Automatic transmission - how the automatic transmission works

The invention of automatic transmission dates back to the first half of the twentieth century, and its creation is partly due to one of our people, from then Yugoslavia.

This form of transmission has come a long way in adaptation and today it is being refined, especially in the last few years when electronics in the auto industry are taking primacy.

Only nowadays, manual transmission it is slowly disappearing from the scene, giving way to some modern and extra-efficient solutions in the form of some automatic-electronic transmission. Of course, we all know that these two types of (manual and automatic) transmissions have the same purpose, but they work in a completely different way.

So, the goal is to get the engine speeds to "align" or to the wheels themselves, in the most efficient way possible. Both ways have their advantages and disadvantages, but about that at the end of the text…

What they are, though, is that automated cars are much easier to drive and, moreover, more durable. In manual transmission models, the driver itself determines when and how much gear it will shift to the next gear, while the automation always has its "program" of operation and there may not be any possible load on the aggregate due to the driver being overly aggressive or something like that. The aforementioned “program” of work is very complex and requires numerous operations at one particular time, so that we will not go into that issue too much. In the next part of the text we will see some basics of work and the most important parts of a standard automatic transmission.

Comparing the automatic to the manual transmission, we will notice that there are almost no technical similarities in them!

The whole automation circuit used to be very large, but today, with the presence of electronics, all of this has been reduced significantly. It used to be extremely difficult to reconcile all the important factors for the normal operation of such a transmission, such as eg. engine speed, vehicle speed, accelerator pedal pressure, etc.

For each of these factors, there was a certain device that would be part of that one, large assembly within the automatics. Today, with the presence of the ECU and other electronic circuits, all this is easier, but more importantly, more accurate. This is one big difference between manual and automatic transmission - the manual itself takes the driver into account these factors and everything is, therefore, much more individual.

As far as technical differences are concerned, the basic one is related to the clutch. What you probably know is that the automatic transmission has no clutch and the manual has only one. However, what is true is that the automatic transmission does not have only a clutch pedal, while at least (four) clutches are included within the assembly of its system! I said that the automation is much more complicated…

The next major difference is in the combination of the engine and gearbox. We all know how simple it is in the manual world - this joint contains the "output" of that single clutch, which, with the help of "basket" and other assemblies, moving the pedals simply separates the engine from the transmission.

Automation uses a special and complex converter. It is, of course, in the same location, but it is slightly larger in size. In principle, it is attached to the flywheel ("autput") of the engine itself, while on the other hand, the shaft that goes inside the automatic transmission system (gearbox). The operation of the converter is based on the circulation of the fluid inside it, from the pump to the turbine, which are also, in fact, inside the converter. The pump is started by the engine and therefore it turns in the same direction as the motor output. With the help of sharp and angled pump wings, the fluid is delivered to a turbine directly connected to the transmission.

This is how the circuits inside the gearbox are actuated, that is, the indirect connection of the motor and gearbox is achieved. The main function of this system is to enable the car to be stationary even when the engine is running or when its "output" is turning. That's why it's called a converter - it provides less power at low operating mode, while at higher speeds it also provides the (real) amount of torque required for vehicle movement. Ideally, at such (slightly higher) transmission speeds, ie "output" to the differential and drive wheels, it should rotate at the same speed as the engine, which is the case with a manual transmission. Automatic, however, rarely succeeds, but its goal is always to be closer to the engine. This will be counted at the end of the text in the drawbacks of automation - thus unnecessarily consuming engine power - there is always a lower maximum speed and higher fuel consumption of each vehicle!

Just not to forget to mention the stator, as the fourth important part inside the converter (in addition to the fluid, pump and turbine). It is located in the center of this small system and allows the fluid to flow properly from the turbine back to the pump. Without the stator, the fluid would return to the pump in the opposite direction of rotation and this would also cause the engine to "choke". So, for the most part, fluid is the main link between the engine and the transmission!

And now about the automation itself… As with manual transmission, the main function is to ensure optimum engine performance in all modes. We have a few gears here as well as the manual one, but it's not quite the same.

In automation we have many more (non-classical) gears of different shapes and sizes, and the "speeds" themselves are somewhat more complex. It all comes down to combinations of the work of certain gears.

In manual gear we have different gears for different gears (“gears”) - just like in a bicycle. In automation, different gears are output by a set of the same gears, only differently combined. So the basis of an automatic transmission system is to mention the gear set!

