Torque converter, CVT, dual or single clutch autos, what's the difference?

27 June 2015
 by 
, CarsGuide

Audiophiles bemoan the digital age and its lack of deep, vinyl warmth; cricket purists rate Twenty20 a big fat zero, and yet both forms of disdain are nothing compared to the loathing driving enthusiasts feel for the seemingly ceaseless march towards the dominance of the automatic gearbox.

It matters not that F1 drivers make do with two pedals and some paddles, manual-loving motorists insist that life is meaningless without a clutch and some heel-and-toe dancing on the pedals.

The fact is, however, that the vast majority of car buyers are happy to stick their transmissions in D for Do-little and thus the self-shifters have reached near ubiquity, with the Federal Chamber of Automotive Industries (FCAI) claiming automatics account for more than 70 per cent of new cars sold in Australia.

Frankly, it's a surprise the figure isn't higher, when you consider that in the US, less than 4 per cent of cars sold have manual gearboxes.

You can't even buy a new Ferrari, Lamborghini or Nissan GT-R with a manual gearbox

It's not just laziness that's to blame, either because since the turn of the millennium automatic transmissions have become more and more advanced and fuel-efficient, leaving the manual option to purists and paupers.

And the argument that you can't have driver involvement without a stick shift gets thinner by the day when you consider you can't even buy a new Ferrari, Lamborghini or Nissan GT-R with a manual gearbox (and even the sportiest Porsche models don't give you the option).

So how did the auto become the automatic choice, and what makes them so tempting that people are willing to pay more for them?

Torque Converter

This is the most common automatic option, and can be found in the hugely popular Mazda range, as well as the more high-end Japanese brand, Lexus.

Instead of using a clutch to engage and disengage the engine's torque from the gearbox, traditional autos keep the drivetrain connected at all times by using a torque converter.

Torque-converter automatics have the clear advantage of big torque at low revs

This slightly complex bit of engineering pushes fluid around a sealed case using something called an "impeller". The fluid drives a turbine on the other side of the case, which sends drive to the gearbox.

Torque-converter automatics have the clear advantage of big torque at low revs, which is great for acceleration off the line and overtaking. Acceleration from a standing start is smooth, as are the gear changes, which wasn't always the case in the jerky, early-80s style autos.

So how do the gears actually shift?

You may have heard there term "planetary gears", which sounds a bit grand but basically refers to the gears being arranged around each other the way moons orbit a planet. By changing which gears spin relative to the others, the gearbox computer can change drive ratios and offer up gears that suit acceleration, or cruising.

One traditional problem with torque converters was that they are essentially inefficient, due to their being no direct mechanical coupling between the input and output shafts.

Modern "lock-up" torque converters incorporate a mechanical clutch to ensure a more efficient coupling.

Add a set of shift paddles to the steering wheel and modern torque converters can even do a convincing impression of their clutch-based brethren.

Single-clutch transmission

The next big technical step ahead for the automatic was the single-clutch system, which is basically like a manual with only two pedals.

A computer takes over control of the clutch for you and matches engine revs to provide smooth shifts.

Or at least that was the idea, because in practice these automated manuals could take time to disengage the clutch, change gears and re-engage, making them jerky and annoying, as if a learner driver, or a kangaroo, was hiding under your bonnet.

Mostly they've been superseded, and are to be avoided if buying second-hand

BMW's SMG (Sequential Manual Gearbox) was a ground breaker in this area, but while tech-heads loved it, many people were driven mad by its ineptness.

Some cars still struggle on with the single-clutch system, like Fiat's Dualogic transmission, but mostly they've been superseded, and are to be avoided if buying second-hand.

Dual-Clutch Transmission (DCT)

A dual-clutch system sounds like it should be twice as good, and it is.

These advanced gearboxes, perhaps most famously used by Volkswagen with its DSG (Direkt-Schalt-Getriebe or Direct Shift Gearbox), use two separate gear sets, each with its own clutch.

An effective, modern DCT-equipped car can swap cogs in just milliseconds

In a seven-speed system, gears 1-3-5-7 will be on one rod, with 2-4-6 on the other. This means if you're accelerating in third gear, fourth gear can already be selected, so when it's time to shift, the computer simply releases one clutch and engages the other, for an almost seamless change. An effective, modern DCT-equipped car can swap cogs in just milliseconds.

VW's system is quick, but the dual-clutch boxes found in cars like the Nissan GT-R, McLaren 650S and Ferrari 488 GTB offer staggeringly fast shift times, and almost no loss of torque between each one.

As hard as it is for a purist to swallow, this also makes them faster, and easier to drive, than any manual could manage.

Constantly Variable Transmission (CVT)

It might sound like the ultimate automatic solution, but the CVT can rub some people the wrong way.

A constantly variable transmission does exactly what it says on the label. Instead of swapping through a set number of pre-determined gears, a CVT can vary its drive ratios on the fly, almost infinitely.

Picture a traffic cone mounted on an axle, with a second empty axle, parallel to the first. Now put a rubber band around the axle and the cone.

CVTs can keep an engine spinning at peak efficiency

If you move the rubber band up and down the traffic cone, you'll change how many times the empty axle has to rotate to complete one rotation of the cone. By moving the band up and down, you'll change the drive ratio.

Because the ratio can vary without moving any gears, CVTs can keep an engine spinning at peak efficiency.

In practice, this means that when you accelerate in a CVT-equipped car, it makes a constant buzzing sound instead of the traditional rise and fall in revs.

It's very fuel-efficient, but it just doesn't sound as exciting as an engine should. Again, that's a purist opinion, and some people don't notice the difference at all, except at the petrol pump.

So which should you choose?

Modern automatics offer better fuel economy than manuals, thanks to a larger choice of drive ratios. Most manual gearboxes have six forward gears, although Porsche's 911 offers seven.

Modern twin-clutch systems use seven gears, torque-converter autos are pushing as high as nine and CVTs can produce a near-infinite number of ratios, meaning they offer the best fuel economy.

With shift speeds that embarrass the fastest manual driver, automatics can also accelerate faster.

It's not just the ultra-fast dual-clutch systems, either; ZF's nine-speed torque-converter gearbox offers gear changes it claims are "below the threshold of perception".

Many car makers are scratching manual gearboxes entirely

It looks like curtains for the humble manual; which has become the slow, thirsty and left-leg-consuming option.

Many car makers are scratching manual gearboxes entirely, so it's not even a base-model option for saving a few dollars any more.

It's hard to believe, but driving a manual might seem as absurdly retro to your grandkids as vinyl records do today.

What's your transmission preference? Do you still drive a manual? Tell us what you think in the comments below.

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