[arunpools]

There is a paradigm shift in the Indian car buyer’s attitude towards Automatic Transmissions. People are opting for automatics for all sorts of reasons but mainly, we Indians have more money to spend and we are annoyed by today’s traffic situation in most of our cities and urbanized neighbourhoods.

I think that many people out there are confused about which automatic type they should choose and why. A plethora of information on this topic can be found floating around on the web for everyone’s consumption. However, I have concluded that most of the information available on the web is quite superficial and lacks the kind of depth that one would like to find. Just to make my point, I don’t think many people know that CVT-equipped vehicles may also have torque convertors in them! Many people avoid Torque Convertor automatics citing the inefficiency of torque converters. Think again!

The following is not a comprehensive account of the various types of automatic transmissions out there by any stretch of the imagination. Neither, this is meant to be advice for buyers on which is the best technology to choose. However, I have tried to make this to be that extra bit of info that I have found missing, that might help people make a more informed decision when they make the plunge or help effectively use the technology that they have opted for. Above all, you might read this simply to better appreciate these tremendously complicated technologies as much as I did researching on this topic.

Okay, I know I’m at the risk of stating the obvious (or blogging on the topic), for TeamBHP is a place where knowledgeable people hang out. Nevertheless, I have decided to write a blog on this technology. So, here goes nothing.


A Modern Conventional Automatic from ZF. Courtesy of Ritchyblack - Stefan Krause.

Note

In this blog, there are no references to any pictures (except one), diagrams or photos. This is intentional because all the pictures, diagrams, animations etc., are just a click away anyways .

Like in many other reads that one might find on this technology, I have to start by stating the fundamental reason why an Internal Combustion Engine (ICE) equipped automobile needs a transmission in the first place. This might be trivial info to most of the guys here, but for completeness in the following discussions, an explanation is found essential.

The need for a transmission

All engines (electric and hybrid car vehicles included except direct drive electric systems) need some form of transmission to couple the prime mover (engine) and the wheels of the vehicle. The need arises because of the characteristics of the engine in question. Engines may produce decent torque only when they rotate between a certain range of engine speeds, the engine would offer good efficiency when run in a certain speed range (which may be different from the maximum power-producing speed range). Depending on different driving conditions of load and vehicle speed, there has to be some form of a mechanism to connect what the engine can deliver and what the vehicle demands. For brevity, discussion on EV transmissions can be kept for another time and confine our discussion to the internal combustion engines (ICEs). That said, transmissions in ICEs do the following things,

1. ICEs produce no torque at zero engine speed. Thus, the transmission has to be able to mate the stationary wheels of the vehicle with the engine that is already rotating at idle or slightly higher engine speed.
2. ICEs produce useful torque and acceptable efficiency only about a specific engine speed range. This speed range will be a portion of the overall range the engine is designed to operate. The power produced by the engine, which is proportional to both torque output and engine speed, would first increase with speed, reach a peak and then drop (or run flat) with a further increase in engine speed. A transmission strapped between the engine and the wheels in a vehicle would have to meet what the engine can deliver and what the driver is demanding within the operating envelope of the vehicle.

There are fundamentally 4 types of automatic transmissions in the mass market. The following enumerates the important ones,

a. Conventional Automatic (CA)
b. Continuously Variable Transmission (CVT)
c. Dual Clutch Automatic (DCA or DCT)
d. Automated Manual Transmission (AMT)

Conventional Automatic

This is the most common type of automatic transmission on the world stage. This was also the first automatic transmission type to ever hit the road in mass-market consumer vehicles. There are two reasons why this type took the title for the first auto. For one, automatic gear change could be achieved without an electronic computer (in the forties, they did not have any electronic computer that could fit in a car) and a device called a torque convertor, which took place of the clutch in a manual transmission, could smooth out shift shocks and get the car moving without stalling the engine.

The conventional automatic also sported an ‘epicyclic’ gear train, instead of the usual gear pairs and dog teeth arrangement. In an epicyclic gear train all the gears in the gear train are always in mesh unlike in a manual where despite having all the gears in mesh, the selection of different gears involves disengagement and engagement of dog teeth. Different gear ratios in epicyclic setup are selected by allowing or disallowing different parts of the gear train from rotation with respect to another component of the train or with respect to the case of the transmission. This meant that in the conventional auto, gear shifts could be achieved by actuating a combination of clutches and brakes. For actuating the clutches and brakes, hydraulic piston-cylinder arrangements were utilized which got actuated via hydraulic a selector-valve position. Positive displacement pump(s) was kept running for providing the hydraulic pressure. Also, in earlier autos, the clutches and brakes had only two positions –fully ON or fully OFF.

In the early auto, gear shifts were achieved using a clever centrifugal governor connected to the wheels of the vehicle. The governor pulled on a lever attached to the hydraulic selector valve. The faster the car went, the higher the force applied to the valve by the governor. The more the force, the more the valve moved. The further it moved, the higher the gear selected. However, the force on the hydraulic valve was countered by the position of the accelerator pedal also. A higher accelerator pedal input also meant a higher force on the selector valve in the opposite direction to the governor. What all of this meant was, when the accelerator was pressed hard, shifts would happen at the higher vehicle (and thus engine speeds) and under part throttle conditions, shifts could happen at lower effective engine speeds. This arrangement also allowed downshifts when the accelerator is pressed hard while in a higher gear.

Okay, none of this would work if not for the torque converter that sits between the engine and the gearbox input shaft. The torque converter would allow its input shaft to rotate with the engine while the output shaft is completely stopped (when the vehicle is stopped in drive mode). Also, the device produces more torque than it receives at its input shaft when stalled. This aspect of the device, from which it gets its name, aids in getting the car going from a standstill. Sort of acts like an additional gear reduction over the first gear. The torque multiplication by the torque converter can be as high as 2.5 times. However, the choice of this value is manufacturer and setup dependent. Because this device works by utilizing dynamic forces of oil flowing in a continuous loop between a pump and a turbine having no direct mechanical connection between, shift shocks are effectively dampened as well.

