John is a fervent writer, gamer, and guitar lover. Former automatic-transmission repairer, welder and hobbyist game developer.
The average road goer rarely finds cause to concern themselves with the exact details of how their vehicle works. So long as they know how to work it at their end, they don't really need to know what's happening under the hood. That being said, there are many curious souls out there who just like to know how things work, especially things they use from day to day.
In this article, we'll be looking specifically at semi-automatic transmissions in cars (as opposed to motorbikes, commercial vehicles, and other uncommon vehicles), and focusing on how they work. If you want to know more about the differences between semi-automatic and other types of transmission, check out "Vehicle Transmission Types and Their Differences" for a more detailed comparison.
For this article, we're going to be focusing on modern semi-automatic transmissions.
The first aspect of modern semi-automatic transmissions that we'll discuss, and perhaps the most significant, is that they are not actually "automatic" transmission at all in the sense that an automatic transmission is. Semi-automatic transmissions are just manual transmissions ("stick" if you prefer) without the clutch pedal. And when I say they are just manual transmissions, I'm speaking literally. The hardware, often the entire gearbox, is the very same that would be used for a manual transmission, whereas fully automatic transmissions operate in an entirely different way.
Where the semi-automatic transmission differs from a manual transmission, and the part that makes it semi-automatic, is that instead of cables and pipes running from the transmission to the driver so that they can affect gear changes, you have actuators and computers doing all that hard work for you.
The advantage of a semi-automatic is that it retains the advantages of a manual transmission (namely low cost and better fuel efficiency), with the ease of use one gets from driving a fully automatic vehicle (which tend cost an arm and a leg as well as sucking up more fuel).
A Brief Note on Dual Clutch Transmissions
There are a number of transmissions that employ a dual clutch technology that technically fall under the umbrella of semi-automatic. For the sake of simplicity, we won't be covering them in this article.
By now you're probably wondering, "well, what's the catch? Why not just use semi automatic transmissions all the time?"
The truth of the semi automatic transmission is that it's really a half way measure. It's bolting extra bits on to one component in order to make it act like another, and like most half way measures, it's not as effective as a dedicated solution. Semi automatic transmissions tend to have "clunky" gear changes when driven in automatic mode, and standard procedures—such as replacing a clutch—can often require expensive diagnostic equipment.
Of course, life is full of compromise, and enough people are willing to take this half way measure that it's offered as an option in many vehicles.
A Little History
Semi-automatic transmissions have actually been around for a very long time, as early as the 1930s, in fact. Of course, the systems employed back then were very different from what you see in modern vehicles, but then, so was everything else about cars.
Rather than doing away with the clutch pedal entirely, many of the early semi automatics aimed to reduce the need to use the clutch. These early methods often involved replacing the standard dry clutch with a fluid coupling or centrifugal clutch, but ultimately fell out of favour as technology advanced.
How it Works
The transmission itself is conceptually the same as any other type of transmission in that it takes the kinetic energy produced by the engine in the form of a turning input shaft, and, through the use of various gears, transfers it out where it can turn the wheels.
Use of a transmission greatly increases the efficiency of an engine. An engine with no transmission would likely need to produce up to four times as much power to move a vehicle from a standstill than one with a gearbox!
In between the transmission and the engine is a clutch which allows controlled "slipping," allowing the transmission to not only engage (be driven by the engine) and disengage (spin or not spin independently of the engine), but also partially engage. This allows for smoother gear changes without the need for pinpoint accurate timing when changing gear.
In a regular manual transmission, the driver would control both the clutch and gear shifting by means of a pedal to engage and disengage the clutch, and a lever to shift gears. Semi-automatic transmissions do away with this by replacing the gear lever with a set of actuators, and the clutch pedal with a hydraulic motor. A computer (sometimes referred to as "TCM," or "Transmission Control Unit") monitors various inputs, such as the vehicle speed, engine torque, accelerator pedal position, and more in order to determine when to change gear and in which direction.
When it decides a gear change is necessary, it will engage the clutch which in turn disengages the transmission from the engine, then activate the shift actuators to effect a gear change, then finally disengage the clutch so that engine is once again driving the transmission.
How to Tell If It's Semi Automatic
There are times when it's not always easy to tell whether a particular vehicle is semi-automatic... unless, of course, you know of some telltale signs that are unique to a semi-automatic gearbox. This is, of course, assuming you don't have any documents about the car, or an owner who can tell you.
The first sign, and the easiest to spot, is the gear lever. A manual transmission will usually have the gear numbers all laid out in a "fork" pattern, whereas an automatic if it has a gear lever at all, will show the typical "PRND" layout (for Park, Reverse, Neutral, and Drive).
A semi-automatic may have a lever or push-button controls, but it's the selection that's the giveaway. Semi-automatics don't layout the gears like a manual does, but they also differ from automatics. Semi-autos have no "Park," and instead of a "Drive" selection, it they tend to have "Automatic." Though the actual selection can vary, most modern cars present a "+" and "-" for moving through the gears manually. So the selection will be neutral, reverse, automatic, and manual +/-. Check the picture below if that was a bit much to follow.
The next thing you can do is take a look under the hood. This may be tricky or impractical if the vehicle has a lot going on in there and you can't see the transmission itself for other components. In general, though, you're looking out for linkage cables and transmission cooler pipes as signs that it's not a semi-automatic.
What Cars Have a Semi-automatic Transmission
It would be impractical to attempt to list every vehicle that qualifies as a semi-automatic, especially if we were to count historical vehicles, but we will touch on a few modern vehicles that use this system.
- Opel (or Vauxhall, depending on where you are in the world) have used their "Easytronic" semi-automatic transmission in a variety of their cars, though typically in the smaller vehicles such as the Corsa.
- Ford also made use of the Easytronic transmission in some of their smaller vehicles, such as the Fiesta, and the Fusion.
- Alpha Romeo/Fiat made use of a similar system to Opel's Easytronic which went by the name of Selespeed, which often comes with paddle shifters.
- Smart represents one of the more common uses for a semi-automatic transmission as it was the only options for a while in their popular town car, the Fortwo.
So now you know a little more about what a semi-automatic transmission is... or do you? Why not take the short quiz below and test yourself.
For each question, choose the best answer. The answer key is below.
- Which gear lever selection is a semi automatic?
- P R N D
- 1 2 3 4 5 R
- A N R M+/-
- Semi automatics have a clutch pedal
- Semi automatics are more ______ than full automatics
- A N R M+/-
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
© 2016 John Bullock