Showing posts with label Transmission System. Show all posts
Showing posts with label Transmission System. Show all posts

Future developments in the Automotive Transmission System

1.   The CVT will gradually replace the conventional automatic transmission due to its high fuel efficiency and smooth gear shift.
2.    The technology of semi-automatic transmission systems will also be improved to perform smooth gear shift and extend the cars' lifetime, without losing fast acceleration and fuel efficiency.
3.      The torque converter with fluid coupling may be improved, or may no longer be used for cars in the future due to its low-efficiency power transfer.
4.  Auto Shift Manual Transmission – This transmission system combines the advantages of an automatic transmission with the flexibility and low fuel consumption of a manual transmission. This is an advanced Shift-By-Wire electronic control system technology. Shift-by-wire totally eliminates mechanical lever shifting, keeping both of driver's hands on the wheel.
The clutch is used only for starting and stopping. Once the vehicle is in motion, Auto Shift operates like an automatic transmission, with the efficiency of a manual transmission.
5.     Adaptive transmission control - ATC has also been invented by using a computer to recognize and memorize different drivers' styles, and determining the best shifting timing for different drivers.
6.    A transmission system is needed for a vehicle due to the internal combustion engines property of running at high pressure at high speed but low pressure at low speed. If someday an engine with different properties is invented, the transmission system may no longer be necessary, but can still get the vehicle to reach its maximum speed in a couple of seconds.
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CONTINUOUSLY VARIABLE TRANSMISSION

 CVT is an “infinite speed” transmission which can change steplessly through an infinite number of effective gear ratios between maximum and minimum values. Unlike traditional automatic transmissions, continuously variable transmissions don't have a gearbox with a set number of gears, which means they don't have interlocking toothed wheels. The word gear in CVT refers to a ratio of engine shaft speed to driveshaft speed. Moreover, CVTs change this ratio without using a set of planetary gears.

Different types of CVTs –
1.     Pulley-based CVTs
2.     Toroidal CVTs
3.     Hydrostatic CVTs

The most common CVT design uses a segmented metal V-belt running between two pulleys. Each pulley consists of a pair of cones that can be moved close together or further apart to adjust the diameter at which the belt operates. The pulley ratios are electronically controlled to select the best overall drive ratio based on throttle position, vehicle speed and engine speed.


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Sequential Transmission

A sequential transmission is a type of  transmission used on motorcycles and high-performance cars for auto racing, where gears are selected in order, and direct access to specific gears is not possible. Cars with SMTs have a manual transmission with no clutch pedal; the clutch is automatically engaged.
In a race car, the motion of the shift lever is either "push forward" to up-shift or "pull backward" to downshift. If you are in a gear and you want to go to a higher gear (e.g. from 2nd to 3rd), you push the shift lever forward. To go from 3rd to 4th, you push the lever forward again. To go from 4th to 5th, you press it forward again. It is the same motion every time. To drop back down a gear, say from 5th to 4th, you pull the lever backward. In European mass-produced automobiles, the shift lever , the motion of the shift lever is either "push forward" to up-shift or "pull backward" to downshift moves forward and backward to shift into higher and lower gears, respectively. In Formula One cars, there are actually two paddles on the sides of the steering wheel, instead of a shift lever. The left paddle up-shifts, while the right paddle downshifts. On a motorcycle, you do the same thing, but instead of moving a lever back and forth with your hand, you move a lever up and down with your foot.

Advantages of using Sequential Transmission-
1. It provides a direct connection between engine and transmission, allowing 100 percent of the engine's power to be transmitted to the wheels.
2. The SMT provides more immediate response and ensures that the engine RPMs do not drop when the driver lifts off the accelerator (as happens with an automatic), giving her more precise control over power output.
3. It uses a solid coupling, as opposed to a fluid coupling (torque converter)
4. The sequential shift lever takes up less space in the race car cockpit.
5. The sequential shift is quicker.
6. The sequential shift is consistent. You do not have to think before gear change.
7.  The hand location is consistent; the shift lever is always in the same place for the next shift.
8. The gearbox reduces the risk of blowing up engine due to mis-shift.
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Dual-Clutch Transmisson

A dual clutch transmission, commonly abbreviated to DCT uses two clutches, but has no clutch pedal. Sophisticated electronics and hydraulics control the clutches, just as they do in a standard automatic transmission. In a DCT, however, the clutches operate independently. One clutch controls the odd gears (first, third, fifth and reverse), while the other controls the even gears (second and fourth) as shown in figure. Using this arrangement, gears can be changed without interrupting the power flow from the engine to the transmission.