However, a torque converter, in its pure form, is not an efficient mechanism to transfer power. When the speed difference between the pump (input device) and the turbine (output device) reaches about 80-90%, the torque converter would reach its maximum power transfer efficiency. The efficiency itself is about 90% which means, even with the torque convertor working at its highest efficiency point, at least 10% of the input power would get wasted in the device itself . The resulting churning of the oil inside the device would increase the oil temperature. The heat is effectively removed by flowing the hot oil from the torque converter through air cooled oil cooler.
The output shaft speed from the torque converter would always drag behind the pump (engine) speed. This problem was circumvented by the addition of a Torque Converter Lockup Clutch. This clutch would mechanically couple the torque converter pump and turbine once suitable conditions are met, allowing it to have 100% power transfer efficiency. The strategies used for closing the locking up of the clutch have also evolved over the years.

In earlier iterations after the introduction of the lockup device, clutch closure was triggered by hydrodynamic forces inside the torque convertor itself and positions of various hydraulic shift logic valves in the transmission etc. No electronics were at play. Even with this rudimentary control, fuel efficiency could be improved considerably.

The fundamental principles of a conventional automatic have remained the same over the years. However, what has changed by leaps and bounds are the control strategies of various clutches, and brakes inside the device. Today with the advent of microcontrollers, shift strategies based on centrifugal governors and accelerator pedal position acting on a shift selector valve like a tug of war can be dispensed with. Moreover, the ON and OFF only positions of the clutches and brakes could now be gradual engagements and disengagements achieved via the use of electro-mechanical servo-hydraulic valves acting on a Pulse Width Modulated supply controlled by a microcontroller. With the microcontroller AKA Transmission Control Module/ Unit or TCM/TCU), the number of inputs that it can take for making a shift decision could be many. Engine speed, accelerator position and rate of its application, the selected drive mode, the transmission temperature etc., can all be decision-making parameters.

Under TCU control, the lockup clutch could be closed much more aggressively than before for improving fuel efficiency. A shift from one gear to the next can even be made without opening the lockup clutch in the torque converter (shift shocks are kept in check by gradually engaging and disengaging relevant clutches and brakes under the precise control of the TCU). Also, the TCU can stop the transmission from making an upshift going down an incline unless the driver presses the accelerator. With a closed lockup clutch, engine braking can be as good as in a manual transmission car. To know what I’m talking about, put the transmission to Manual mode if it has one.

It is to be noted that the lockup clutch would open inadvertently when the vehicle speed drops below a lower limit causing sudden loss of engine braking like slowing down to a speed breaker or something. In such very low-speed conditions, with the lockup clutch remaining fully open, torque convertor efficiency would be appalling. This is also the reason why conventional autos return terrible fuel mileage in stop-and-go traffic.

The bottom line is that conventional automatic transmission systems have evolved over 80 years to what it is now. Developers and designers have had all this time in improving various systems and components, making this one of the most reliable auto transmission technologies ever to hit the road in mass-market vehicles. This technology is relatively costly to put together because of its high complexity and returns lower fuel efficiency figures than some of its competing systems like CVT or even DCT. Though complex, the cost of this transmission is kept in check by economies of scale. Moreover, conventional automatics are rugged, smooth to drive, and can take a beating in the most demanding of applications. This makes it the go-to technology for SUVs (the real deals not imposters), off-road vehicles and other larger vehicles etc.

This technology is not limited to large lumbering giants with no high-speed performance requirements, with the right design and engineering, the shifts can be lightning fast. There are many examples in today’s auto industry where manufacturers have moved back to Conventional Autos from Dual Clutch Systems.

2020 VW Polo GT TSI with 6-Speed Convectional Auto which they put in place of 7-speed DSG point to a similar conclusion. Even in the higher-end spectrum like BMW M3 Competition (which used to sport a 7-speed DCT called Steptronic) now has 8 Speed ZF conventional automatic in some International markets. Earlier Conventional Autos sucked a lot of juice from the engine. Performance car manufacturers want a solution for more direct coupling between the engine and wheels. So first they invented the Automated Manual (BMW, Lamborghini, Ferrari etc). Further down the line, they progressed to Dual Clutch Transmission technology and now many of them are back to advanced Conventional Autos.

Dual Clutch Automatic

The Dual Clutch Transmission or DCT AKA DSG was invented when Formula 1 race-going teams wanted a super quick and lightweight transmission for their cars. DCT is like two manual transmissions with two clutches in a single unit. One of these transmissions only has odd gears (1, 3, 5, 7 etc.) and the other has even gears. The clutches close one at a time, thus power flows from the engine to the wheels through one of the clutches, let us say odd gear clutch. While one is in operation, the other clutch remains open. With the even gear clutch remaining open, the even gearbox can keep the upcoming gear selected. So, when the time comes for the next gear to engage, it is just a matter (at least theoretically) of opening the odd clutch and engaging the even clutch. This process can be accomplished very fast, in a few tens of milliseconds.

The DCT is controlled by computers like all modern autos today. While shifting from one gear to the next, the clutch connected to the outgoing gear opens gradually while the clutch connected to the incoming gear is already engaging. The torque transmitted by each of the clutches would even equal at a certain point! This strategy reduces (to the point of nil) power drop during a gear shift procedure and gives the fastest vehicle acceleration. One must note that there is no torque converter in a DCT, and the speed difference between the outgoing clutch and the incoming clutch is adjusted by slipping the clutches. I remember driving a Volkswagen Polo DSG a long time back. This was pure joy, I could hardly perceive gear shifts since the RPM would hardly drop from one gear to the next. I have also driven Volkswagen Ameo Diesel DSG, Kia Seltos DCT and others but I don’t remember experiencing the same performance though. Probably the underwhelming performance is a result of their manufacturers trading performance for improved longevity of the transmission.

