John is a fervent writer, gamer, and guitar lover. He is a former automatic-transmission repairer, welder, and hobbyist game developer.
There are many brands and models of transmission used in your average road-going cars, but they can broadly be boiled down to a few different types. In this article, we’ll take a look at each of those types and the significant differences between them. Don’t worry; we won’t get too technical. But first…
What Transmissions Do
Before we look at the differences between types of transmissions, it’s important to understand the basics of what a transmission does. This will be a very simplified overview, and if you’re already familiar with the concept of gear ratios, you can probably skip this part.
Simply put, gear ratios exchange speed for power, or vice versa. If an engine is turning at 1000 RPM (revolutions per minute), and the gear ratio is 2:1, the wheels will turn at 500 RPM, however, the power required to achieve that is much less than if the wheels were turning at the same rate. This allows the vehicle to get moving when an engine alone would lack the power. As the vehicle gets moving and is assisted by its own momentum, the gear ratio can be shifted up, eventually reaching 1:1—the wheels turning at the same rate as the engine—and higher. Of course there are more factors to take into account, but in essence, this is what the transmission does.
The other significant component in this set up is a clutch. A clutch is an interface between the engine and the transmission which allows the driving force of the engine to be disengaged, such as during a gear change. A clutch will allow “slippage,” a partial transfer of drive. This allows for smooth gear changes rather than an immediate (and likely bone-jarring) shift from one gear to another. Now, onto the different types of transmission.
If you’re in the US, you might call it “driving stick.” More common in the rest of the world, however, the manual transmission has many advantages over alternative transmission types. Let’s look at the pros and cons.
- Low net weight
- Low energy loss (through friction)
- Better reliability when compared to complicated automatic transmissions
- Ease of use
Essentially, a manual transmission has everything going for it except for the fact that it is simply much more difficult to use over any of the alternatives in this article.
So how does it work?
The insides of a manual transmission are a mass of gears and cogs and meshes that provide the different gear ratios. Drive from the engine passes through the clutch and into the transmission where it travels along the gear train, finally leaving the transmission and continuing on to the wheels. Gear selection is handled entirely by the driver through means of a gear lever which, in turn, controls a selector fork (or forks) which physically move the internal components of the transmission.
It is possible to move through the entire selection of gears without using a clutch by timing the gear changes perfectly, but for the sake of usability, the clutch allows the driver to disengage drive entirely, change gear, and then ease the clutch back in to re-engage drive. This is all judged entirely by the driver, hence the increased difficulty in using a manual transmission.
Essentially, a semi-automatic transmission is a manual transmission with little actuators attached to it to do all the physical stuff that the driver would ordinarily be doing. Often times the transmission itself is a manual transmission that has had the additional semi-automatic components fitted to it. Let’s look at the pros and cons.
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- Low net weight
- Low energy loss (through friction)
- Inexpensive compared to full automatic transmissions
- Easier to use
- Driving experience noticeably inferior to fully automatic vehicles
Actuators attached to the outside of the transmission take the place of a gear lever, selecting the appropriate gear for the driver. As the transmission itself is often just a standard manual gearbox, there will generally be two actuators to simulate both the up and down motions of a gear lever. The clutch is controlled by a hydraulic pump which simulates the driver pressing the clutch pedal. All of this is controlled by a control module (essentially a small computer) that decides when to change gears.
Although semi-automatics achieve some of the improved comfort of driving an automatic while retaining a lot of the performance advantages of a manual, the gear changes in semi-automatics tend to be awkward and not nearly as smooth as a fully automatic transmission.
Fully Automatic Transmission
This is the real deal. Fully automatic transmissions are the height of driving comfort. With an automatic vehicle you do not need to concern yourself with gear changes, and often won’t notice them at all when the transmission is behaving correctly. It’s not all upside, however.
- Vastly more comfortable driving experience
- Considerably easier to use
- Inefficient (power loss)
- Considerably more expensive to repair when faulty
A fully automatic transmission is almost a mirror image of a manual in that, while a manual has a lot of gears and one clutch, an automatic tends to have a lot of clutches and only one or two gears. In an automatic, each clutch is part of a metal drum. That metal drum acts as a gear, providing the necessary ratios for the transmission. When a particular drum is not needed, the clutch inside it is disengaged and the transmission can turn independent of that particular drum. When the clutch is engaged that drum is then locked onto the drivetrain, performing whatever task it is intended for.
Unlike manuals, there is often no simple layout for an automatic, by which I mean no “1st gear” and “2nd gear” clutch. Instead, there will often be an arrangement of, for example, “A” clutch, “B” clutch, and so on. Gear ratios will be achieved by engaging a particular combination, such as “A” clutch alone for first gear, and “A” and “C” clutch for second gear.
Automatic transmissions are hydraulic, meaning that the fluid that is in the transmission is not simply a lubricant like the fluid in a manual. A component called a hydraulic control unit, or “valve body” directs the flow of oil to where it needs to go, engaging and disengaging clutches. This is all controlled by the TCM (transmission control module) the computer that decides what the transmission should be doing.
The design of an automatic means that there are noticeable power losses over a manual alternative due to the increased friction inside the gearbox. It also means that there is often very little that can go wrong that doesn’t require some major (and expensive) repair work.
Dual Clutch Transmission
A relatively new player in the transmission game and one that is rapidly gaining ground in the automotive market is the dual clutch transmission (DCT). The goal of this transmission is to take all the best bits from both worlds and combine them in a way that the semi-automatic failed to.
- Comfortable driving experience
- Ease of use
- Improved efficiency over fully automatics
- Lower net weight than fully automatics
- Clutch replacement requires specialist training/tools
- Prone to expensive electronics failures
Essentially, a DCT is a manual transmission inside, using mechanical gears to achieve the necessary ratios. Unlike a semi-automatic, however, it also contains a hydraulic control unit—or valve body—which controls the applications of the clutches and the movement of the gear selector forks. It’s the “dual” in dual clutch transmission that sets them apart, however. Unlike a regular manual or semi-automatic transmission, a DCT has two clutches that are each responsible for a different set of gears. Typically, one clutch will handle odd-numbered gears while the other will handle even-numbered gears. This greatly reduces shifting time as a clutch can begin engaging before the other has finished disengaging.
Of course, the use of manual-style gears means an improvement in efficiency over its fully automatic predecessors. The only real downside is that this kind of transmission can be susceptible to electronics failures, specifically in the control module, which can be costly to repair. That kink aside, this transmission has been slowly growing in market share and is expected to be the most common type of transmission in the next decade.
Dual Clutch Transmission - How It Works
These are the different kinds of transmission you are likely to find in a typical, road-going vehicle. Although I will add the caveat that this probably doesn’t include supercars, agricultural tractors, haulage vehicles, or other special-use vehicles, the principles involved in those transmissions are broadly the same.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2016 John Bullock