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10/05/2011
The gearbox

From the engine to the wheels (Part two)


Unless it is for very special uses, the internal combustion engine, whether it runs on petrol or diesel, does not have the characteristics that will allow it to drive wheels with a fixed gear ratio. This is why a component is mounted between it and the transmission to vary this ratio: the gearbox.

Nicodemo Angì

Obviously, transmitting engine power to the wheels is necessary to make the vehicle move, but it is not enough, given that our propulsion system imposes the use of other mechanical components that will restrain its "fiery" temper.

 

We now know more about the clutch: in the previous article we attempted to explain the use and appearance of a component that is essential for transmitting engine power to where it is needed - to those wheels that Pneurama readers know all about.
Obviously, transmitting engine power to the wheels is necessary to make the vehicle move, but it is not enough, given that our propulsion system imposes the use of other mechanical components that will restrain its "fiery" temper.
To define the latter, we must remember that the power it produces is definable by certain "qualities", just like temperature defines the quality of heat. In the case of engines (in general, and not just the internal combustion engine), they are power and torque.
The first quantifies the engine's ability to do the job more or less quickly, whereas the second tells us how much "force" the engine delivers at a certain number of rotations.
In other words: a more powerful engine will lift the same weight more quickly or make the same car (or boat or train) move faster than a less powerful engine: we could mention hundreds of examples, but the concept is always the same.
Torque, on the other hand, is rotational movement and is expressed as the ratio between force and the distance from the point of application, which is the axis around which the movement takes place: this is expressed as F/d and then as kg/metre or Newton/metre (Newton is the currently used international unit for measuring force). To understand what it is, just unscrew a bolt with a spanner: the force is your hand and the distance at which it is applied is the length of the spanner.

 

Variable ratios
If we consider that the power delivered by an internal combustion or endothermic engine (there are also exothermic engines such as steam engines, but they are not of any practical interest to us here) is created by fuel exploding inside cylinders, it is not difficult to understand that the more explosions there are during a unit of time - for example, one minute - the more power is produced by the engine.
The idea is right: as rotational speed increases, the power available to the engine shaft also increases; this increase is not indefinite because at a certain number of rotations, the power stops increasing and starts to decrease. The reason is simple: it becomes more difficult for the engine to "breathe", friction and losses increase and the power delivered decreases instead of increasing. The torque produced by the engine is quite different in that it tends to decrease as rotation increases: this gives an idea of engine output and, in effect, specific consumption (how many grams of fuel does an engine consume every hour to produce one horsepower, a figure that is independent of the number of cylinders and maximum power) is minimum at maximum torque rotation. When the output - and therefore the torque - decreases too much as rotation increases, the increase in useful stages during the unit of time will not compensate for this drop and so the power decreases even if rotation increases.
So it is fairly obvious (as the graphs of torque and power confirm) that every engine performs better if its rotations are neither too high nor too low. For a "normal" petrol engine, the revolutions per minute are between 1,500 and 6,000, but a diesel engine, which cannot turn as fast, has reached its peak at around 4,500-5,000 revs/min.
The presence of the gearbox is linked to the need to keep the engine running within this speed range, with a maximum/minimum ratio of 4 (for a petrol engine), when speed can vary during a much wider interval: between the 130 km/h motorway limit and the 5 km/h of walking pace the ratio is in fact 26. It is understandable that the ratio between the engine revolutions and those of the wheels (directly proportional to the speed of the vehicle) cannot be constant but must change.
The majority of gearboxes have pairs of gears of different sizes: one gear in each pair is driven by the engine and the other is connected to the wheels (not directly, but by the rest of the transmission).
The different sizes of the various gears means that each one must have a different number of teeth and it is this difference that determines the gear ratio of the relevant pair. It is easy to understand how this quantity is expressed: if the gear connected to the engine has 20 teeth and the other one has 40, when the first one has completed its rotation, the second one will have completed only half a turn. So, we can say that the gear ratio is 1:2; in other words, the gear that is driven (the one connected to the wheels) completes half a turn for every complete turn of the drive gear.
It must be pointed out that there are also multiple-ratio gear systems by which the driven gear turns the engine faster; the wheels will turn more slowly than the engine because the differential is reduced, i.e. a drive/driven ratio of less than 1.
There are different practical reasons for the arrangement of the pairs of gears, but in all of them one of the components of each pair (in general there are two for each gear) is an integral part of a gear shaft (it can also be machined out of the shaft itself) but the other one is a toothed cog and meshes with its shaft only when required.
The development of the technique has not only perfected the design and construction of the gears but also of the cogs by fitting them with synchronizers (like small clutches): they match the speed of the gear to that of the shaft before they engage. This technology is so advanced that the mechanical output of a pair of gears is currently more than 90% and very little power is dissipated by the complete gearbox.

 

Gears or belts, the purpose is to change
These pairs can be selected manually - using the lever we all know well - or automatically with electronically-coordinated servomotors; they release the clutch, select the gear and then engage the clutch again, a mechanism that is similar to that of a manual shift. A recent and highly rated variation is the dual-clutch gearbox in which the pairs of gears are distributed on two shafts instead of one: each shaft has its own clutch. This means that traction is never interrupted because the next gear can be selected while the previous one is still engaged: the change from one to the other only involves the opening of one clutch and the closing of the other, actions that are coordinated by an electronic "controller".
If they are well designed, automatic transmissions with torque convertors have always had the advantage of smoother gear changing because the converter itself (as we saw in the previous article) is hydraulic and not rigid like a mechanical converter, and because the pairs of gears are replaced with epicyclic gearing. These gears have parallel axes: starting from the centre, the toothed shaft is called the sun and it is surrounded by planets which contemporaneously mesh with it and another external gear called the crown. The gear ratio varies depending on whether movement is taken from the planetary gears, the crown or the sun; the selection is made by locking one or other of these components with the clutches or brakes. The smoothness of the change is due to the fact that the gears are always engaged - which avoids potential grinding - while the blocking of the shafts can be gradual by activating the clutches and brakes.
With the exception of Honda motorcycles, hydraulic speed changers are used only on industrial vehicles, whereas expandable pulley changers are used in some cars and the majority of scooters.
It is a gear that uses a flexible belt made of rubber or of metal rings and plates to transmit movement from a pulley driven by the engine to one connected to the drive wheel. The drive and driven pulleys are divided in halves which can come to together: the closer they are the more the pulley will operate in a wide diameter. When the vehicle is stationary and the engine is at minimum, the halves will away from each other, the drive pulley will "widen" and the belt will move close to the axis of the pulley itself. The opposite occurs with the driven pulley: its halves will be close together - a calibrated spring would do this - and the belt will be away from the axis; the centrifugal clutch on this pulley will not mesh and the wheel will not be driven.
With acceleration two things happen: the clutch will engage, drive the wheel, and the weights on the drive pulley hub will be forced to the outside by the centrifugal force, bringing the pulley halves together and forcing the pulley to operate with a wider diameter. Given that the belt cannot stretch it will tend to move towards the axis of the driven pulley, act against the power of the spring and separate the two halves, with the result that the belt winding diameter will now be greater for the drive pulley than that of the driven pulley. The gear ratio will simulate a high gear in the sense that the engine will turn slowly compared to the wheel, as if we had engaged fifth or sixth gear. The ratios can also be varied manually by using a small motor that maintains the distance between the drive pulley halves.

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