REPUBLIC OF YEMEN

UNIVERSITY OF ADEN

FACULTY OF ENGINEERING

DEPARTMENT OF MECHNICAL ENGINEERING

Prepared by:

Shadi Gamal Ali Abdul Galeel 91002 B4M

Wael Hassein Abdul –Gabar 9698 B4M

Automatic transmission (also called automatic gearbox)

Is a type of motor vehicle transmission that can automatically change gear ratios as

the vehicle moves, freeing the driver from having to shift gears manually. Most

automatic transmissions have a defined set of gear ranges, often with aparking

pawl feature that locks the output shaft of the transmission stroke face to keep the

vehicle from rolling either forward or backward.

Similar but larger devices are also used for heavy-duty commercial and industrial

vehicles and equipment. Some machines with limited speed ranges or fixed engine

speeds, such as some forklifts and lawn mowers, only use a torque converter to

provide a variable gearing of the engine to the wheels.

Besides automatics, there are also other types of automated transmissions such as

a continuously variable transmission (CVT) and semi-automatic transmissions, that

free the driver from having to shift gears manually, by using the transmission's

computer to change gear, if for example the driver were redlining the engine. Despite

superficial similarity to other transmissions, automatic transmissions differ

significantly in internal operation and driver's feel from semi-automatics and CVTs.

An automatic uses a torque converter instead of a clutch to manage the connection

between the transmission gearing and the engine. In contrast, a CVT uses a belt or

other torque transmission scheme to allow an "infinite" number of gear ratios instead

of a fixed number of gear ratios. A semi-automatic retains a clutch like a manual

transmission, but controls the clutch through electrohydraulic means.

Automatic transmissions components:

There are three basic parts of automatic transmission, the torque converter, the gear

system and hydraulic control system.

How an automatic transmission works

Automatic transmissions – they're pretty much black magic. The sheer number of

moving parts makes them very difficult to comprehend. Let's simplify it a bit to get a

basic understanding of how it all works in a traditional, torque converter-based

system. Your engine connects to your transmission at a place called a bell housing.

The bell housing contains a torque converter for automatic transmission-equipped

vehicles as opposed to a clutch on manual vehicles. The torque converter is a fluid

coupling whose job it is to connect your engine to your transmission and thus to your

driven wheels. The transmission contains planetary gearsets which are in charge of

providing different gear ratios. To get a good understanding of how the whole

automatic transmission system works, let's have a look at torque converters and

planetary gearsets as shown in figure 1.

Figure 1. Automatic transimmision

Torque Converter

First and foremost, your engine's flex plate (basically a flywheel for an automatic)

connects directly to a torque converter. So when the crankshaft rotates, so does the

torque converter housing. The goal of the torque converter is to provide a means by

which to connect and disconnect the engine's power to the driven load. The torque

converter takes the place of a clutch on a conventional manual transmission as shown in

figure 2.

Figure 2. torque converter

The major components of a torque converter are: the impeller, the turbine, the stator,

and the lock-up clutch. The impeller is part of the torque converter housing, which is

connected to the engine. It drives the turbine via viscous forces. The turbine is connected

to the transmission input shaft. In essence, the engine turns the impeller which imparts

forces on a fluid, which then rotates the turbine, sending torque to the transmission. 

Figure3. Cross section of torque converter

The hydraulic system:

The transmission fluid flows in a loop between the impeller to the turbine. The fluid

coupling suffers from severe churning losses (and consequent heat buildup) as the fluid

returning from the turbine has a component of its velocity that opposes the rotation of

the impeller. That is, the fluid returning from the turbine works against the impeller's

rotation and thus against the engine.

The stator sits between the impeller and turbine. Its goal is to minimize churning losses

and to increase torque output by redirecting the fluid as it returns from the turbine to the

impeller. The stator directs the fluid so that the majority of its velocity is in the direction

of the impeller, helping the impeller move, and thus adding to the torque produced by

the motor. This ability to multiply torque is why we call them torque converters, not

fluid couplings.

