Automotive Transmission Systems
The transmission systems is the final stage of the engine generated power before it hits the wheels.
The whole system is responsible to couple engine and wheels, driving and adapting the output shaft rotation to a desired speed/torque ratio, allowing a wider range of speed and better performance as the engine has its own RPM limit (redline) and maximum performance value. As displayed in the figure below, from the back of the engine to where the rubber meets the road, the “Drive Train” encompasses the most complicated systems of a vehicle.
The Drive Train has two basic functions:
- It transmits power from the engine to the drive wheels.
- It varies the amount of torque.
There are two set of gears in the drive train
- The Transmission
- The Differential
Rear Wheel Drive
- The transmission is usually mounted to the back of the engine.
- A drive shaft connects the rear of the transmission to the final drive which is located in the rear axle and is used to send power to the rear wheels.
- Power flow on this system is simple and straight forward going from the engine, through the torque converter, then through the transmission and drive shaft until it reaches the final drive where it is split
Front Wheel Drive
- The transmission is usually combined with the final drive to form what is called a transaxle.
- The engine on a front wheel drive car is usually mounted sideways in the car with the transaxle.
- Front axles are connected directly to the transaxle and provide power to the front wheels
- Power flows from the engine, through the torque converter to transmission that is alongside the engine.
- From there, the power is routed through the transmission to the final drive where it is split and sent to the two front wheels through the drive axles.
- Allow the vehicle to start from rest, with the engine running continuously
- Let the vehicle stop by disconnecting the drive when appropriate
- Enable the vehicle to start at varied rates, under controlled manner
- Vary the speed ratio between the engine and wheels
- Allow this ratio to change when required
- Transmit the drive torque to the required wheels.
- Manual Transmission
- Dual-clutch transmission (DCT)
- Continuous Variable transmission (CVT)
- Automatic transmission
Transmission - Manual
H pattern is usually used to control the manual transmission. The name is derived because it has H architecture, means the gear lever works like an H.
- The green shaft comes from the engine through the clutch;
- 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 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 too.
- 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 spines, 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.
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 synchronizer their speeds before the teeth need to engage, like this:
Transmission – Dual Clutch (DCT)
- Audi and Porsche have used this type of transmission in races since 1985.
- In this transmission 2 clutches work sequentially. One of them is connected to the odd gears and to the reverse gear, and other connected to the even gears.
- It enables the vehicle to change gears very quickly , promotes lower consumptions and longer live than the other transmissions
- The synchronizers of this gearboxes are done with a more abrasive material of friction to get very lower synchronization times and to allow down shifts like 6th to 2nd. In other car this isn’t possible.
Transmission - Automatic
To allow an easy and more optimum driving experience and to transfer the need of shifting gears from the driver to the car, automatic transmission was invented, as well as for the same objective as all the other types of transmission: convert the narrow range of engine speeds into a wide range to the output.
Therefore, if the car would needs to shift gear it should be able to press the clutch pedal autonomously first. Well, that’s not like that... Automatic transmission cars doesn’t have clutch at all.
In manual transmission system there is need to use clutch to keep engine running, in automatic transmission there is same need ,thus, another device “Torque Converter” was created. It is this device that will do the job and its operation is based on fluid movement.
Consists of three major parts:
Turbine is directly connected to the engine flywheel, so it spins at the same speed. The stator is the middle part that will redirect the fluid from the turbine to the pump, forcing this one to rotate nearly at the same speed as the turbine that will transmit the movement forward as it’s directly connected to the transmission. In low engine rpm’s the force of the fluid is so low that almost no rotation will be generated in the pump.
An automatic transmission produces all of different gear ratios with one set of gears. This set of gears is called Planetary Gearset and consists of ring gear, planet gear and sun gear. Combination of these act as an input and output to produce the gear ratios available in the car. In cars a compound planetary gear set is used. In this gear set has an extra sun gear with different diameter which helps in creating more gear ratios than with one.
Transmission – Continuous Variable
In continuous variable transmission can generate infinite gear ratios without any kind of gears, actually it uses a pair of pulleys capable of changing its diameter, thus, changing gear ratio and allowing infinite variability lowest gears without discrete steps or shifts . This was invented by Leonardo Da vinci in 1490 and modified since to suit the current conditions.
Pulleys are made of two 20-degree cones facing each other varying its distance, where a belt adjusts its position generating a continuous gear ratio. Is preferred this belt is made from rubber and is V-shaped allowing to increase frictional grip. The belt is an essential component of the CVT transmission system as it holds all the torque transferred by the engine to the wheels. Now-a-days it’s made of nine to twelve bands of steel combined with steel pieces making this a very strong although flexible belt.
Advantages of CVT
- Smoother drive – constant and stepless acceleration;
- Improved fuel efficiency – always keeps the car in optimum power range;
- No gear shifting – great response to changing conditions (speed, throttle…);
- Better acceleration – less power loss than in a typical automatic transmission;
- Better control over emissions - better control of engine’s speed range.
Disadvantages of CVT
- Some CVT’s in production vehicles have seen premature failures;
- In vehicles, without intervening electronics, a CVT will go to high rmp on wide throttle opening, producing a disquieting noise;
- CVT’s aren’t as capable of handling torque as other transmission systems.
- Some of the Existing cars with CVT Transmission are - Nissan Sunny, Renault Scala and now is all New Honda City coming in CVT Transmission.
Nevertheless they are very reliable, and are being increasingly used in lower priced cars.
This is all about automotive transmission system explained in a nut shell. For More Information: http://discuss.spareshub.com/