DCT technology is possible only because of clever computers. DCT has to rely on calibrated slipping of its automated mechanical clutch (or clutch pack) to get going from a standstill. To smoothly get going from a standstill, the computer has to figure out how to engage the 1st gear clutch smoothly taking into account wear and tear and other conditions. Since DCT does not get any torque multiplication as in a torque convertor, the transmission has to rely on increasing the engine RPM above idle to start creeping forward. In a conventional automatic in Drive Mode torque is always applied to the drive wheels. One has to apply sufficient brake force to keep the car from creeping forward while stopped. In DCT however, even in Drive Mode, once the car has come to a complete stop the clutch is completely kept open. DCT detects the requirement to move ahead by checking whether the brake pedal is pressed or not. When the driver lifts the foot from the brake pedal, DCT would increase the engine RPM and slowly start engaging the appropriate clutch. This would work just fine in normal forward-driving conditions. However, in tight parking manoeuvres where precise control of power and brake are needed, DCT might become cumbersome to handle. While in a Conventional Auto, or even in a CVT with a Torque Convertor, one just has to modulate the brake force.

In stop-and-go traffic conditions, constant slipping of the clutch would deposit heat on it, which further deteriorates (albeit temporarily) clutch performance if the temperature rise is not kept in check. DCTs come in two varieties as far as the types of the two clutches are concerned. The low-cost ones come with dry single plate clutches while the more serious ones come with wet multi plates. While cooling of the wet multi-plate type DCT can be lesser of an issue to handle, it is the dry clutch ones that are to be taken care of under extreme conditions.

The bottom line here is that DCT is fantabulous technology for race cars and high performance sports cars. They are very quick in gear shifts with the least amount of power loss between shifts, they are relatively lightweight, low on parasitic losses compared to conventional autos and can handle oodles of torque and power. But when manufacturers adapted this racing technology to everyday cars, they had the gargantuan task of making this technology reliable and cost-effective – not attributes that race cars have to bear. One must take into consideration that the likes of Porsche 7 Speed PDK have wet multi plate clutches and other advanced technologies capable of handling all the abuse that a race car has to bear. While that may not be the case in DCT avatars found in more humbler vehicles.

There is a lot of loose talk floating around on the web about DCT technology being more complex than Conventional Autos. This is pure garbage. Think about the 10-speed Conventional Auto by Ford. Even the six-speed ones are at least as complex as a comparable DCT. Most of the DCT incarnations that we drive around in our country today are dry-clutch ones. Manufacturers have packaged their vehicles with this technology and have advertised them as a vehicle for adrenaline junkies. At the end of the day, DCTs in a sub 25 lakh vehicle is a technology that gives you good fuel mileage (compared to a conventional auto), reasonable to good driving dynamics and reasonable cost. Though many a time, at the expense of some long-term (sometimes even short-term) reliability, comfort and fine vehicle control.

Continuously Variable Transmission

All the transmission technologies provide stepped gear ratios to keep the car engine running at the required performance ‘point’. Theoretically, the more the number of gear ratios, the better will be the performance of the transmission in keeping the engine at its optimum operating ‘point’. Increasing the number of gear ratios in a traditional automatic or a DCT beyond let us say 8 or 9 has obvious cost and complexity penalties. A Continuously Variable Transmission circumvents this problem by allowing infinite ratios between two fixed ratio limits.

There are many different CVT technologies in the industry. But pulley and belt/chain-based CVT are by far the most common in consumer vehicles. A CVT achieves these infinite ratios by running a belt/ chain between two variable-diameter pulleys. One of the pulleys is connected to the engine side through a decoupling device and the other to the wheel side. Variable diameter of the pulleys is accomplished by various methods. In small scooters and similar low-power applications, the diameter is varied by centrifugal flyweights on the drive pulley and counter springs on the driven pulleys. However, in modern car CVTs, the forces generated by centrifugal forces and springs are simply not enough to make the pulleys move or even if they did, this type of control simply isn’t sophisticated enough for the demanding requirements of a car transmission.

So, for controlling the size of the pulleys, hydraulic actuators under the electronic control of the TCU are employed. It must be borne in mind that the CVT depends on the friction between the cone pulley surface and the sides of the belt or chain to transfer torque from the drive pulley to the driven pulley. This friction depends on the clamping force with which the cones hold the CVT belt. Precise control of this clamping force is necessary to hit the right balance between wear and tear and torque capacity. In a car, there can be situations when torque demand would change suddenly, like when the car tyre suddenly regains traction after slipping on slush. This would mean that the hydraulic actuators have to respond not only to slow changes in traction demand but also the sudden ones (Audi Multitronic).

Though a clutch in manual transmission uses friction for transferring torque, the friction area available for applying the torque is substantially higher than what is available between the cones of a CVT and its belt. Also, normal friction materials used in clutches cannot be employed by CVTs. Also, the contact area available for transferring torque between the pulley and its chain depends on the effective diameter of the pulley. A lower effective diameter pulley results in a lower friction contact area.

In scooters, fibre-reinforced rubber belts are employed. F used in a car transmission, rubber would simply burn out under the forces. So, in cars, either a ‘metallic push belt’ or a ‘pull chain’ is used. Metallic CVT Push Belt was originally developed for DAF 600 by Hub Van Doorne. I would refrain from explaining how a push belt is designed or put together. But for the sake of this discussion, a metallic push belt is what it is – it is capable of transferring force from one CVT pulley to the other by way of push or compression force rather than tension or pull force as in the traditional sense of a belt and pulley system. The metallic push belt is ‘rigid’ for all practical purposes in compression (and tension too) likewise; the metallic chain is rigid in tension. Except for how the forces are transferred, both types achieve the same goal in a CVT setup. The push belts are more common though. One example of chain-type CVT is Audi’s Multitronic.

