Read At Home: How A Car Gearbox Works

2023 Author: Natalie MacDonald | [email protected]. Last modified: 2023-11-26 13:59

We have already talked about how the internal combustion engine works. It's time to see how the unit that helps transfer engine energy to the wheels is arranged and works - the gearbox. Sit down more comfortably: now we'll tell you everything

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Today, cars use different types of gearboxes, radically different in design - each with its own advantages and disadvantages. But first, let's answer the most important question …

Why does a car even need a gearbox?

Due to the peculiarities of the internal combustion engine. At low revs, it does not always have enough strength (torque) to rotate the wheels and move the car. To help the motor, you need to give it the opportunity to spin faster at a low speed of the car - for this, the engine is connected to the wheels through transmission.

The simplest gear is two gears of different sizes, meshed with teeth. Imagine one has three times as many teeth as the other. Then, in one revolution of the large gear, the small gear will already make three revolutions. Conversely, by connecting the engine to a small gear, and the wheels to a large gear, we will make them spin three times slower than the crankshaft. Another plus: the torque that turns the wheels will also be three times the engine torque.

But when the speed of the car doubles, the engine speed will increase by six times. And it cannot rotate too fast - the fuel simply will not have time to burn. Therefore, as the engine accelerates, it will need another pair of gears - with a less dramatic difference in the number of teeth (it is called the gear ratio). Modern passenger cars have 5-6 different gears (or steps), and some have nine. And the gearbox is the unit in which they are all brought together.

So the gearbox is just a bunch of gears?

Yes and no. In reality, everything is more complicated. In addition to the transmissions themselves, mechanisms are also needed that allow these transmissions to be changed. And gears are just one type of transmission. For more than a hundred years of the existence of cars, many mechanisms have been invented - from the simplest pulleys, between which a drive belt was thrown (similar to how it is done with a chain on bicycles), to completely exotic designs. And today four types of gearboxes are used in cars: mechanical, hydromechanical, robotic and variator. Each one works in its own way.

Manual Transmission

The device of a manual transmission is closest to the example that we considered at the very beginning. The gears in it are just a pair of gears: a leading plus a driven, with different gear ratios. All drive gears are mounted on the input shaft from the engine, and the driven gears are mounted on the output, which transmits torque to the wheels. But the former are rigidly connected to the shaft, while the latter can rotate freely independently of the shaft.

If the driven gear (in the figure they are below) of any of the gears is nevertheless fixed on the shaft - it is rigidly connected to it, then the engine will turn the wheels with the corresponding gear ratio.

To fix the gears on the output shaft in the gearbox, gear couplings are used that can move along its axis. The driver depressing the clutch disconnects the input shaft (and with it the driven gears) from the engine. Then the driver engages the gear with a lever - moving the clutch to the desired gear. There is also a friction ring - a synchronizer between the gear and the clutch. The movement of the clutch forces it to press against the gear and slow it down or accelerate it to the speed of rotation of the shaft. When the speeds of rotation of the gear and the clutch are equal, the clutch is connected with its teeth with the gear and the gear is engaged. The driver only has to carefully release the clutch by reconnecting the gearbox to the motor.

How does the reverse gear work? In order for the car to go backward, the output shaft of the box must begin to rotate in the opposite direction. To do this, the gears on the input and output shafts are simply connected by another, intermediate. And since we turn on the reverse gear when the car is stationary (when the shafts in the box do not rotate), the reverse gear does not require a synchronizer and a clutch. Usually, the intermediate gear itself moves along its axis, clinging to the rest with its teeth.

At first, by the way, all the gears on the first manual transmissions in cars worked in the same way - but on the fly it was much more difficult to engage them than synchronized ones, the wear of the teeth increased. This is how the gearboxes we are used to today were born.

Robotic transmission

Over time, engineers wanted to make life easier for drivers by shifting clutch and gear shifting to automatic. This is how robotic boxes appeared.

The simplest version of the "robot" is a conventional mechanical box, in which the couplings are moved using servo drives at the command of the electronics. Other servos will release and release the clutch at the right time. But in practice, in the simplest "robots", electronics change gears like a novice driver - roughly and slowly.

Much better work preselective robotic boxes … In them, the gears are switched on in the same way - with couplings with synchronizers, and the torque is transmitted by gears from the input shaft to the output. Only the preselection box uses four shafts. On one pair there are gears of even gears, on the second - odd gears. There are also two clutches, each for its own input shaft. Roughly speaking, a preselective robotic box is two ordinary boxes combined in one.

How it works? While the car is driving, for example, in third gear, the automatic system in advance (hence the name "preselective") includes the fourth gear - if the car is accelerating, or the second - if the car is slowing down. In this case, the clutch, which is connected to the "even" shaft, is open, and this part of the box has no connection with the engine. The transition from third to fourth gear occurs at the moment when one clutch is disengaged and at the same time the second is activated. After that, the fifth (or first) stage is launched on the now inoperative “odd” driven shaft, and at the right moment the clutch castling occurs again. The electronics know how to close and open the clutches quickly and almost imperceptibly for the driver, therefore the gear change here is lightning-fast and practically without breaking the traction.

