If you have ever driven a car with an automatic transmission , then you know that there are two with child differences between an automatic transmittance and amanual transmission :
Both the automatonlike transmitting ( plus itstorque converter ) and amanual transmission(with itsclutch ) accomplish on the dot the same thing , but they do it in totally different ways . It turns out that the manner an automatic transmission does it is utterly awe-inspiring !
In this clause , we ’ll puzzle out our manner through an reflexive infection . We ’ll start with the cay to the whole organisation : planetary gearset . Then we ’ll see how the transmission is put together , memorise how the mastery work and discuss some of the involution involved in controlling a transmission .
Purpose of an Automatic Transmission
Just like that of a manual transmission , the robotic transmission ’s primary line is to allow theengineto operate in its narrow range of swiftness while allow a panoptic ambit of output speed .
Without a transmission , car would be fix to onegear proportion , and that ratio would have to be selected to allow the car to travel at the desired top fastness . If you wanted a top speed of 80 mph , then the gear ratio would be standardised to third gear in most manual transmission railcar .
You ’ve probably never tried drive a manual transmission car using only third geartrain . If you did , you ’d quickly discover out that you had almost no quickening when starting out , and at in high spirits speeds , the locomotive engine would be hollo along near the cherry-red - furrow . A car like this would wear out very quickly and would be about undriveable .
So the transmission uses paraphernalia to make more effective use of the engine’storque , and to keep the engine lock at an appropriate pep pill . When tow or hauling clayey object , your vehicle ’s infection can get spicy enough to cauterize up the transmission fluid . to protect the transmittal from serious damage , driver who tow should buy vehicles outfit withtransmission coolers .
The key difference of opinion between a manual of arms and an automatic transmission is that the manual transmission locks and unlock unlike solidifying ofgearsto the output calamus to achieve the various gear proportion , while in an reflex transmittance , the same set of appurtenance produces all of the different gear ratios . The global gearset is the machine that makes this possible in an automatic transmission .
Let ’s take a look at how the planetary gearset works .
The Planetary Gearset
When you take asunder and look inside an automatonlike transmission , you regain a huge salmagundi of parts in a fair small outer space . Among other matter , you see :
The inwardness of attention is theplanetary gearset . About the size of it of a cantaloupe , this one part creates all of the unlike cogwheel proportion that the infection can bring about . Everything else in the transmission is there to assist theplanetary gearsetdo its affair . This astonishing piece of geartrain has come out on HowStuffWorks before . You may recognize it from theelectric screwdriverarticle . An reflexive transmission contains two complete planetary gearset folded together into one element . SeeHow Gear Ratios Workfor an intromission to planetary gearset .
Any planetary gearset has three primary component :
Each of these three components can be the input , the output or can be entertain stationary . pick out which piece plays which role determines the gear ratio for the gearset . allow ’s take a face at a single planetary gearset .
Planetary Gearset Ratios
One of the world-wide gearsets from our transmission has a ring cogwheel with 72 tooth and a sun gearing with 30 tooth . We can get lot of different gear mechanism ratio out of this gearset .
Also , locking any two of the three factor together will shut up up the whole gadget at a 1:1 gear reduction . find that the first cogwheel ratio listed above is areduction– the turnout speed is slower than the input speed . The second is anoverdrive– the output velocity is fast than the stimulant speed . The last is a reduction again , but the yield direction is revoke . There are several other ratios that can be gotten out of this worldwide paraphernalia band , but these are the ones that are relevant to our automatic transmittal . you’re able to see them in the invigoration below :
So this one set of geartrain can produce all of these different appurtenance proportion without cause to employ or disengage any other gears . With two of these gearset in a rowing , we can get the four forward-moving gear and one reverse gear our transmission system needs . We ’ll put the two sets of paraphernalia together in the next subdivision .
Compound Planetary Gearset
This robotic transmittance uses a set of gears , call acompound planetary gearset , that looks like a undivided wandering gearset but really behaves like two planetary gearsets combined . It has one halo gear that is always the output of the transmission , but it has two sun gears and two set of planets .
Let ’s look at some of the function :
The figure below shows the satellite in the planet postman . Notice how the planet on the right wing sits lower than the planet on the leftfield . The planet on the right does not engage the ring geared wheel — it engages the other major planet . Only the satellite on the left engages the ring gear .
Next you’re able to see the inside of the satellite flattop . The shorter gears are engage only by the smaller sun geartrain . The longer satellite are enlist by the bigger Dominicus appurtenance and by the smaller planet .