As pointed out before, CVT has infinite ratios between two fixed ratio limits (let us say, CVT can select any ratio between 4:1 and 1:2). So, just like any other transmission technology, this one also needs a device to replace the clutch for starting (for ratios more than 4:1 in the above example) from a standstill or for extremely low-speed movements of the vehicle. There are two methods employed to achieve this, an automated clutch and a torque converter. In Audi’s Multitronic, wet mutilate clutches are employed, but it seems a torque convertor is more common in other CVT vehicles.

It is safe to assume that the CVT TC would also have a lockup clutch like in a conventional auto to save fuel. Since CVTs also may contain a torque convertor, calling a conventional Auto a torque convertor auto would be wrong.

There are beautiful things that can be achieved with a CVT. When you press the pedal to the metal, the transmission would allow engine speed to reach safe max power RPM and then it would continuously adjusts the CVT ratio to reach the over-drive ratio or till the foot is taken off from the accelerator. With the engine kept at its highest power RPM without any engine speed changes caused by gear shifts, the car would have the highest acceleration (compared to CVTs Stepped gear brothers), at least in theory. Moreover, if the driver wants to keep operating in high fuel efficiency mode, his intentions are detected by the TCU from various inputs (part throttle, rate of change in accelerator position, vehicle speed and load etc) and keep the engine at the most efficient engine speed point. This is what makes CVTs the most efficient of all transmission technologies.

But for many CVT users out there, their car being extra fuel-efficient has not been a reality. This has to do with energy losses in the transmission and even the driving style itself. An isolated CVT V belt gearbox has more energy losses than a comparable isolated conventional manual gearbox. Moreover, if equipped with a torque convertor, the losses in TC would add up to a significant portion of the inefficiency of the overall system. Inefficiency would be more evident in stop-and-go driving conditions.
The lacklustre performance of CVT reported by many users in most cases has more to do with perception than with reality. Sure with a CVT, the engine sounds weird but, we have to take into consideration that many CVT-equipped cars have small low powerful engines too and are anyways dull. Likewise, the rubber band effect that many complain about also seems to be a similar thing. It seems to be like when some might get a rush when the turbo kicks in (in a car with pronounced turbo lag) in comparison to a car with a linear power delivery of the same power and engine speed category.

To add, there is no slack in the CVT metallic belt or chain for it to stretch like a rubber band! Car Companies have tried to even put virtual discrete selectable ratios (Honda ‘7 Speed CVT’, Audi ‘8 Speed Multitronic’ for example), the very thing that CVT tries to avoid, in their cars so that their customers don’t complain.

CVTs have found a place in Formula 1 history too. In 1993 Williams team successfully put a CVT in an 850 HP F1 car and put the track ablaze. It is rumoured that FIA banned the use of all types of transmissions other than fixed-geared ones right after Williams showcased this new technology, effectively stopping Williams with the CVT from hitting the tracks. FIA is weird! One must remember F1 and similar high-tech events are breeding grounds for new advancements in automobile technology. FIA killed this one.

The bottom line here is that CVT had the potential to be a more significant player in the car market. But thanks to the general public perception that CVTs are bad to drive and unreliable, throw in FIA’s tantrums to the fray, and you have CVT where it is. It was Nissan and its unreliable CVTS that did the most damage to CVT’s reputation in the international automobile industry. There are so many references on the web on Class Action Suits filed against Nissan for all sorts of problems with their CVTs. But there are also stories of many happy customers who have easily driven their CVTs cars to 3,00,000 km as well.

The performance of CVT-equipped vehicles and transmission life highly depend on how these are treated. Overcooking this one would lead to premature transmission failure. CVT transmission oil needs regular attention. Under stress, CVT tends to chip small metallic flakes into the oil. If these things keep circulating back to the friction surfaces would mean even more damage and possibly lead to a ‘damage-runaway’ situation.

Unfortunately, CVT would not see much change in its status quo since EVs are here already. Most engine and subsystem designers have stopped doing any research towards advancements in diesel engines and are only making some small technical improvements in the petrol ones. With the doomed future of ICE, CVT would wither as is. Such a shame!

Automated Manual Transmission

As a child, I remember thinking about why car companies couldn’t replace the gear level and clutch pedals with actuators. When I learned how to drive I realized that though it would be relatively easy to make a gear shift actuator, it was the automation of the clutch that needed to be figured out. Though I was well aware of all the other auto technologies out there, I would brush them off as being very expensive and unreliable. But when AMT technology hit our shores I dug out research papers to understand how they figured out the clutch. To my absolute delight what I discovered was how advancements in computers, control systems and technologies to analyze interactions of complex mechatronic systems could bring simplicity to the common man.

AMTs, to the uninitiated, are Manual Transmissions with actuators (hydraulic or electromechanical) for changing gears and actuation of the clutch. The actuators in turn are under the direct control of the electronic brains of the TCU. In most cars with AMT, the automation module is a strap-on device that sits atop the car’s standard manual transmission. This makes it possible for its manufacturer to have much-higher parts sharing between its manual and AMT variants bringing the overall cost of the AMT-equipped version of the car affordable to the consumer.