A two-part transmission shaft is at the heart of a DCT. Unlike a conventional manual gearbox, which houses all of its gears on a single input shaft, the DCT splits up odd and even gears on two input shafts. The outer shaft is hollowed out, making room for an inner shaft, which is nested inside. The outer hollow shaft feeds second and fourth gears, while the inner shaft feeds first, third and fifth.

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Semi-Automatic Transmission

A semi-automatic transmission (also known as clutchless manual transmission, automated manual transmission, flappy-paddle gearbox, or paddle shift gearbox) is a system which uses electronic sensors, processors and actuators to execute gear shifts on the command of the driver. This removes the need for a clutch pedal which the driver otherwise needs to depress before making a gear change, since the clutch itself is actuated by electronic equipment which can synchronise the timing and torque required to make gear shifts quick and smooth.

The two most common semi-automatic transmissions are-
1.     Dual-clutch Transmission
2.     Sequential  Transmission
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Comparison between Manual and Automatic Transmission

Both the automatic transmission and a manual transmission accomplish exactly the same thing, but they do it in totally different way.

Advantages of manual transmission over automatic transmission-

1.    It is easier to build a strong manual transmission than an automatic one. This is because a manual system has one clutch to operate, whereas an automatic system has a number of clutch packs that function in harmony with each other.
2.     Manual transmissions normally do not require active cooling, because not much power is dissipated as heat through the transmission.
3.     Manual gearshifts are more fuel efficient as compared to their automatic counterpart. Torque convertor used to engage and disengage automatic gears may lose power and reduce acceleration as well as fuel economy.
4.     Manual transmissions generally require less maintenance than automatic transmissions. An automatic transmission is made up of several components and a breakdown of even a single component can stall the car completely.

Advantages of automatic transmission over manual transmission-

1.      The manual transmission locks and unlocks different sets of gears to the output shaft to achieve the various gear ratios, while in an automatic transmission; the same set of gears produces all of the different gear ratios. The planetary gearset is the device that makes this possible in an automatic transmission.
2.      Automatic cars are easier to use, especially for the inexperienced car driver. Manual system requires better driving skills, whereas with an automatic, the clever system does it all on its own. This holds a greater advantage for new and inexperienced drivers and also helps during congested traffic situations where it becomes difficult to change gears every second.
3.    Automatic transmission requires less attention and concentration from the driver because the automatic gears start functioning as soon as the system feels the need of a gear change. For car with manual gear shifts, the driver has to be more alert while driving and better coordinated.
4.     There is no clutch pedal and gear shift in an automatic transmission car. Once you put the transmission into drive, everything else is automatic.
5.   Automatic cars have better ability to control traction when approaching steep hills or engine braking during descents. Manual gears are difficult to operate on steep climbs.
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Clutches & Bands, Torque Converter, Valve Body


1.     Clutches & Bands - These are friction devices that drive or lock planetary gearsets members. They are used to cause the gearset to transfer power. In other words, they are used to hold a particular member of the planetary gearset motionless, while allowing another member to rotate, thereby transmitting torque and producing gear reductions or overdrive ratios. These clutches are actuated by the valve body their sequence controlled by the transmission's internal programming.
2.     Torque Converter - It is a hydraulic device that connects the engine and the transmission. It takes the place of a mechanical clutch, allowing the transmission to stay 'in gear' and the engine to remain running whilst the vehicle is stationary, without stalling. A torque converter is a fluid coupling that also provides a variable amount of torque multiplication at low engine speeds, increasing "breakaway" acceleration.
3.     Valve Body - The valve body is the control center of the automatic transmission.  It contains a maze of channels and passages that direct hydraulic fluid to the numerous valves.  Depending on which gear is selected, the manual valve feeds hydraulic circuits that inhibit certain gears. For instance, if the shift lever is in third gear, it feeds a circuit that prevents overdrive from engaging.  The valve body of the transmission contains several shift valves. Shift valves supply hydraulic pressure to the clutches and bands to engage each gear.  The shift valve determines when to shift from one gear to the next


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Planetary Gear Sets


The planetary gear set is the device that produces different gear ratios through the same set of gears. Any planetary gearset has three main components:
·         The sun gear
·         The planet gears  and the planet gears' carrier
·         The ring gear
Each of these three components can be the input, the output or can be held stationary. Choosing which   piece plays which role determines the gear ratio for the gearset.