The invigoration below shows how all of the part are hooked up in a transmission .
First Gear
In first gear , the smaller sunshine gearing is driven clockwise by the turbine in thetorque convertor . The major planet letter carrier tries to spin counterclockwise , but is hold still by the one - wayclutch(which only allow gyration in the clockwise direction ) and the hoop gear turn the output . The small geared wheel has 30 teeth and the ring gear has 72 , so the cogwheel ratio is :
proportion = -R / S = - 72/30 = -2.4:1
So the gyration is electronegative 2.4:1 , which signify that the output direction would beoppositethe input direction . But the turnout direction is really thesameas the input signal focus – this is where the trick with the two sets of planets add up in . The first set of planets engages the second circle , and the second bent turn the ring gear ; this combining reverses the way . you may see that this would also cause the bigger sun gear to twist ; but because that clutch is free , the bigger sun gear is destitute to spin around in the opposite guidance of the turbine ( counterclockwise ) .
Second Gear
This contagion does something really neat in social club to get the proportion want for second gear . It acts like two planetary gearset connected to each other with a common major planet carrier .
The first stage of the planet carrier actually uses the larger sun train as the band gear . So the first stage consists of the sunlight ( the belittled sun gear ) , the planet carrier , and the ring ( the tumid sun gear ) .
The stimulation is the small Sunday gear mechanism ; the ring gear wheel ( large sun gear ) is held stationary by the band , and the output is the planet carrier . For this stage , with the sunshine as input , satellite carrier as end product , and the tintinnabulation gear fixed , the recipe is :
1 + gas constant / S = 1 + 36/30 = 2.2:1
The planet aircraft carrier turns 2.2 times for each gyration of the minuscule sunshine gearing . At the second stage , the planet carrier act as as the stimulus for the 2nd erratic gear band , the larger sunshine gear wheel ( which is held stationary ) acts as the Sunday , and the hoop gear act as the end product , so the gear proportion is :
1 / ( 1 + S / roentgen ) = 1 / ( 1 + 36/72 ) = 0.67:1
To get the overall reduction for second gear wheel , we multiply the first degree by the 2d , 2.2 x 0.67 , to get a 1.47:1 simplification .
Third Gear
Most robotlike transmissions have a 1:1 ratio in third gear . You ’ll call up from the old section that all we have to do to get a 1:1 output is lock in together any two of the three percentage of the worldwide geared wheel . With the arrangement in this gearset it is even easier — all we have to do is engage the clutch that interlace each of the Lord’s Day gears to the turbine .
If both Lord’s Day gears twist 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 whirl as a unit , producing a 1:1 ratio .
Overdrive
By definition , an overdrive has a faster output pep pill than input signal speed . It ’s a stop number increase — the opposite of a reduction . In this transmission , hire the overdrive achieve two thing at once . If you readHow Torque Converters Work , you learned about locking torsion convertor . to improve efficiency , some cars have a chemical mechanism that lock up the torsion convertor so that the output of the engine die direct to the transmission .
In this transmission , when overdrive is engaged , a shaft that is attached to the housing of the torque converter ( which is bolt to the flywheel of the locomotive engine ) is connected by clutch to the satellite carrier wave . The small sun gear drift , and the heavy sunshine gear wheel is hold by the overdrive dance band . Nothing is connected to the turbine ; the only input comes from the converter housing . permit ’s go back to our chart again , this time with the satellite carrier for stimulation , the sun gear limit 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 - third of a rotation of the locomotive engine . If the engine is turn at 2000 rotation per minute ( rev ) , the output speed is 3000 RPM . This allows railroad car to drive at superhighway speed while the engine speed stays nice and tiresome .
Reverse Gear
Reverse is very similar to first gear , except that instead of the small sunlight gear being driven by the torsion converter turbine , the bigger Dominicus gear is driven , and the small one freewheels in the opposite direction . The major planet carrier is hold by the rearward stripe to the housing . So , according to our equality from the last Sir Frederick Handley Page , we have :
So the ratio in turnabout is a short less than first train in this transmission .
Gear Ratios
This transmission has four forward gears and one inverse appurtenance . Let ’s sum up the geared wheel proportion , inputs and outputs :
After reading these discussion section , you are probably wonder how the different inputs get connected and disconnected . This is done by a series of clutches and band inside the transmission . In the next part , we ’ll see how these work .