I fondly remember test-driving the Dzire AGS for my dad. He wanted an upgrade from his manual car for he was done with clutching and declutching through the traffic. I was pessimistic about Dzire’s drive quality since a lot had been said by the auto Journos. I thought I would snap my neck (pun intended) during the test drive. To my surprise, the car drove incredibly well and had an extra smooth creep function to boast too. I could hardly feel any gear shifts under moderate acceleration. Even when the car was pushed, the gearbox maintained complete composure and the gear shifts were reasonably quick without any fuss. Many of you guys might be thinking I’m crazy but, I’m not, and neither is my dad who has been driving a Dzire AMT since 2017. He has some 50k km on it and the vehicle returns 17+ km to the litre on the twisty and windy roads in his part of the country. I must add that I drive a conventional auto-equipped car and have the right reference for comparison.

Many people who find AMTs to have rough shifts are very much correct in their experiences too. There are some good examples in the market today to stay well clear of. But there are good ones too like the AMT variants of Dzire, the latest Baleno etc in which you would find AMT from Magneti Mareli. I’m not a big fan of Maruti Suzuki in particular because of their below-average engineering quality but, they have figured out their AMT for sure. I’m in no way trying to say that AMT is comparable to a conventional auto when it comes to gear shift quality but, as a package, this technology is not something to be made jokes about or condescended as cheap and unrefined.

The TCU in an AMT has to mimic a human driver’s dexterity in applying the clutch and changing gears. So just like a human driver would, when AMT wants a gear shift to be made, it would cut fuel to the engine, apply the clutch, make the shift and release the clutch. This makes it possible for the driver to drive without lifting off of the accelerator pedal during a gear change. For this, AMT-equipped vehicles (modern ones), get throttle-by-wire. All CRDI Diesel engines are throttle by wire already. There is no mechanical cable running between the accelerator pedal and the fuel pump anymore. However, in petrol engines, throttle by wire requires the butterfly valve in the throttle body to be controlled by a servo motor (now almost all petrol cars, irrespective of whether it is auto or manual, have throttle by wire for meeting emission norms). In a conventional automatic, since gear shifts are made very quickly, direct control of the engine throttle seems not to be a necessity, though almost all modern TCUs communicate with the vehicle's Engine Control Unit for better overall control.

One must note that engagement of the next gear in AMT requires the gearbox synchronizer to do its job as well (this is the case with DCT as well. Just that in DCT, the next gear can be kept preselected). This leads to an extended pause in power from the engine in AMT, especially under high engine RPM shits, leading to deceleration for a short while. This is what causes the infamous head node in AMT cars. A human driver also has to deal with all these technicalities of a gear shift in a manual but he completes the procedure with or without realizing it. In an automatic, however, the driver does not know when a gear shift is going to happen and when it does inadvertently, he notices it.

Try changing gears in AMT in manual mode, head nodes would be hardly noticeable though all the processes that are taking place during a complete auto gear change and the manual mode gear change are the same. It is just that the gear shift in the manual mode happened when we expected it to happen.
The pause or cut in power from the engine would be even more noticeable when going up a steep incline fully loaded. A downshift in such a condition would be close to being violent. This is because; the TCU knows that the downshift has become necessary under the fully loaded condition from falling engine speed and accelerator pedal position inputs. It has to make this shift quickly or the car would lose speed even further. In such a situation the TCU first cuts engine power declutches and increases engine RPM to a level for easy gear synchronization to the lower gear and engages the clutch. During an overtaking manoeuvre also a similar situation may arise when the TCU decides to go for a downshift albeit less violent. I have found myself shifting over to manual mode in my dad’s DZire AMT every time I have to make those quick overtakes. After the overtaking manoeuvre is completed, I go back to Drive mode. The shifts in the manual mode feel direct, natural and easy.

In my Convectional Auto, auto downshifts are very fast and without much fuss (it is certainly not completely devoid of it) however, its manual mode is useless except in 1st and 2nd gears. Also, conventional autos, the epitome of perfection to many, from Yester generations used to hunt in certain conditions going back and forth between two gears, confused about which gear to stick to.

AMTs were used by BMW, Lamborghini, Ferrari, Volvo etc. BMW E60 M5 and E63 M6 both were equipped with 7-speed SMG III AMT. SMG stands for Sequential Manual Gearbox, which essentially was the manual transmission with strap-on automation. This device had a manual mode via paddle shifters and a fully auto mode. This is an M63, mind you. To add, Volvo trucks and buses that come with Volvo's I Shift were AMT from 2001 to 2014. As per Volvo, there are millions of Volvos out there with this system. Bosch provides AMT solutions to truck and bus OEMs. Many people believe AMTs are a low-cost, low power and low-torque engine option. Think again! It is just that the car that you bought has a messed up AMT.

The bottom line is an Automated Manual is a fully automatic transmission in all respects. When you have to decide on which transmission to go for, just don’t listen to what people are talking about. Go and get a test drive and feel for yourself. Keep in mind that all modern transmissions have intelligent brains in them. They learn your driving style and adapt. AMT will also take some time to learn your driving style.

AMT will have all the problems of a DCT and then some. Temperamental crawl speed performance, overheating and possible judder in extreme stop-and-go traffic, difficulty in taking off from steep inclines etc. Except for difficulty in taking off from inclines, DCTs also would have almost all the above problems, since most DCTs have higher-powered engines and or have Hill Hold. Likewise, a CVT may go kaput at 80k kilometres. Many times a CV Transmission rebuild may not even be possible. In that case, CVT can be quite a bit expensive if you get unlucky.

In normal sane driving conditions all the transmissions, including AMT, would just be fine. Also, the AMT option will not cost a ton upfront when buying a new car. Fuel mileage would be in the good to excellent range depending on your driving style and when it breaks down, it won’t break your bank. The sales number of AMT is a testament to the success of this technology in the Indian market.

Bottom Line of all the Bottom Lines

All automatic transmission technologies are complex to varying degrees over their manual counterparts. They all target to bring convenience and some fun to the driver while keeping the costs down. All the manufacturers consider various things in deciding on which type to offer on which car model in their lineup. For example, Mahindra gives the AMT option in their XUV 300 meanwhile, all the larger vehicles in their lineup get conventional autos. While the AMT option would add about 50k to the price of the XUV 300, conventional auto or a DCT would have added 100-150k to the same car. Add the price premium and lower fuel efficiency of the CA option, XUV 300 with a CA would probably make no sense to the buyer.

