CVT Vs. Dual Clutch

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There are two kinds of car buyers: those who shop with their heads, and those who buy with their hearts. Neither the continuously variable nor dual-clutch transmission designs are particularly new, but it took modern high-speed computing to make them truly viable in the marketplace. And now that they are, several marques, including Volkswagen and Audi, are offering them as options in the same chassis. So begins a new chapter in that age-old head-vs.-heart automotive cage match.

Dual Clutch Transmission Basics

  • Imagine, if you will, that your car has two manual, three-speed transmissions placed side by side; both the left- and the right-hand transmission connect to the engine at the front and to the driveshaft at the back. Now imagine that the transmission on the right contains gears one, three and five and the transmission on the left contains gears two, four and six. After accelerating through and just before shifting out of first gear via the right-hand transmission, the left-hand transmission would automatically select second gear and apply the clutch. With power to the driveshaft now coming from two inputs, the clutch on the right-hand transmission releases. On and on you go through the gears, with the computer automatically pre-engaging the next gear before the last one releases. The net result is acceleration through the gears with no lag between them.

Continuously Variable Basics

  • Continuously variable transmissions -- transmissions that can change the ratio of input to output speed without stepped gear-sets -- have been around at least since the time of DaVinci, and they're more common than you might think. There are more than a dozen different kinds of CVT out there, but the underlying principles remain the same. A variable-diameter pulley CVT changes gear ratio using a belt and two pulleys that automatically get larger or smaller with engine or road speed. The VDP is the simplest, cheapest and most common type, but it is limited in its ability to transfer power without slipping. On the other end of the scale you've got hydrostatic CVTs, which are immensely strong but are also usually too heavy for use in automobiles. Between them you'll find auto-oriented designs like the toroidal roller, ratcheting CVT, variable-tooth and cone-type.

CVT Pros and Cons

  • Above and beyond all else, CVTs are designed to operate smooth as Isaac Hayes on roller skates -- and for the most part, they do. Because they lack fixed gear ratios, CVTs won't induce the sudden deceleration and acceleration that comes with shifting gears. CVTs also offer a hypothetical advantage in performance, fuel economy and emissions compliance because they can keep the engine operating at either peak efficiency or power-output rpm regardless of actual road speed. Oddly enough, the CVT's major obstacles at this point have less to do with engineering than with public acceptance. Because there's no direct relationship between engine speed and road speed, a car equipped with a CVT will accelerate something like a boat. The engine roars and there's a surge in acceleration, but the perception of outright power and performance just isn't there.

DCG Pros and Cons

  • You can play Isaac Hayes as loud as you want to, enough to shake the very Earth with smooth baritone thunder, but you're still going to walk away disappointed if you're in the mood for Sepultura. What the DCG lacks in technical performance it makes up for with attitude and the perception of performance through neck-snapping shift speed. While Ferrari and Lamborghini may play around with the idea of dumping manuals and DCGs and installing CVTs, it's never going to happen. These are, after all, the same kinds of transmissions used in Formula One cars, and that's got a cachet that the CVT just can't match. Of course, that's not to say that the DCG can't compete in the real world of fuel economy and acceleration -- it is still basically a manual transmission, after all -- it only means that the DCG needs a few more gear ratios and a bit more driver talent to do so.

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References

  • Heavy Duty Drivetrains: The System and Component Application; Charles Jones
  • Today's Technician: Manual Transmissions and Transaxles; Jack Erjavec
  • Continuously Variable Transmission; John Maten and Bruce Anderson
  • Five Hundred and Seven Mechanical Movements; Henry Brown
  • Photo Credit Jupiterimages/Comstock/Getty Images
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