In our never-ending pursuit to help our readers to be as mechanically inclined as they can be, we posted an article—just a few short weeks ago—detailing the various different types of engine configurations found in cars today and in the past. And while engines do play the largest role in powering a car, they do not make up the entire puzzle that is a car’s drivetrain.
Instead, we need another piece of equipment to translate the engine’s power into forward motion, and that piece is the transmission. So, as a follow-up of sorts to that last article, let’s make our way down the driveline and talk about transmissions! This article will go over how a transmission works and the different types of transmissions used, as well as their applications in various makes and models.
What is a transmission and how does it work?
Before we get into explaining how a transmission works, we first need to understand one very important thing about engines: they are optimized to work best at certain RPMs. Every engine has what is called a power band, and this is the RPM range (usually close to redline) within which the engine produces the most power and torque for maximum acceleration.
Conversely, every engine also has an RPM range where it is most fuel efficient, and this is where your car likes to be during normal driving, for obvious reasons.
Now with that out of the way, we can look into why a transmission is so important. A transmission’s job is to use its different gears to keep your engine in whatever RPM range is optimal based on the driving conditions. If you want acceleration, it will select a gear that brings your engine into the power band.
If you want efficiency, it will select a gear that allows your engine’s RPMs to remain low. Without a transmission with different gears, your car would be either really slow with a high top speed (one long gear) or really fast but have a very low top speed (one short gear). Both of these scenarios are rather undesirable, so a transmission allows your car to have both a reasonable top speed as well as reasonable acceleration because it has numerous gears (the best of both worlds!).
A transmission is connected to the engine via a number of different methods (which we’ll discuss shortly), and takes the engines output, its power, as an input. The engine’s power will then be used to spin whatever gear is selected at the time, and then send that power out to the wheels to move your car.
Different Types of Transmissions
Now that we understand why transmissions are important as well as how they work, we can get into discussing some of the different types of transmissions used in most road cars. While transmissions do differ in their internal components, the main difference between the below-mentioned types is the way in which they are connected to the engine. Transmissions must briefly disconnect from the engine in order to switch gears, and the different types of transmissions go about doing this in a variety of ways. And of course, they can vary in the method used to actually select gears (i.e. automatic vs. manual gear selection).
Automatic transmissions are by far the most common type of transmission found in cars on the road today. This is largely due to their ease of use, allowing drivers to throw it in drive and motor away.
Automatics are distinct from other types of transmissions in that they connect to the engine via a torque converter. Torque converters use fluid inside of them to connect and disconnect from the transmission, as well as deliver the engine’s power to the transmission. As the name would suggest, gear selection with an automatic transmission is done automatically.
Manual transmissions have fallen out of popularity recently, largely due to the lack of demand for them in most markets. However, they remain the transmission of choice for car enthusiasts, as they’re just some much fun to use! Manual transmissions are also the most labor intensive to drive though, as it requires the driver to manually select gears, as well as decide, via the use of a clutch pedal, when the engine should and should not be connected to the transmission. These types of transmissions are connected to the engine via the coupling of a clutch and flywheel, both of which are friction disks. The clutch spins with the transmission, and the flywheel spins with the engine, and when the clutch pedal is released, the two join together, and the engine and transmission are connected. Gear selection is also done manually using a shift lever, allowing the driver to decide which gear is best for the given conditions.
Automated manual transmissions
Automated manual transmissions operate just about exactly as the name would suggest. They use the clutch and flywheel coupling of a manual, but the engine engagement and gear selection of an automatic. In other words, they have roughly the same hardware as a manual, but the convenience of an automatic in that the driver doesn’t have to control the clutch and gear selection can be done automatically.
These types of transmissions are mostly used in performance applications because of their quick-shifting ability (although advancements in traditional torque converter automatics are starting to catch up). Additionally, this category encompasses the high-performing dual-clutch transmission. Dual-clutch transmissions have two clutches, meaning that one gear can be engaged and the other can be ready to engage, with a separate clutch, in order to reduce shift times to an absolute minimum.
CVT transmissions (continuously variable transmissions) are the least performance-oriented of all the different types here, but they work in a very interesting way. They consist of a series of pulleys, and are also connected to the engine via a pulley. These pulleys are continuously variable in that they can keep the engine RPM at a very specific point, rather than just within a range. In other words, they use pulleys in place of gears, and these pulleys are more flexible in their ability to control engine RPM. This means that they are very efficient, and can help cars achieve strong fuel-economy ratings. For this reason, they’re typically used in cars with smaller engines designed purely for efficient commuting.
As with many things, different automakers use different names to describe roughly the same thing. For example, Audi, BMW, and Mercedes all offer all-wheel drive systems, but they call those systems Quattro, xDrive, and 4Matic, respectively. The same concept applies to transmissions, so let’s take a second to break down some of the common names for some common transmissions.
- Automatic Transmission Names
- Tiptronic: Tiptronic was the name used by VW/Audi to describe their shiftable automatic transmissions. They were manually shiftable via paddle shifters and the gear lever, but used all the hardware of a normal automatic.
- Steptronic: Steptronic is BMW’s name for their shiftable automatic transmissions, very similar to Audi’s Tiptronic
- Automated Manual Transmissions
- DSG: DSG stands for direct shift gearbox, and it is the term that VW and Audi use to describe their dual-clutch transmissions.
- PDK: PDK stands for Porsche Doppelkupplungsgetriebe, which is German for Porsche Double Clutch. This term is used to describe Porsche’s widely renowned double-clutch transmission.
- SMG: SMG, which stands for sequential manual transmission, was used by BMW to describe their single-clutch automatic transmissions used in the 2000’s in their E46 M3 and E60 M5.
- DCT: DCT stands for dual-clutch transmission, and it is BMW’s name for their, well… dual dual-clutch transmission.
While the above list is far from all-inclusive, it can serve to show two main points. The first is that manufacturers will always find a way to make their product sound different than others, even if it’s mostly the same. Secondly, it’s really only German luxury brands that take the time to give their transmissions bespoke names, and in most cases they only do so for their performance-oriented gearboxes. While other brands do it, the names are not nearly as heavily marketed as they are with the Germans.
As we can see, transmissions are not only important, but complicated, and highly variable in their operation. However, most of the information in this article can be boiled down to the following premise: if you want to shift yourself, buy a manual transmission car, and if you don’t, buy anything else. While those that want to be more in tune with their car will make a more nuanced decision, the general populous can be satisfied by that general rule.