As with anything technical, there is a learning curve associated with driving automatic transmission-equipped vehicles too. Modern TCUs try to learn one’s driving style from various inputs that it receives. Many people who have been sane drivers when in a manual car, would turn a little too enthusiastic in an automatic. They floor the pedal more often, out-accelerate everyone at a traffic light and may indulge in erratic driving patterns with more braking and accelerating manoeuvres. I’m also guilty of doing this to some extent when I moved from a manual to an auto. While driving a manual the driver gets instant feedback from the vehicle by way of vibrations or noise while doing things that the transmission or the engine dislikes, an auto transmission’s TCU tries to mask these by changing the transmission and engine operating parameters. This most often is at the expense of the life of transmission components, fuel economy, shift quality etc. In a CA this might involve keeping the TC lockup clutch open till a higher engine RPM or more aggressive engagement of its multi-plate clutch packs, in a DCT or AMT this may be higher than normal slipping of the clutch or operating a CVT at a higher transmission loss regime.

Learning to drive a particular type of automatic is an important aspect of owning it. Not all automatic transmission technologies are equal, but they all promise to give a level of increased comfort when handled properly. If pushed beyond their normal operating envelope often, all these systems can also become a pain or a total nightmare to their owner.

[Dr.Car]

Originally Posted by arunpools

There is a paradigm shift in the Indian car buyer’s attitude towards Automatic Transmissions. People are opting for automatics for all sorts of reasons but mainly, we Indians have more money to spend and we are annoyed by today’s traffic situation in most of our cities and urbanized neighborhoods.
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What a nice thread especially for someone like me who also researched a lot about automatics a few years ago when we bought the first automatic car which was an AMT. But, I was simply unable to find such high quality information anywhere. Hats off to you sir

Just one thing I'd like you to highlight is the difference between normal CVTs and E-CVTs which are nowadays being used in Hybrid vehicles like the Toyota Hyryder, Innova Hycross and Honda City hybrid since most articles I have read about them state that they are very different from normal CVTs and thus, have completely different characteristics. Also, could you please explain why any other automatic type can't be used for these Hybrid vehicles. Thanks in advance

[arunpools]

Its a AMT 'head nod' not a 'node' in my blog! I'm out of luck and cant edit my original post! So my apologies (It seems i will have to learn to use the Assembly Line)

[MasterFoxD]

Whoa! Someone did his research well. And this should give some AMT nay-sayers proper sleepless nights.

But fun and jokes apart, I really appreciate the thorough detail with which you have gathered and shared the myriad but nuanced intricacies of automatic transmissions. May be too much technical jargon for some people (though we are all true-blood auto-buffs), but nevertheless something for everyone.

In my opinion, the DCT fad was really built up by the Polo GT, which to some level did bring about a smile to everyone who drove it (though I should admit I never got to), and of course, amply fuelled by the auto-media. But one should never forget the fact that all these cars had good engines in the first place. And my take is no amount of transmission wizardry can make a poor engined car feel good to drive (or maybe not, with Tata Altroz DCA being a case in point?), but the vice versa being very possible.

Looking forward to more of such auto-gyaan from learned fellow bhp-ians. Kudos to you, bro! (yeah, literally)

[Narendranath]

That was a fantastic exposition of the mystery world of automatic transmission technologies for dummies. Anyone contemplating purchase of a new automatic car should go through this post before making the purchase decision. A truly painstaking and thorough analysis.
Thank you sir for your serious effort...

[shankar.balan]

Thank you @arunpools!
Superb post and great research.
Im wishing to just add on a tag to an youtube video which visually illustrates the AT. I think this will be useful when viewed in conjunction with your post.

https://youtu.be/2Ld6xhJuGBk

[SedateGuy]

One of the most detailed and informative article on automatic transmissions. Loved the unbiased perspective on AMTs.

[arunpools]

Originally Posted by Dr.Car

Just one thing I'd like you to highlight is the difference between normal CVTs and E-CVTs which are nowadays being used in Hybrid vehicles like the Toyota Hyryder, Innova Hycross and Honda City hybrid since most articles I have read about them state that they are very different from normal CVTs and thus, have completely different characteristics. Also, could you please explain why any other automatic type can't be used for these Hybrid vehicles. Thanks in advance

Thank you very much. Coming to the topic of E-CVTs, Honda, Toyota, General Motors etc all have different Hybrid systems. A Hybrid drive train tries to put its ICE engine at its maximum efficiency point throughout the vehicle’s operating speed envelope. Moreover, ICEs in hybrids run on Atkinson Thermodynamic Cycle. In Atkinson Cycle, the expansion stroke is longer than the compression stroke of the engine. This allows for better extraction of heat energy produced from the burning fuel. Simply put, the increased thermodynamic efficiency is a direct result of the increased distance the high-pressure gases push the pistons before getting expelled into the atmosphere through the exhaust port. Atkinson Cycle Engines are very much similar in construction to normal engines. Except in this one, the intake port remains open well into the compression stroke, thereby reducing the effective compression stroke (thereby relatively increasing the expansion stroke).

If this rather simple arrangement improves fuel economy, then why not all engines run on this principle? The answer is, Atkison Cycle engines have to operate at a high engine RPM range to generate acceptable performance figures. The operating speed range itself can be quite narrow. Moreover, these engines also have a lower power-to-cubic-capacity ratio. So, you have a high-revving, low-power engine that threatens to fall off acceptable performance margins the instant engine RPM is changed (at least for the sake of discussion this is true). Hence, this as a standalone engine has limited applicability in vehicles.

