Tech Talk – Types of Transmission:

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Not all transmissions that change gears themselves are automatics, and in fact not all transmissions have gears at all...
Not all transmissions that change gears themselves are automatics, and in fact not all transmissions have gears at all…

There are only 2 types of gearbox used in cars, right?  Wrong.  Contrary to popular belief, there’s more to a gearbox than just whether it’s an automatic or a manual – not all gearboxes that shift gears themselves are automatics, in fact – so we thought we’d cover the basics of gearboxes for you, and give you a little insight as to what gearboxes exist and what defines them/separates them from the others.

 

First of all, there are two basic groups under which gearboxes may be classified: Manual and Automatic.  Under these categories there are several different types of transmission that we’ll be discussing:

 

Manual Transmission: Automatic Transmission:
   
Traditional Manual Torque-Converter Automatic
Sequential Manual Continuously Variable Transmission (CVT)
Automated Manual Transmission/Semi Automatic Transmission  

 

Manual Transmissions:

 

Manual Gearbox:

 

The ‘purist’s choice’, the traditional manual gearbox is one many of us grow up knowing; widely regarded as the driver’s transmission for its ability to immerse one in the driving experience.

 

It’s a simple concept to understand and is characterised by the fact that it has a shift lever that requires manual operation, in addition to a clutch pedal which is needed to engage and disengage the gearbox from the engine itself in order to select a gear.  Most notably, a traditional manual gearbox allows drivers to skip gears, either shifting up or down by more than one ratio at a time.  This is in contrast to the Sequential Manual…

 

Sequential Manual Transmission:

 

Those who have spent any time riding a motorbike will know a sequential ‘box all too well.  The sequential manual transmission, much like a traditional manual, features a clutch and requires the driver to shift gears themselves.  Unlike a regular manual transmission, gear shifts with a sequential gearbox must follow a logical sequence and ratios can not be skipped, i.e. to shift from 4th to 2nd you must engage 3rd gear.

 

Some sequential manual gearboxes, particularly those found in race cars and modern motorbikes, are equipped with synchromesh between all ratios that enables gear shifts without the use of the clutch pedal.  In these setups, the clutch is used purely to pull away from a standstill in 1st gear.

 

Semi-automatic/Automated manual transmission:

 

The thought behind the automated manual transmission is that it combines all the best attributes of a manual transmission (fuel economy, no mechanical lag etc) with the ease of use of an automatic.  It is essentially a manual transmission mechanically, but instead of the clutch being operated by the driver, it, and gear selections, are controlled by a computer.  In manual mode, the gearbox will not automatically shift, but the engine will bounce off the rev limiter until the driver prompts a shift either via a gear shift lever of paddle-shift mechanism.

 

Automated manual transmissions are not strictly single clutch gearboxes – and in many cases automated manuals are dual clutch units.  Dual clutch gearboxes are, in layman’s terms, essentially two automated manual ‘boxes joined together with one shared output – whereby one gearbox controls odd gears and the other, even gears.

 

In the modern motoring era, automated manual transmissions are some of the most commons gearboxes around – VW’s DSG, BMW’s M-DCT, Audi’s S Tronic, and Porsche’s PDK are all examples of current dual-clutch automated manuals, whilst BMW’s SMG and Alfa Romeo’s Selespeed are classic examples of single clutch automated manual transmissions.

 

Contrary to popular belief, the fact that a clutch engages the gearbox with the engine, means that any dual-clutch or automated manual is not an automatic gearbox.

 

Historically, automated manuals had to be driven in a very specific manner in order to avoid jerking and rough gear shifts, however in recent years through the improvement of dual-clutch technology, refinement has been vastly improved and as such, DCT’s are now the first choice of many a manufacturer.

 

Automatic Transmissions:

 

Traditional torque converter automatic gearbox:

 

The torque converter automatic gearbox is the conventional automatic transmission.  In this setup, the gearbox is never mechanically connected to the engine by means of a clutch as in a manual transmission.  Instead, drive is sent from the engine to the gearbox via a fluid coupling mechanism called a torque converter.  The engine’s rotation causes a rotation of fluid within the torque converter which in turn rotates the gears in a turbine-like manner.  This type of gearbox is also colloquially referred to as a ‘slushbox’ or ‘slush-type gearbox’ due to the liquid coupling between engine and gearbox and the lack of a solid mechanical linkage.

 

Naturally, the pros to a torque-converter auto are the ease of use, and reduced mechanical wear, but slushboxes are also traditionally heavier on fuel, have higher emissions, and are also prone to mechanical lag due to the rotation of fluid as opposed to a solid mechanical coupling.  This can dull the driving experience with delayed responses to throttle inputs, and as such isn’t the preferred choice for a ‘driver’s car’.

 

In recent years, automatic gearboxes have improved massively from a performance perspective – with manufacturers such as ZF developing brilliant slush-type gearboxes that rival even the best dual-clutch ‘box.  The ZF 8-speed in particular, found in many an Audi, Jaguar and BMW vehicle, is highly impressive from a performance perspective.

 

Continuously Variable Transmission (CVT):

 

Found on many Japanese vehicles nowadays, a CVT gearbox theoretically has an infinite amount of gear ratios, and simultaneously only one.  This gearbox uses either a belt or chain, linked to specific types of pulleys that can change their size, changing the ratio appropriate to the vehicle speed, whilst maintaining the same engine speed (rpm).

 

Theoretically, the CVT gearbox is incredibly efficient as it can ensure that an engine is always kept at its optimum operating speed, maximising power output whilst minimising fuel consumption.  Many manufacturers now equip their CVT’s with ‘traditional gear shift simulation’ techniques that give you the feeling that the vehicle is actually shifting gears.

 

The CVT does have some strange quirks, like the fact that with a planted throttle, the revs stay the same while speed increases, but on the flip side they do have rather horrendous overtaking acceleration traditionally.

 

In recent times, Subaru have done much to overcome the negatives of driving a CVT and are currently the front runners in CVT technology. Their CVT found in the latest WRX is particularly impressive.

 

No Gearbox At All:

 

Direct Drive System:

 

There is one type of ‘transmission’ that doesn’t actually fit into the manual or automatic classification.  A direct drive system doesn’t have any sort of gearbox at all, and instead, as the name suggests, drives the wheels directly via the engine.  This generally requires an engine with massive amounts of torque, from very low down.  In vehicles with direct drive systems, speed increases proportionally to the engine speed.

 

At present there is only one mainstream manufacturer that uses direct drive – Koenigsegg in the Regera Hypercar.  Use of a direct drive system means that they can reduce weight and complexity from the drivetrain system – allowing them the freedom to equip mild hybridisation to improve the driving experience without making a vehicle that is overweight.

 

Conclusion:

 

There you have it – the basics to different types of gearboxes and what differentiates them from one another.  Now, next time you’re asked, you can correctly state whether or not a gearbox is a manual, automatic, or neither of them at all.

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