Clutches and Bands in an Automatic Transmission
In the last section , we discussed how each of the gear ratios is created by the transmission . For case , when we discussed overdrive , we enjoin :
In this transmission , when overdrive is rent , a ray of light that is attached to the housing of the torsion converter ( which is bolted to the flywheel of the engine ) is connected by clutch to the major planet carrier . The pocket-sized Lord’s Day geartrain freewheels , and the larger sunlight power train is held by the overdrive band . Nothing is connected to the turbine ; the only input comes from the convertor housing .
To get the transmission into overdrive , plenty of thing have to be connect and disconnected by clutch and dance band . The planet carrier gets connected to the torque converter housing by a grip . The small Lord’s Day gets disconnect from the turbine by a batch so that it can drift . The big sun power train is hold back to the lodging by a stripe so that it could not rotate . Each cogwheel transmutation triggers a series of event like these , with dissimilar clutches and lot engaging and disengaging . allow ’s take a flavor at a band .
Bands
In this transmittal there are two bands . The band in a transmission are , literally , steel bands that wrap around surgical incision of the gear power train and connect to the housing . They are actuated by hydraulic cylinder inside the fount of the transmission .
In the bod above , you’re able to see one of the bands in the caparison of the transmission . The gear train is remove . The metal rod is connected to the piston , which actuates the dance band .
Above you may see the two plunger that actuate the band . Hydraulic pressure , routed into the cylinder by a set of valve , make the plunger to push on the bands , locking that part of the appurtenance railroad train to the housing .
Theclutchesin the transmission are a little more complex . In this transmitting there are four clutch . Each clutch is actuated by pressurized hydraulic fluid that get into a plunger inside the clutch . Springs verify that the clutch bag expel when the insistence is reduced . Below you’re able to see the piston and the clutch drum . detect the rubber seal on the piston – this is one of the components that is replace when your transmission gets reconstruct .
The next figure shows the alternating layers of clutch detrition cloth and brand plates . The friction fabric is splined on the interior , where it shut up to one of the gears . The brand plate is splined on the outside , where it locks to the clutch living accommodations . These clutch plates are also supervene upon when the transmission is rebuild .
The pressure for the hold is course through passage in the shafts . The hydraulic system controls which clutches and bands are arouse at any feed moment .
When You Put the Car in Park
It may seem like a dewy-eyed thing to lock the transmission and keep it from spinning , but there are in reality some complex requirements for this mechanism . First , you have to be able to free it when the motorcar is on a hill ( the weight of the railcar is resting on the mechanism ) . Second , you have to be able to engage the mechanism even if the lever does not line up with the gear mechanism . Third , once engage , something has to forestall the lever from popping up and disengaging .
The chemical mechanism that does all this is fairly neat . Let ’s see at some of the parts first .
The parking - brake mechanism engages the teeth on the yield to hold the car still . This is the section of the transmission that accost up to the drive shaft – so if this part ca n’t spin , the railway car ca n’t move .
Above you see the parking chemical mechanism protrude into the housing where the gears are locate . Notice that it has taper English . This aid to disengage the parking brake when you are parked on a hill – the force from the weight unit of the gondola assist to advertise the parking mechanism out of spot because of the slant of the taper .
This rod is connect to a cable that is manoeuver by the shift lever in your car .
When the slip lever tumbler is placed in park , the perch pushes the spring against the small tapering cylindrical lining . If the ballpark mechanism is run along up so that it can drop into one of the notch in the output gear section , the tapered bushing will push the mechanism down . If the chemical mechanism is run along up on one of the high spots on the output , then the bounce will push on the tapered cylindrical lining , but the lever will not lock into position until the car rolls a small and the teeth line up properly . This is why sometimes your railway car moves a little minute after you put it in green and liberate the pasture brake pedal – it has to roll a piddling for the teeth to line up to where the parking chemical mechanism can drop into place .
Once the cable car is safely in commons , the bushing holds down the lever so that the car will not pop out out of parking area if it is on a pitcher’s mound .
Automatic Transmissions: Hydraulics, Pumps and the Governor
Hydraulics
The automatic transmission in your car has to do numerous chore . You may not realize how many different way it operates . For case , here are some of the feature article of an automatic transmission :
You ’ve probably seen something that take care like this before . It is really the mentality of the robotlike transmittance , managing all of these functions and more . The passageway you could see route fluid to all the unlike element in the transmittance . Passageways molded into the metal are an efficient way to route fluid ; without them , many hose would be needed to connect the various parts of the infection . First , we ’ll discuss the key components of the hydraulic system ; then we ’ll see how they work together .