However, somebody had this bright idea that if they strap together an Atkinson ICE, an electric motor and a small-capacity battery together in a car, then better fuel efficiencies can be achieved by using the best of all three systems. So, the first thought that you may get is to connect the engine directly to an electric generator and put the power produced by the generator back into a traction motor connected to the wheels of the vehicle. The battery can be put in between the generator and motor to provide some electric power cushioning. The battery can also be used to store power that is generated during regenerative braking as a bonus. This setup is known as a Series Hybrid setup. In this, the Atkinson ICE (A-ICE) can run at its best efficiency point and keep producing electricity and feed the traction drive motor. The traction motor can draw power from the battery and the generator together as and when needed. When the battery is full, the ICE can go completely off and wake up when needed. Clever? Not so much, since the combined efficiency can often offset the advantage gained from the Atkinson ICE.

So, an arrangement that allows the A-ICE to drive the wheels directly in addition to the series motor-generator setup, as discussed before, is adopted. The engine does not permanently drive the wheels, it only comes into action when the vehicle speeds match with engine speed (remember Atkinson Cycle Engine has a narrow operating speed range) inside the transmission. This arrangement is called a Series-Parallel Hybrid. Series – because, the engine, the generator and the motor are all working back to back to power the wheels of the car. Parallel – because, the engine, in certain operating scenarios (usually only at high vehicle speeds), drives the wheels directly in parallel with the motor. The electric motor is designed to operate at low vehicle speeds because of its high torque characteristics at low speeds. Meanwhile, at very high vehicle speeds, the electric motor may even be completely taken off (it would still be rotating albeit without any power output) and only the Atkinson ICE would be powering the wheels. Engagement and disengagement of the engine from the wheels are by operating automated clutches. This arrangement can be found in Honda Hybrids. The reason why Honda calls this tech an E-CVT may be because the engine can remain at its optimum efficiency point while the electric motor in Series - Parallel mode with the engine can take the car through its full range of speeds. This is similar in principle to what a common pulley and belt CVT in an ICE-only vehicle tries to achieve.

Toyota Hybrids are generally Parallel-only hybrids. In other words, the engine is either completely off or it is powering the wheels in parallel with the electric motors to varying degrees. In this setup also, there are two electric motors of which one is a Motor-Generator (MG) unit and the other is a Traction-only Motor. However, the connection between the electric motors, the engine and the final drive to the wheels is via an Epicyclic gear set called a Power Split Device (PSD). This is in contrast to the automated mechanical clutches used in the Series-Parallel setup.

An Epicyclic gear set, similar to that in a Conventional Automatic Transmission, consists of three gear kinematic elements and a gear carrier (I’m assuming you have Googled the term already, if not check the figure given below). These are known as the Sun Gear, the Planet Gear, and the Ring Gear. The planet gears are placed in a gear carrier called a planet gear carrier. In a parallel hybrid transmission, the A-ICE is connected to the planet carrier of the PSD meanwhile, the Motor-Generator (MG) is connected to the sun gear and the Traction Motor is connected to the Ring Gear. The Ring Gear in turn is connected to the final drive of the transmission. In other words, if the Ring Gear is rotating, the traction motor is also rotating, and so are the wheels of the car.



It must be remembered that normal hybrids rely completely on the engine to produce power. It may use electric motors that run from electric juice stored in its battery. But that electric juice came from the A-ICE in the first place. The PSD setup allows the splitting of power produced by the engine in two ways (or Power Flow Paths). The engine can generate electricity via the MG unit and that power can either get stored in the battery or go to the Traction Unit to power the car via the PSD (Path 1). Or, the engine can directly drive the wheels through mechanical gears in the PSD (Path 2). So, by choosing AICE Engine Speed, the MG speed, the direction of MG torque (the direction of torque determines whether it acts like a motor or a generator) and the Traction Motor power, the vehicle power requirements can be met.

Under high acceleration, all three devices would produce power together to accelerate the car (Sort of Path 3). During this time, energy stored in the battery would be used to power both motors. It must be kept in mind that Hybrids (normal Hybrids, not the more fanciful PHEVs) get small capacity batteries and those would drain pretty fast in a high power demand mode.
At very low vehicle speeds, the engine would remain off and all the traction would be provided by the traction motor. When the battery charge levels drop below a limit, the engine would come alive and charge the battery via the MG unit. Once fully charged the engine would go back off. Simply put, the engine remains working at its max efficiency point or it remains completely off at very low vehicle speeds.

At low to medium vehicle speeds, the power produced by the A-ICE (or most of it) would go to the wheels through the Traction-Motor power flow ‘Path’ via the PSD. Meanwhile, at high vehicle speeds, most of the engine power would go straight to the wheels taking the direct geared ‘path’ via the PSD. And at medium to high vehicle speed range, the engine power would be split up appropriately between the direct gearing and the Traction Motor power flow paths.

All this wizardry is possible because of the clever (yet very simple) Power Split Device and the advanced computer brains of the hybrid power train. The continuously variable combinations of engine power split between traction motor path and direct coupling path, all the while allowing the engine to remain operating at its maximum efficiency point, would have earned this setup the E-CVT name.

It is not that you 'cannot' put a normal automatic transmission in these cars, but it is you don't have to since you have got all that you expect from a transmission in the E-CVT setup itself.

