The Dual-Clutch Transmission

By Zach Bowman


In the search for ever-better fuel economy and ease of use, carmakers are looking at every individual piece of their products to improve efficiency. One major component of the power/fuel economy puzzle is the vehicle’s transmission. Standard automatic and manual transmissions, along with the rest of the driveline components typically draw around 15 percent of the engine’s power. That’s gusto that never makes it to the ground and fuel that’s burnt for no reason. To help address the problem and reduce drag on the vehicle’s engine, manufacturers have begun to incorporate more sophisticated gearboxes, one of which is the dual-clutch transmission.

Audi's 7-speed dual-clutch

Before we go diving into the nitty-gritty of how various transmissions function, we should start with what one does. Your car’s engine can only spin so fast, so engineers have come up with a variety of gear ratios to allow the engine’s work to be multiplied. Multiplying the work means rolling along at higher speeds. A modern transmission typically houses anywhere from four to six forward gears and a single reverse gear, selected either mechanically (a manual), hydraulically (an automatic) or electronically (dual-clutch).

To really understand how a dual-clutch transmission works, it’s necessary to get a basic grasp of the mechanisms at play in the other two most common variants. First up is the manual transmission. Hands down, this is the oldest type of gearbox out there – thought to have been invented sometime in the late 1800’s. This system is made up of a clutch and a gear selector, both of which are operated by the driver.

For illustration purposes, we’ll look at a two-speed transmission. In this type of gearbox there are three shafts: an input shaft coming from the engine, an output shaft going to the rest of the drivetrain and a layshaft connecting the other two via gears to the side. In our model, the output shaft is the one with our forward gears on it, though they sit on bearings so they can spin freely regardless of how fast the output shaft and wheels move. Still with me?

That means that all of your gears are constantly moving, they’re just not positively connected to the output shaft. That happens thanks to something called a dog gear that spins at the same speed as the output shaft. When you put our model transmission in gear, the dog gear connects with one of our gears spinning around the output shaft and a positive connection is made. Just like that, you’re moving down the road. Changing gears simply disengages the dog gear from one forward gear and connects it with another.

An automatic transmission selects all of your gears for you. If you thought the manual transmission was complicated, get ready for some head scratching. Automatics typically rely on what’s referred to as a planetary gear set. Think of your elementary school lessons on how the planets orbit the sun. Staring straight on at a planetary gear set gives you the same view, with a large “sun" gear in the middle several small planet gears aligned along a carrier “orbiting" the sun and ring gear around the whole kit and caboodle. The really tricky part is that an automatic transmission may have several of these planetary gear sets linked together.

Automatic vehicles use a torque converter to help translate engine speed to gear selection. As the engine climbs in RPM, the torque converter generates more and more fluid pressure in the transmission. That fluid then pushes through a series of spring-loaded valves inside of the transmission which then actuate a series of bands and clutches to engage or disengaged sections of the planetary gear. This allows for a wide variety of gear ratios depending on the number of planetary gears inside of the transmission.

Now that we’ve got a grasp on two of the most common types of transmissions out there, we can take a look at the dual-clutch version. Essentially, this type of gearbox is comprised of two manual transmissions melded together. One holds all of the odd gears and the other takes care of the evens. Unlike a manual, however, the clutches are handled by sophisticated computer wizardry – not the driver’s left foot.

With gears aligned on two different shafts, two gears can be engaged precisely at the same time, allowing for a constant flow of power. This sort of set up wasn’t easily (or cheaply) produced until just recently when the combination of inexpensive electronics and stout internal components made the technology available for mass production. The result is the best of both the manual and automatic type gearboxes. If the driver wants, he or she can let the car do all the thinking and it will select which gear is appropriate for the situation. If the driver’s looking for a little more control, most dual-clutch cars allow for a “manual" mode, where the driver selects gears via paddle shifters or a console-mounted selector. Keep in mind that there is no physical connection between those shifters and the transmission.

There’s more to it than just car control, too. Typically, automatic transmissions sell better than their mechanical counterparts because anyone can drive them. The drawback is that automatics are more expensive and less efficient than their manual counterparts. A dual-clutch gearbox blends the ease of use of the automatic with the efficiency of a manual, generally producing better fuel economy. What’s more, for the performance minded out there, dual-clutch gearboxes shift instantaneously – shaving precious moments off of lap times.

More and more manufacturers are beginning to embrace the dual-clutch transmission, especially in performance cars. Nissan’s mighty Nissan GT-R boasts a six-speed variant and Mitsubishi employs the dual-clutch box in their all-wheel drive Lancer Evolution X thanks to its lightening fast shifts. With all of the benefits the dual-clutch has to offer, it wouldn’t surprise us to see the gearbox become more and more common as manufacturers search for ways to get the most out of their vehicles.

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