The stator sits on a one-way clutch. It can rotate in one direction only when the turbine

and impeller are moving at approximately the same speed (like during highway driving).

The stator either rotates with the impeller or not at all. Stators don't always multiply

torque, though. They provide you with more torque when you're either at stall (applying

the brakes at a stop light, for example) or while accelerating, but not during highway

cruising. 

In addition to the one-way clutch in the stator, some torque converters contain a lock-up

clutch whose job it is to lock the turbine with the torque converter housing so that the

turbine and impeller are mechanically connected. Eliminating the fluid coupling and

replacing it with a mechanical connection ensures that all of the engine's torque is

transmitted to the transmission input shaft.

Planetary Gears

Figure 4. Planetary Gears

So, now that we've figured out how the engine sends power to the transmission, it's time

to figure out how in tarnation it changes gears. On a conventional transmission, changing

gears is the job of a compound planetary gear set. Understanding how planetary gear sets

work is a bit tricky, so let's have a look at a basic planetary gear set.

A planetary gearset (also known as an epicyclic gear set) consists of a sun gear in the

center, planet gears that rotate around the sun gear, a planet carrier that connects the

planet gears, and a ring gear on the outside that meshes with the planet gears. The basic

idea behind a planetary gear set is this: using clutches and brakes, you can prevent

certain components from moving. In doing so, you can alter the input and output of the

system and thus change the overall gear ratio. Think of it this way: a planetary gear set

lets you change gear ratios without having to engage different gears. They're all already

engaged. All you have to do is use clutches and brakes to change which components

rotate and which stay stationary.

The final gear ratio depends on which component is fixed. For example, if the ring gear

is fixed, the gear ratio will be much shorter than if the sun gear is fixed. Knowing full

well the risks associated with ploppin' an equation on here, I'm gonna put one in anyway.

The following equation will tell you your gear ratios depending on which component is

fixed and which are in motion. R, C, and S represent the ring gear, carrier, and sun gear.

Omega simply represents the angular speed of the gears, and N is the tooth count. 

The way it works is thus: let's say we decided to keep the planet carrier stationary and

make the sun gear our input (thus the ring gear is our output). The planets are able to

rotate, but they cannot move since the carrier cannot move. Omega_c is zero, so the left

side of the equation above is gone. This means that when we rotate the sun gear, it sends

torque through the planet gears to the ring gear. To figure out what the gear ratio would

be, we simply solve the above equation for Omega_r/Omega_s. We end up with

-N_s/N_R, that is, the gear ratio when we fix the carrier and make the ring gear our

output and the sun gear our input is simply the ratio of the number of teeth between the

sun gear and ring gear. This is negative, since the ring spins in the opposite direction of

the sun gear. You can also lock the ring gear and make the sun gear your input and you

can lock the sun gear and make the carrier your input. Depending on what you lock,

you'll get different gear ratios, i.e. you'll get different "gears." To obtain a 1:1 gear ratio,

you simply lock the components together (you only have to lock two to do this) so that

the crankshaft spins at the same speed as the transmission output shaft.

So how do the brakes and clutches move to change gears? the torque converter is also in

charge of driving the transmission fluid pump. The fluid pressure is what activates

clutches and brakes in the planetary gearset. The pump is often a geroter type pump (a

gear pump) meaning that a rotor spins in a pump housing and as it spins, it "meshes"

with the housing. This "meshing" creates chambers that change in volume. When the

volume increases, a vacuum is created- this is the pump inlet. When the volume

decreases, the fluid is compressed or pumped by the meshing of the gears- this is the

pump exit. A hydraulic control unit sends hydraulic signals to change gears (via band

brakes and clutches) and to lock the torque converter.

Note that most modern automatic transmissions use a Ravigneaux compound planetary

gearset. This gearset has two sun gears (a small and a large), two sets of planets (inner

and outer), and one planet carrier. This is essentially two simple planetary gearsets in

one.