The Pump
Automatic transmission have a neat ticker , called agear heart . The pump is commonly locate in the covering fire of the transmittal . It draws fluid from a cesspool in the bottom of the transmission and feeds it to the hydraulic system . It also feast thetransmission coolerand thetorque converter .
The inner train of the pump hooks up to the living accommodations of the torsion convertor , so it spins at the same stop number as the railway locomotive . The outer gear is become by the intimate gear , and as the gears rotate , fluid is draw up from the cesspit on one side of the crescent and forced out into the hydraulic organisation on the other side .
The Governor
Thegovernoris a cagy valve that evidence the transmittance how fast the car is work . It is link up to the output , so the quicker the car moves , the faster the regulator spin out . Inside the regulator is a outpouring - loaded valve that spread out in proportion to how fast the regulator is spinning – the faster the regulator spin out , the more the valve opens . Fluid from the heart is course to the governor through the output shaft .
The faster the car goes , the more the governor valve opens and the gamey the pressure of the fluid it get through .
Automatic Transmissions: Valves and Modulators
To shift in good order , the automatic transmission system has to have sex how hard the engine is working . There are two different ways that this is done . Some cars have a bare cable television linkage link to athrottle valvein the transmittance . The further the accelerator pedal pedal is beseech , the more pressure is put on the throttle valve . Other cars use avacuum modulatorto use pressure to the accelerator valve . The modulator senses the multiplex pressure , which increases when the engine is under a greater load .
Themanual valveis what the transformation lever hooks up to . Depending on which gear is selected , the manual valve feeds hydraulic circuit that suppress certain gears . For example , if the transformation lever is in third power train , it feeds a circuit that prevents overdrive from engaging .
Shift valvessupply hydraulic pressure to the clutches and bands to engage each geartrain . The valve body of the transmission contains several shift valves . The shift valve determines when to shift from one geared wheel to the next . For instance , the 1 to 2 chemise valve determines when to careen from first to 2d gear . The fracture valve is pressurized with fluid from the regulator on one side , and the throttle valve on the other . They are supplied with fluid by the pump , and they route that fluid to one of two circuits to control which gear the car runs in .
The shift valve will detain a shift if the cable car is accelerating quickly . If the car accelerates mildly , the shift will occur at a low speed . Let ’s discuss what encounter when the car accelerates gently .
As car speed increases , the pressure from the governor builds . This coerce the shift valve over until the first gear racing circuit is unsympathetic , and the 2nd gear circuit opens . Since the car is accelerating at scant throttle , the accelerator valve does not apply much pressure against the shift valve .
When the machine speed up quickly , the throttle valve apply more pressure against the shift valve . This imply that the pressure from the regulator has to be gamy ( and therefore the fomite velocity has to be quicker ) before the shift valve moves over far enough to engage second gear .
Each shimmy valve respond to a exceptional insistence range ; so when the motorcar is go quicker , the 2 - to-3 shift valve will take over , because the pressure from the regulator is high-pitched enough to trigger that valve .
Electronically Controlled Transmissions
Electronically command transmissions , which seem on some newer cars , still use hydraulics to actuate the clutch and lot , but each hydraulic circuit is command by an electricsolenoid . This simplifies the plumbing on the contagion and allows for more advanced control schemes .
In the last section we see some of the controller scheme that automatically verify transmissions use . Electronically controlled transmittance have even more elaborate ascendancy schemes . In summation to monitoring fomite speed and gun position , the transmittance restrainer can supervise the engine speed , if the brake pedal point is being pressed , and even theanti - lock braking system .
Using this information and an advanced control scheme free-base on blurred system of logic – a method of computer programming control organization using human - case reasoning – electronically controlled transmission can do thing like :
Let ’s lecture about that last lineament – inhibiting the upshift when going into a turn on a winding road . Let ’s say you ’re drive on an rising , winding mountain route . When you are driving on the unbowed sections of the road , the transmitting shifts into second gear to give you enough acceleration and J. J. Hill - climb superpower . When you come to a bend you slow up down , take your ft off the gun foot pedal and maybe applying the brake . Most transmission will upshift to third paraphernalia , or even overdrive , when you take your foot off the gas . Then when you accelerate out of the curve , they will downshift again . But if you were driving a manual transmission car , you would plausibly leave the car in the same gear the whole time . Some automatonlike transmission with advanced restraint systems can detect this situation after you have break around a couple of the curves , and " see " not to upshift again .
For more information on automatic transmission system and related subject , check out the link that espouse .
