AA Buyer's Guide: Lost in transmission
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We often receive queries from AA Members on our online forum about the reliability of vehicle transmissions, particularly when customers are in the market for a new vehicle.
Gone are the days where choices were limited to choosing between manual and automatic. Now consumers face an increasing number of acronyms, and not everyone is going to know their CVTs from their DCTs.
Continuously variable transmission (CVT)
CVTs are becoming increasingly popular, to the point that they are often mistaken for a “standard” automatic transmission.
A CVT employs two V-shaped pulleys that are driven by a flexible metallic belt, to continually change through the range of ratios. A CVT doesn’t have fixed “gears” as such, but some do have pre-set steps that can be selected for a more conventional feel.
As a CVT car accelerates, the belt drives up the surfaces of the pulley, shifting seamlessly through ratios. This takes away the need for a clutch and, as a CVT is comprised of fewer elements, cars with CVT transmission are often cheaper to manufacture.
Cars with CVT may feel slow to react under sudden acceleration or when carrying a lot of weight. The technology is well suited to light passenger cars as it provides a smooth and continuous power, but more manufacturers now include this type of transmission on larger vehicles due to advancements in CVT technology. On top of that, they offer great fuel economy, as torque is retained during gear changes.
Dual-clutch transmission (DCT)
DCT combines the benefits of a conventional manual transmission, with the use of dual clutches and a physical gear selection.
DCT uses two sets of gears - one holds the odd numbers, the other the evens – and it "pre-selects" the next gear, allowing the shift to be almost instantaneous, and quicker than both a manual or automatic.. This results in less torque loss when changing. Each gear is used with a clutch to both engage and disengage. It’s often a wet clutch, like that of a conventional automatic.
Over time, DCTs have improved significantly, although they must stay calibrated or their performance can deteriorate. A good DCT can feel a lot like a regular manual transmission, but it also boasts strong performance benefits, making this a good option for someone after a proper “driver’s car”.
Carmakers market this technology under a number of brand names; Volkswagen calls its version DSG, for example, while for Porsche it’s PDK.
Automated manual transmission (AMT)
AMTs offer manufacturers a cheaper way to utilise an existing manual transmission in a car rather than creating a new automatic.
Cars sporting automated manual transmissions are less frequently found in New Zealand, but if you drive an older Alfa Romeo, Fiat or Peugeot, you might be more familiar with AMT than you first thought.
Normally, AMTs have a manual transmission setup that consists of controlled actuators and sensors. The actuators typically control the clutch and gear selection using electric or electro hydraulics. The sensors monitor gear selection, engine speed and clutch control.
Unlike a conventional manual transmission, there is no physical connection between the driver and the gearbox, so a computer regulates the gear selection and clutch control. Sometimes, they have a fully automatic mode, but because everything is operated through a computer, there are delays between tasks, which leads to power loss and occasionally harsher operation. Because of this, the AMT can’t really be considered a “true” automatic, especially since it requires calibration.
The operation of AMTs isn’t the fastest, but it may appeal to drivers who enjoy the excitement of a gearstick without having the hassle of controlling the clutch.
What about Battery Electric Vehicles (BEVs)?
BEVs with high torque motors do not require a transmission, so many will wonder if this is the end for gearboxes as we know them.
Transmission manufacturer ZF is currently developing a unit that can fit into a range of vehicles and can be mated to a motor with a maximum power rating of 140kW. This is said to improve efficiency by five per cent.