Submitted 9/2/2009

What is a Dual Clutch Transmission?

The system may have several different names, as branded by individual manufacturers, but a dual-clutch transmission with an automated clutch essentially does the same thing in all variations. Dual Clutch Transmissions (DCT) systems work by using a system of twin clutches which shifts gears automatically with the direction of the driver and assistance of a computer. Without a clutch pedal, the DCT system is able to shift faster than a manual, while still allowing more control and power than an automatic transmission. It takes the best from both autos and manuals and creates a technically superior transmission.

What’s wrong with my standard manual transmission?

The standard transmission is wonderful, and allows the driver to be more engaged and have more control than an automatic transmission. The problem is, every time the driver presses in the clutch pedal to shift, the driver disconnects the link between the transmission and the engine, causing a short period where no power is going to the wheels. Manual transmissions also require the driver to take a hand off the wheel to shift, reducing focus on other driving aspects such as vehicle placement and braking.

Sequential Manual Transmission (SMT)

The first semi-auto transmission was the Sequential Manual Transmission. The SMT was designed to automate the clutch of a manual transmission while still giving the driver full control of gear selection. Drivers use a mechanism such as paddles to select gears, and the SMT takes care of all clutch work for them. Like a manual, the SMT creates a direct link between the engine and the transmission, unlike an automatic transmission, which uses a torque converter. Torque converters have fluid coupling, allowing some slippage and loss of power which is not present in a manual’s direct coupling via clutch. This is why you won’t see an automatic in a real race. The downside to manual transmissions and SMTs is that to shift gears, the transmission must disengage, creating a short period of time when power is not being sent to the wheels.

The Dual Clutch Improvement

Dual Clutch Transmissions were designed to give drivers the direct coupling of a manual, the ease of automatic shifts, and the addition of constant power delivery by eliminating cuts in power during shifts. Like an SMT, the driver manually selects gears using paddle shifters or buttons, and is the transmission automates the clutch.

Think of a dual clutch transmission as two separate transmissions working together as one. One transmission takes care of odd numbered gears (1, 3, 5) while the other takes care of even numbered gears (2, 4, 6). Unlike manual and sequential manual transmissions, there is no break in power while switching gears. When accelerating from a standstill, the first transmission is used, with first gear engaged. The second transmission immediately readies the second gear while the car is still in first gear, and when the driver selects an up-shift, the second gear is ready and is almost immediately engaged, thus transferring power without a break. The first transmission now selects the third gear, to be instantly ready when called upon. A computer predicts which gear will be selected next, depending on inputs such as acceleration or breaking, and engages the proper gear ahead of the shift. Using this system, the transmission will always be spinning the next gear, creating a seamless and ultra-fast shifting experience.

Advantages of the DCT

DCT’s eliminate the loss of power that manual transmissions have during shifts and also increase the speed of a shift. A DCT can shift in under 10 milliseconds. While this may not be necessary for everyday driving, it is for supercars. Today’s supercar engines are so powerful that the only way to make them accelerate faster is by adding more tire grip via all-wheel drive or by decreasing the time it takes to shift gears.

Both SMT and DCT transmissions also have the ability to perfectly match engine revs in a downshift. In a manual transmission, the driver must make sure that the engine is revved high enough while downshifting, or the car will lurch forward as the engine and transmission speed come back in line. A Dual Clutch Transmission completely eliminates this shift shock with perfect rev matching, without requiring the advanced (and difficult to master) heal-toe driving technique used with a manual transmission. (See how Nissan solved this problem in their manual transmission using SynchroRev Match Technology).

Disadvantages of DCT

Some drivers prefer the experience of a manual transmission with a proper manual clutch, and will never give that up. Having a clutch gives a driver the ability to modulate engine revs, making it possible for the driver to easily kick out the back end of a rear-wheel drive car or to burnout on a launch. This full control is irreplaceable.

Other limitations are inherent because a DCT uses a fixed amount of gears, and therefore the engine can’t always remain in its most powerful range. Continuously Variable Transmissions (CVT) have been created to fix this, but are often limited to engines with lower power outputs. Also, these transmissions have not been well received by enthusiasts due to the non-traditional feel, and have been mostly implemented in hybrids and other fuel conscious vehicles.

In the end, a DCT transmission allows for the best possible acceleration and shifting, so long as the driver agrees to give up some manual control.

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