This is why such boxes have become common in sports cars.

Hydromechanical transmission

The box, which is colloquially called "automatic", because it also switches the steps without the participation of the driver. But the basis of its design is not a pair of gears, but a planetary gearbox, which is more complex (see figure). It has a central (sun) gear, a ring gear - a ring with teeth on the inner surface, and several satellites - small gears that mesh with the sun and the crown at the same time. The rotation axes of the satellites are interconnected by another part - a carrier. In this case, the sun gear, ring gear and carrier can rotate around one imaginary axis.

The uniqueness of the planetary gearbox is that its gear ratio is not fixed, as in the simplest gear with two gears, but can change depending on how its parts rotate. When the ring gear is stationary, the carrier spins several times slower than the sun. But if the crown begins to rotate in the same direction, the gear ratio from the sun to the carrier will decrease - and when the crown "catches up" the rest, it will become equal to unity. And then, with an increase in its speed, the transmission will increase, that is, the output shaft will start spinning faster than the input!

And if you transfer the rotation from the input shaft of the gearbox to the output through several such "planetary gears", connecting their different parts in different combinations - and, due to this, forcing them to spin at different speeds, you get a gearbox with several gears. You can even force the output shaft to turn in the opposite direction for reverse. Moreover, for a 6-speed "automatic", only three planetary gearboxes are enough, and with four you can already make 10 gears!

However, in the “automatic” it is much more difficult than in the “mechanics” to change gears - that is why the automation is engaged in this, and not the person. Each gear has its own combination of parts that need to be connected together. Or with the box body, stopping their rotation. To do this, automatic transmissions use multi-plate friction clutches, which work on the same principle as the clutch: when the “sandwich” of the discs is compressed, they stop sliding relative to each other - and the speeds of rotation of the parts with which the discs are connected are aligned. The clutches in a traditional "automatic" machine are compressed under oil pressure - and this pressure is created by a pump driven by the engine. When and to which clutch to supply oil, the electronics decides, which opens different valves in the valve body and directs the fluid through different channels.

Friction clutches operate smoother than gear clutches and do not require the motor to be completely disconnected from the gearbox when shifting gears. Therefore, the torque from the engine is transmitted to the gearbox not by the clutch, but by the torque converter - because of it the gearbox is called hydromechanical … The torque converter has three coaxial wheels with blades like a turbine: a driving wheel rotating with the motor, a driven one connected to the input shaft of the "machine", and the so-called reactor, which can be stationary or freely rotate in one direction. The space between these wheels is filled with oil. The blades of the driving wheel spin the oil flow, in the reactor the flow changes its direction and, in turn, spins the driven wheel. There is no rigid connection between the engine and the gearbox, so the "automatic" switches smoothly, without rough jerks and blows.

This is perhaps the most sophisticated gearbox used in automobiles. And if earlier "automatic machines" were not liked for their slowness and low efficiency (part of the engine's energy was spent on useless mixing of oil in the torque converter), today this type of transmission is very close to traditional manual transmissions in terms of efficiency, and to robotic ones in terms of gear change speed.

Variable speed drive

And in the variator there are no gears at all, which is why it is called continuously variable transmission … More precisely, there is a transmission, as it were, but only one, but with a variable gear ratio. And the torque is transmitted from the drive shaft to the driven one not through the direct engagement of the rotating pulleys, but through the belt connecting them. Just like the foot drive of the sewing machine at our grandmothers worked.

But both the pulleys and the belt of the variator are not simple. Each pulley is two cones on one shaft, with their vertices facing each other. The gap between them, in which the belt is located, can change - it becomes larger or smaller. If the cones are moved apart, the belt will be closer to the axis of their rotation and when the pulley rotates, it will move along a small radius. Conversely, as the cones approach each other, the belt will be forced outward and the radius will increase.

The difference in radii on the drive and driven shafts determines the gear ratio of the belt drive. Thus, it can be changed - by shifting the cones on one shaft and at the same time pushing them apart on the other. The main thing is that the belt on the pulleys does not slip - for this it is recruited from a multitude of transverse metal plates, connected into a loop with steel belts. Sometimes a special chain is used instead of a belt, but this does not change the principle of operation of the variator.

The cones move-move apart in about the same way as the friction clutches in the "automatic machine" are compressed - by oil pressure, which is regulated by the electronics. The variator also has other common units with the hydromechanical box. For example, a torque converter, which allows a car with a CVT to start and stop. Or a planetary gearbox - to engage reverse.

Today, CVTs are often used in low-cost vehicles as a more affordable alternative to traditional automatic transmissions.

So which box is the best?

There is no unequivocal answer to this question, therefore today they exist in the automotive world in parallel. One works very smoothly, but has less efficiency - which means that the car with it consumes more fuel. The other switches quickly and there is a minimum of losses - but too expensive and not very reliable. Each has its own advantages and disadvantages. An automaker, when deciding which box to put on a future car, takes into account many different factors: its price, operating conditions, and even the tastes of buyers. Please tell us which box do you prefer and why?