Compound Planetary gearset - The automatic transmission uses a set of gears, called a compound planetary gearset that looks like a single planetary gearset but actually behaves like two planetary gearsets combined. It has one ring gear that is always the output of the transmission, but it has two sun gears and two sets of planets.
The figure below shows a compound planetary gearset:

First Gear- In first gear, the smaller sun gear is driven clockwise by the turbine in the torque converter. The planet carrier tries to spin counterclockwise, but is held still by the one-way clutch (which only allows rotation in the clockwise direction) and the ring gear turns the output. The small gear has 30 teeth and the ring gear has 72, so the gear ratio is:

Ratio = -R/S = - 72/30 = -2.4:1
So the rotation is negative 2.4:1, which means that the output direction would be opposite the input direction. But the output direction is really the same as the input direction -- this is where the trick with the two sets of planets comes in. The first set of planets engages the second set, and the second set turns the ring gear; this combination reverses the direction. You can see that this would also cause the bigger sun gear to spin but because that clutch is released, the bigger sun gear is free to spin in the opposite direction of the turbine (counterclockwise).


Second Gear - The two planetary gearsets connected to each other with a common planet carrier.
The first stage of the planet carrier actually uses the larger sun gear as the ring gear. So the first stage consists of the sun (the smaller sun gear), the planet carrier, and the ring (the larger sun gear).
The input is the small sun gear; the ring gear (large sun gear) is held stationary by the band, and the output is the planet carrier. For this stage, with the sun as input, planet carrier as output, and the ring gear fixed, the formula is:
1 + R/S = 1 + 36/30 = 2.2:1
The planet carrier turns 2.2 times for each rotation of the small sun gear. At the second stage, the planet carrier acts as the input for the second planetary gear set, the larger sun gear (which is held stationary) acts as the sun, and the ring gear acts as the output, so the gear ratio is:
1 / (1 + S/R) = 1 / (1 + 36/72) = 0.67:1
To get the overall reduction for second gear, we multiply the first stage by the second, 2.2 x 0.67, to get a 1.47:1 reduction.
Third Gear- Most automatic transmissions have a 1:1 ratio in third gear. To achieve a ratio of 1:1 engage the clutches that lock each of the sun gears to the turbine. If both sun gears turn in the same direction, the planet gears lockup because they can only spin in opposite directions. This locks the ring gear to the planets and causes everything to spin as a unit, producing a 1:1 ratio.
Overdrive Gear- An overdrive has a faster output speed than input speed. When overdrive is engaged, a shaft that is attached to the housing of the torque converter (which is bolted to the flywheel of the engine) is connected by clutch to the planet carrier. The small sun gear freewheels, and the larger sun gear is held by the overdrive band. Nothing is connected to the turbine; the only input comes from the converter housing. This time with the planet carrier for input, the sun gear fixed and the ring gear for output.
Ratio = 1 / (1 + S/R) = 1 / (1 + 36/72) = 0.67:1
So the output spins once for every two-thirds of a rotation of the engine. If the engine is turning at 2000 rotations per minute (RPM), the output speed is 3000 RPM. This allows cars to drive at freeway speed while the engine speed stays nice and slow.

Reverse Gear- Reverse is very similar to first gear, except that instead of the small sun gear being driven by the torque converter turbine, the bigger sun gear is driven, and the small one freewheels in the opposite direction. The planet carrier is held by the reverse band to the housing. So, according to our equations from the last page, we have:
Ratio = -R/S = 72/36 = 2.0:1
So the ratio in reverse is a little less than first gear in this transmission.

Summary of Gear Ratios- Transmission having four forward gears and one reverse gear.