Other parts of the system are clutches and straps, as well as their hydraulics, then various valves and formwork. All these parts are there to allow the gears to function properly. Each automatic transmission system usually has two sets of these gears, while each set has three types of gears. These are central, outer and ring gears, with always more outer ones.

To this list should be added an external gear carrier, which also rotates. We will explain the locations with the help of the attached cross-section of one set of gears - the central one in the center is orange, around it are the outer ("planetary") with an all purple carrier, while all of them are "encircled" by a ring, blue. Each of these gears can rotate and each of them can be "input" (to receive rpm from the converter, that is, the motor) and "output" (to send rpm to the differential, ie wheels) and all of them are directly, directly or indirectly , connected. In fact, one may be an input, the other an output while one of them is always idle. By combining this, thanks to the different sizes and shapes of these gears, we get different gears! I said that each system usually has two sets of these gears - so we get even more combinations, ie. such a system provides us with four optimal "speeds" and a reverse. This system contains two central (different sizes), two sets of outer ones and ONE enlarge ring gear (which connects them all inside). Now, here is where the easier part of the text should end, and we should already move slightly into the sphere of explaining the possible combinations of all these gears, which specifically makes the gears.

I will try to simplify this whole story to the maximum and we will tell only what goes on in the very process of functioning of this system. As I mentioned earlier, two of the three sets (+ outer gear mount) always move while the 'lock' of any two sets creates a 1: 1 reduction ratio. You may have read that in the text about the manual transmission system, but this relationship is between the speed of the engine output and the transmission. So 1000 rpm to 1000 rpm! Such a ratio is common at slightly higher speeds and is present mainly in the third gear in automatic. This ratio can always be easily calculated using existing calculation formulas, which are specific to the automatic transmission system and depend solely on the types of gears that are active and the number of their gears. So, let's start in the first place - in the first gear with automatic input is a central gear with 30 teeth (usually), so it is driven by a converter, an engine. In this gear, the outer gear carrier is static so that the connection to the ring gear (72 gears), which will be output, is only external. The formula for calculating this ratio is: - 72/30 = -2,4. We notice that the ratio is -2,4: 1, therefore negative! This should not confuse and say that the output moves in the opposite direction to the input - however, this is not true because external gears, as intermediaries, contribute to the correct rotation of the output (ring gear)!
It's something like the reverse gear ratio of a manual transmission, with that little gear between the two big ones that changes the direction of rotation ...

It is worth mentioning that there are two types of these relationships - reduction, when the output makes less revs than the input and "overdrive", which is the opposite of the reduction (autput faster than the inputs). So the first gear in automatic is reduction, as is the case with the second gear. The second stage is the most complex in the whole story and consists of two active combinations of gear work! In fact, we now have two inputs and two outputs. How?! First, we have that standard input from the converter which, in this case, drives the smaller center gear. Then we have a larger center gear which is fixed by a belt and does not rotate, and as an output is a carrier of external gears. The output, therefore, rotates and has a reduction ratio of 1 + 36/30 = 2,2: 1 (the ratio of the number of gears of the larger and smaller center gears). That output is passed on and forms the input for the next set of gears, where the final output is actually a ring gear that is driven through the outer gears. In the second part of this story we have an “overdrive” ratio: 1 / (1 + 36/72) = 0,67: 1! Now, how is the whole other "speed" nevertheless reducing? Well, because 0,67: 1 is not a second-rate final ratio - that first reduction ratio (2,2: 1) must also be taken into account. This is done by multiplying 2,2 by 0,67 to get a final ratio of 1,47: 1. Sounds pretty banged up, doesn't it?

The third gear is already 'lemonade' compared to these first two ... I have already mentioned that the third gear almost always uses a 1: 1 ratio by fixing two elements within the whole system. We will do this very easily with the help of one of the 'clutches' that will fix both the center gears, which otherwise represent a direct connection to the turbine inside the converter! This eliminates the effect of the turbine and easily establishes a 1: 1 (direct) connection to the engine. So the whole transmission is spinning at the same pace as the engine! After the third degree, we have the only degree that is in overdrive mode - the fourth. Logically, we will again avoid operating the turbine, which in principle only helps us at lower speeds and lower gears, as well as when the vehicle is stationary. We avoid it by eliminating the central gears, which are directly connected to the same… By their 'locking' (with the help of clutches and straps), we leave only the engine flywheel at the converter, which is directly connected to the input, which in this case outer gear mount. It, with the help of the external gears itself, drives a large ring gear, which is output ... The formula for this 'overdrive' mode is the same as the second part of the second gear: 1 / (1 + 36/72) = 0,67: 1. So at 2000 rpm the transmission will spin at 3000 rpm! This allows for a very quiet and cultivated open-road ride ... As far as the reverse is concerned, it is very similar to first gear. The only difference is that a larger central gear (36 teeth) is used as an input instead of a smaller one. in this case, the special belt holds the outer gear carrier, so that a real negative reduction ratio is still possible - the output rotates in the opposite direction to the input. The formula says: - 72/36 = - 2,0: 1.

