drive—or maybe with one reduction gear to match the electric motor’s optimal revs with the vehicle’s road speed.

End of story, sort of. But there’s still a lot to be noted about the gearing of internal-combustion engines.

For years, a three-speed manual gearbox was de rigueur: 1st gear for startup, 2nd gear for acceleration and 3rd gear, direct drive, for cruising. Sportier cars (and heavy-duty trucks) had more speeds for optimized performance.

In fact, even in the early days, race car gearboxes could be fine-tuned to suit the particular circuit. Consider Le Mans, with Tertre Rouge Corner leading onto the 3.5-mile (and once chicaneless!) Hunaudières (aka Mulsanne) Straight.

The circuit at Le Mans, France, home to the annual 24-hour endurance race, as it was 1932-1967.

You’d choose the correct ratio for 2rd-gear acceleration out of Tertre Rouge and ensure that top gear would reach maximum revs just at the braking point for Mulsanne Corner, likely taken in 1st gear, at the straight’s end.

The Hewland transaxle has easily accessible gears for optimizing ratios.

The renowned Hewland gearbox, designed by Brit Mike Hewland in the mid-1950s, was originally a five-speed transaxle with an important feature: By removing its end case, the gears could be easily exchanged for optimal choices at any particular circuit.

Among earliest automatic transmissions, the 1940 GM Hydra-Matic, initially for Oldsmobile, then Cadillac, were four-speeds. Their planetary-derived ratios were 3.82:1, 2.63:1, 1.45:1 and 1.00:1. Later the 1950 Chevrolet Powerglide simplified its internals with only two forward speeds. By the late 1960s, though, automatics settled on three speeds with torque converter.

Speeds? I’ll show you speeds. This ZF automatic has nine forward speeds. Its efficiency of packaging is noteworthy.

For best performance, though, the more ratios, the better. These days, optimized fuel economy is the goal, not simply quickest acceleration nor highest top speed. BMW’s 7 Series ZF automatic had six speeds in 2002. Mercedes-Benz introduced a seven-speed in 2003. The 2007 Lexus LS 460 featured an eight-speed automatic. The 2014 Jeep Cherokee has a nine-speed automatic.

And then there’s the Continuously Variable Transmission, the CVT, with an infinite number of ratios. I’ll leave it and other innovative transmissions for later Tranny Talks. ds

© Dennis Simanaitis, SimanaitisSays.com, 2013