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Automotive Transmissions: Efficiently Transferring Power from Engine to Wheels
(Released January 2008)

 
  by Chao-Hsu Yao  

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Automatic Transmission

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Driving a manual transmission vehicle is not an easy job for a beginner, and it takes time for a new driver to attain the skill. Improper driving always causes the car to stall, and could damage the transmission system. For some disabled people who are not able to use both legs, driving a manual transmission car is impossible. In 1941, Chrysler introduced the first automatic transmission system, which included a fluid coupling between engine and clutch. The gear set is the same as those in a manual transmission box; however, a vacuum cylinder or a hydraulic cylinder is used to perform automatic gear shifting. The clutch selects the gear range only but isn't used when driving. The first range (or low range) contains the 1st and 2nd gears, while the second range (or high range) contains 3rd and 4th gears. To move the car, the clutch and brake must be depressed and a gear range must be selected (low, high, or reverse.) After the gear range is selected, the clutch can be released. To move the car, the brake is released and the gas is stepped on. The gear is changed automatically (between 1 and 2 or between 3 and 4) during driving [6].

The automatic system for current automobiles uses a planetary gear set instead of the traditional manual transmission gear set. The planetary gear set contains four parts: sun gear, planet gears, planet carrier, and ring gear (see Figure 10.) Based on this planetary set design, sun gear, planet carrier, and ring gear spin centrifugally. By locking one of them, the planetary set can generate three different gear ratios, including one reverse gear, without engaging and disengaging the gear set. The shockwave file http://static.howstuffworks.com/flash/automatic-transmission-planetary.swf shows how this mechanism works when you click on the left buttons. Normally an automatic transmission system has two planetary gear sets with different sizes of sun gears with their planet gears intermeshed (see Figure 11.) Only one planet carrier is used to connect both sets of planet gears. This is called a compound planetary gear set. This design can generate four different gear ratios and one reverse gear. The inputs, outputs, and gear ratios are summarized in Table 1[7].

planetary gear set
Figure 10: Planetary Gear Set.
compound planetary gear set
Figure 11: Compound Planetary Gear Set.

Gear
Input
Output
Fixed

Gear Ratio
1st 30-tooth Sun 72-tooth Ring Planet Carrier 2.4:1
2nd 30-tooth Sun Planet Carrier 36-tooth Ring 2.2:1
Planet Carrier 72-tooth Ring 36-tooth Sun 0.67:1
    Total 2nd 1.47:1
3rd 30- and 36-tooth Suns 72-tooth Ring   1.0:1
Over Drive or 4th Planet Carrier
72-tooth Ring 36-tooth Sun
0.67:1
Reverse 36-tooth Sun 72-tooth Ring Planet Carrier -2.0:1
Table 1

In order to lock the gears to perform gear ratio change, a band and clutches are used. The band is connected to an actuator piston by way of a lever link (see Figure 12). The piston pushes the lever link to force the band to lock the gear.

actuator piston, lever link, and band system
Figure 12: The structure of the actuator piston, lever link, and band system.

The clutch lock is much more complicated than the band lock. A set of clutches is shown in Figure 13. It is constructed with a clutch housing (drum), clutch plates (pressure plates), and clutch discs (friction plates).

clutch discs, plates, and drum
Figure 13: Clutch Packs.
From different auto makers, several different types of automatic transmission systems were designed and developed. Figure 14 shows one of the simple automatic transmission system designs. From this design, the sun gear (yellow) is connected to a drum (yellow), which can be locked by a band (red). The ring gear (blue) is directly connected to the input shaft (blue), which transfers power from the engine. The planet carrier (green) is connected to the output shaft (green), which transfers power into the wheels. Based on this design, when in neutral, both band and clutch sets are released. Turning the ring gear can only drive planet gears but not the planet carrier, which stays static if the car is not moving. The planet gears drive the sun gear to spin freely. In this situation, the input shaft is not able to transfer power to the output shaft. When shifting to 1st gear, the band locks the sun gear by locking the drum. The ring gear drives the planet carrier to spin. In this situation, the ring gear (input shaft) spins faster than the planet carrier (output shaft). To shift to higher gear, the band is released and the clutch is engaged to force the sun gear and planet carrier (output shaft) to spin at the same speed. The input shaft will also spin at the same speed as the output shaft, which makes the car run faster than in 1st gear. Using a compound planetary gear set generates more gear ratios with a special gear ratio, over-drive gear, whose gear ratio is small than 1. This will make the gear shift more smooth [8].

gear sets
Figure 14: Planetary Gear Sets with Band and Clutches

How can the band and clutches perform the lock function to select the right gear automatically? Both the band and clutch piston are pressurized by the hydraulic system (see Figure 15). The part connecting the band or clutches to the hydraulic system is called the shift valve, while the one connecting the hydraulic system to the output shaft is called the governor (see Figure 16). The governor is a centrifugal sensor with a spring loaded valve. The faster the governor spins, the more the valve opens. The more the valve opens, the more the fluid goes through and the higher the pressure applied on the shift valve. Therefore, each band and clutch can be pushed to lock the gear based on a specific spin speed detected by the governor from the output shaft. To make the hydraulic system work efficiently, a complex maze of passages was designed to replace a large number of tubes (see Figure 17). For modern cars, an electronic controlled (computer controlled) solenoid pack is used to detect throttle position, vehicle speed, engine speed, engine load, brake pedal position, etc., and to automatically choose the best gear for a moving vehicle.

simple shift circuit
Figure 15: Hydraulic system.

governor
Figure 16: Governor.

exploded view of valve body section
Figure 17: Hydraulic System with Maze Structure.

two fans facing each other
Figure 18
Unlike a manual transmission system, automatic transmission doesn't use a clutch to disconnect power from the engine temporarily when shifting gears. Instead, a device called a torque converter was invented to prevent power from being temporarily disconnected from the engine and also to prevent the vehicle from stalling when the transmission is in gear. Consider two fans facing each other: when one of them is turned on and starts spinning, the other one will also start spinning at a lower speed (see Figure 18). That's because the first fan moves the air to drive the second fan to spin. This is the same idea as the torque converter of an automatic transmission system, except that it uses fluid instead of air as the transportation media.

A torque converter consists of four parts, a pump (impeller), turbine, stator, and transmission fluid. The pump is connected to the engine and transfers engine power to the transmission fluid. The fluid causes the turbine, which is connected to the input shaft, to spin. The stator is used to redirect fluid returning from the turbine before it hits the pump, again to increase the efficiency. In this design, even when the vehicle is in gear but not moving (the turbine is forced to stop), the pump can still keep spinning without causing the engine to stall.

torque converter
Figure 19: The Structure of a Torque Converter.

When the vehicle speed is slow, the turbine is always spinning more slowly than the pump. However, when the vehicle moves at high speed, the turbine can spin at almost the same speed as the pump. Therefore, for modern cars, a "lock up" will occur between pump and turbine when the vehicle is at high speed in fourth gear, which makes the torque converter work more efficiently.

Tiptronic transmission is a special type of automatic transmission with a computer controlled automatic shift. The driver can switch the transmission to manual mode, which lets her shift the gear at her wish sequentially up (+) or down (-) (see Figure 20) without disengaging the clutch. This works just like a manual transmission; however, it still uses a torque converter to transfer power from the engine. Unfortunately, this is less efficient than a manual transmission.

car interior
Figure 20: Tiptronic transmission.

Go To Continuously-Variable Transmission (CVT)

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