What I often mentioned during the previous story, and I didn't get to explain, are the clutches and straps within the automation system. As I said at the beginning of the text, these parts are there to make all these gear sets work. Their function is to enable or not to enable one of the gears. This whole mode of operation is one meaningful system - when the vehicle moves faster and the gear changes, then the clutches and straps mentioned, that is, the hydraulics that drive them and permeate the entire transmission system! The strips are generally circular in shape, aiming to stop the operation (rotation) of a particular assembly (gears or beams), while the clutches are there to connect the gear sets to each other or to an input (converter) or output (to differential).

In the system that I presented to you today, there are a total of 4 clutches. So, in order for all this to be possible, one hydraulic system is needed to move all these belts and clutches. In addition to all these parts, each automatic transmission system must also include an oil pump in the transmission. It's the same pump from the beginning of the story - the one in the converter that directly drives its turbine. However, this pump still drives a lot of things - first of all the entire hydraulic system and the transmission cooler. On older models of automatic transmission, there was also a control valve, which was needed by this system because it monitored the speed of the vehicle, which is a very important factor in the proper operation of this transmission. So, the faster the car would move, the more open this valve would become. This is recognized by the hydraulic system inside the automation and reacts instantly with a possible gear change. And this change of gear is also controlled separately, with the help of special valves, one valve for each gear. Thus, when certain values ​​of all relevant factors are reached, the “first gear” valve will open and thus allow transmission to the second stage. The same applies to the transmission in third or fourth gear "with the help of a special valve.

In terms of the factors themselves, in addition to the speed of the vehicle, the pressure of the engine is important, so practically how fast the engine is turning. Of course, you guessed it, this also controls a special valve, and there are two types, one of which is used in the system. One type controls the pressure on the accelerator pedal itself, while the other type controls the pressure itself in the engine. Of course, today all this is done with the help of electronics, under control ECUSo, the transmission possibilities are much bigger and almost every day, some new, inventive solutions are being constructed… So today the transmission system takes many more factors into consideration - whether the road is slippery, whether it is uphill or downhill, the pace of driving, etc. . This all enhances the efficiency of the entire system, allowing for a smoother and easier ride, with greater acceleration and lower power consumption.

What many people are probably wondering is what all the positions on the automatic gearbox handle mean? Well, from the top down, there is a 'P' which means 'parking', right, and that's when the vehicle brakes through the transmission - the special grooves on the transmission system's output groove so that the differential (and therefore the wheels) cannot rotate. Next is 'R' as 'reverse', or reverse, which we have already talked about ... 'N' is 'neutral', that is, the back of the automatic - then the transmission is completely separated from the engine. The 'D' is the 'drive', of course, when the car is in motion. The numbers '1' and '2' actually indicate the gears you want the car to be in. So if you put in '1' or '2', the car will never exceed the first or second 'speed'. And at the end of the text I said I would mention something about the advantages and disadvantages between manual and automatic transmission.

As I said before, the big disadvantage of automatics is that very rarely the real speed of the engine and the transmission can be reconciled, so that then there is an unnecessary "throw" of engine power, which causes more consumption and lower maximum speed. Of course, there is also a lack of individuality in driving, as you can in no way affect the transmission and will have a hard time getting the most out of the powertrain. On the other hand, the great advantage of automation is that a car with this kind of transmission is less corrupt. This is logical, because then the proper operation of the engine is not the concern of the driver, but the transmission system itself and allows constant optimal use and exploitation of the engine. The benefits are also very easy to use, especially in urban, urban environments. Simply insert it into the drive and don't think about the transmission during the entire ride. It is a great advantage of an automatic transmission and when, God forbid, you happen to injure your left ankle, and you have to drive home or to the doctor on duty. Then you will be glad the absence of the clutch pedal on the left side of the foot controls…

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