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Automatic Transmission

The concept of an automatic transmission is new in India. An automatic transmission is a motor vehicle transmission that can automatically change gear ratios as the vehicle moves, freeing the driver from having to shift gears.  In this transmission system the gears are never physically moved and are always engaged to the same gears. Automatic transmissions contain mechanical systems, hydraulic systems, electrical systems and computer controls, all working together in perfect harmony manually.
Main Components of an Automatic Transmission-
1.      Planetary Gear Sets
2.      Clutches and Bands
3.      Torque Converter
4.      Valve Body





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Synchronized Transmission


Manual transmissions in modern passenger cars use synchronizers to eliminate the need for double-clutching. A synchro's purpose is to allow the collar and the gear to make frictional contact before the dog teeth make contact. This lets the collar and the gear synchronize their speeds before the teeth need to engage as shown in figures.

Synchronized gearbox consists of cone shaped brass clutch engaged to the gear. The cone on the blue gear fits into the cone-shaped area in the collar, and friction between the cone and the collar synchronize the collar and the gear. The outer portion of the collar then slides so that the dog teeth can engage the gear.
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Double clutching

Double-clutching was common in older cars and is still common in some modern race cars. In double-clutching, you first push the clutch pedal in once to disengage the engine from the transmission. This takes the pressure off the dog teeth so you can move the collar into neutral. Then you release the clutch pedal and rev the engine to the "right speed." The right speed is the rpm value at which the engine should be running in the next gear. The idea is to get the blue gear of the next gear and the collar rotating at the same speed so that the dog teeth can engage. Then you push the clutch pedal in again and lock the collar into the new gear. At every gear change you have to press and release the clutch twice, hence the name "double-clutching."
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Five Speed Manual Transmission

The five-speed manual transmission is fairly standard on cars today. Internally, it looks something like this:
There are three forks controlled by three rods that are engaged by the shift lever.  The shift lever has a rotation point in the middle. When you push the knob forward to engage first gear, you are actually pulling the rod and fork for first gear back.
When you move the shifter left and right you are engaging different forks (and therefore different collars). Moving the knob forward and backward moves the collar to engage one of the gears.

Idler Gear or Reverse Gear-
Idler gear is a small gear (purple) and is slid between red and blue gear. At all times, the blue reverse gear in this diagram is turning in a direction opposite to all of the other blue gears. The idler has teeth which mesh with both gears, and thus it couples these gears together and reverses the direction of rotation without changing the gear ratio.


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Working of Manual Transmission

When the gear selector fork is shifted into first gear, the collar engages the blue gear on the right:
In this picture, the green shaft from the engine turns the layshaft, which turns the blue gear on the right. This gear transmits its energy through the collar to drive the yellow drive shaft. Meanwhile, the blue gear on the left is turning, but it is freewheeling on its bearing so it has no effect on the yellow shaft.

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Components of Manual Transmission

The diagram below shows a very simple two-speed transmission in neutral:
·   The green shaft comes from the engine through the clutch. The green shaft and green gear are connected as a single unit. (The clutch is a device that lets you connect and disconnect the engine and the transmission. When you push in the clutch pedal, the engine and the transmission are disconnected so the engine can run even if the car is standing still. When you release the clutch pedal, the engine and the green shaft are directly connected to one another. The green shaft and gear turn at the same rpm as the engine.)
·      The red shaft and gears are called the layshaft. These are also connected as a single piece, so all of the gears on the layshaft and the layshaft itself spin as one unit. The green shaft and the red shaft are directly connected through their meshed gears so that if the green shaft is spinning, so is the red shaft. In this way, the layshaft receives its power directly from the engine whenever the clutch is engaged.
  • The yellow shaft is a splined shaft that connects directly to the drive shaft through the differential to the drive wheels of the car. If the wheels are spinning, the yellow shaft is spinning.



·         The blue gears ride on bearings, so they spin on the yellow shaft. If the engine is off but the car is coasting, the yellow shaft can turn inside the blue gears while the blue gears and the layshaft are motionless.
·      The purpose of the collar is to connect one of the two blue gears to the yellow drive shaft. The collar is connected, through the splines, directly to the yellow shaft and spins with the yellow shaft. However, the collar can slide left or right along the yellow shaft to engage either of the blue gears. Teeth on the collar, called dog teeth, fit into holes on the sides of the blue gears to engage them.
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