[Auto_guy_101]

This is indeed an amazing thread. I wish I had found this when trying to understand automatics a couple of years back. I'm not sure if this is the right thread (sorry mods!) but I have a couple of questions on engine+ gearbox+ region combos

1. VWs DSG push
The VW Polo GT, vento 1.2 TSI, new 1.5 TSI, 1.8 TSI and 2.0 TSI (Octavia, tiguan), come with a DSG - dry or wet clutch. However, I've noticed 2 interesting facts that make me question their choice
- BMW, one of the best drivers cars uses a ZF torque converter in most of their cars - even the fast ones. No one has called a BMW TC car slow or non-sporty
- Some VW offerings in foreign countries (tiguan awd in Canada for example) also use a TC. Even the toureg in 2023 comes with a TC.
Given the general reliability issues and Indian traffic conditions, why are VW pushing DSGs in India? Will a TC not be cheaper (economies of scale) and more reliable especially in the under 50L Vehicles VW- skoda sell here?

2. The Japanese love for CVT
Many Japanese car manufacturers have a CVT as their only automatic option. This ranges from the humble Honda amaze to the likes of the Civic, Innova hycross non hybrid etc. I believe even the old Camry was a CVT and so was the Honda Accord.

While I agree that CVTs are simple, efficient and somewhat more reliable than others, I fail to understand why some rev happy engine's are being paired with a CVT?

I've seen videos of the 4th gen ivtec CVT city and an older Camry being floored and they do pick up speed. However, the use of CVTs here kills the sporty nature. Is there a reason we've not seen a TC Accord?

[arunpools]

Given the general reliability issues and Indian traffic conditions, why are VW pushing DSGs in India? Will a TC not be cheaper (economies of scale) and more reliable especially in the under 50L Vehicles VW- skoda sell here?

A TC or a Conventional Automatic (or simply AT, as I would like to call it) may not be a cheaper option to a DSG. A DCT/ DSG is a lesser-understood and relatively new technology. People tend to conclude DCTs to be more complicated than a comparable AT option because of DCT’s F1 roots. On the other hand, ATs are thought to be simple since they have been around for so many years.

Modern ATs are incredibly complicated with intelligent brains that would put even the best production DCTs to shame. If the DCT had two clutches, a 6-speed AT would have six and that’s a six speed! Many of those clutch packs are under PWM valve control and gives seamless gearshifts without loss of torque at the wheels and shift shocks.

If you find a modern AT sluggish through the gears, it’s simply because its manufacturer chose to put it that way.

BMW M Series uses a ZF AT now after having used Steptronic DCT for many model years. So, they have been there. But now, with the performance of Conventional Automatics (ATs) getting really good, BMW seems to have made come back.

The use of CVTs here kills the sporty nature. Is there a reason we've not seen a TC Accord?

In the Indian market, a dry clutch DCT seems to be the better option for VW. DCT as we know it was introduced by VW in cars that mere mortals use. This technology is a wee bit more fuel-efficient and somewhat cheaper in comparison to an AT option even. I must point out that a DCT provides a more direct connection between the engine and the wheels in comparison to an AT. Moreover, a DCT also gets the performance-oriented-cool-car tag. All of these sell, especially at the lower price bracket.

The choice of a particular AT technology in the mass market vehicles like that from Honda has a lot to do with its manufacturer's experience with a particular type of transmission technology. Especially so, if the technology was developed by the company itself. For example, you would find CVTs in almost all Nissans, even in some of their small SUVs! It’s like Nissans are synonymous with their CVTs, unfortunately for the wrong reasons. Though their larger and purposeful SUVs use ATs as expected, if you were to Google ‘Nissan CVT issues’ you would bump into even Class Action Suits levelled against them by disgruntled customers. CVT's notoriety as an unreliable technology (as opposed to your conclusion of CVT as a reliable one) is mostly because of Nissan and its CVT misadventures.

If the intended purpose of a particular manufacturer’s vehicle model on the global stage is that of a fuel-efficient compact-to-medium-size everyday car, there is a very high chance that you would find a CVT under its hood. Honda would even make a CVT behave like a conventional AT with stepped gears ( thereby defeating the very purpose of a CVT) so that its customers won’t complain about dull CVT acceleration and uninspiring engine notes. I don’t think Honda would have put a DCT or a performance AT under the hood of their Accord just for India, they love their CVTs too much.

[Sumedik]

Excellent analysis !

Originally Posted by arunpools

Honda would even make a CVT behave like a conventional AT with stepped gears ( thereby defeating the very purpose of a CVT) so that its customers won’t complain about dull CVT acceleration and uninspiring engine notes.

How is the gear stepping done in CVT? Is it logical or physical?

[arunpools]

Originally Posted by Sumedik

How is the gear stepping done in CVT? Is it logical or physical?

A change in transmission ratio has to happen physically in any given transmission type. To explain this in case of CVT, there are no discrete gears in it per se. That is why it is called continuously variable in the first place. To make a CVT behave like a geared automatic transmission, its Transmission computer makes the effective diameter of its pulleys move to discrete ratios only. In a normal CVT the pulley sheaves move incrementally from one effective diameter to the other and are not ‘stopped’ at any discrete ratios in between its min and max limits. However, in a pseudo Manual mode, when the driver requests a shift through a paddle shifter (for example), the pulley sheaves would ‘jump’ from one effective ratio to another and does not stop in between the two ratios. This sudden change reduces the engine speed (in case of an up shift) or increases it (in case of a down shift) and gives that distinct engine note variation expected out of a regular automatic doing the same shifts. Likewise, when one accelerates with a fixed the CVT ratio, the engine would sound similar to a TC equipped car.

In effect, the CVT ratio limits are divided into, let us say six, discrete ratios and the driver can select each one by using the paddle shifter. Phew!

[Auto_guy_101]

Originally Posted by arunpools

Modern ATs are incredibly complicated with intelligent brains that would put even the best production DCTs to shame.

Originally Posted by arunpools

BMW M Series uses a ZF AT now after having used Steptronic DCT for many model years

Originally Posted by arunpools

If you find a modern AT sluggish through the gears, it’s simply because its manufacturer chose to put it that way.

Thanks! I didn't know that AT's have gotten complex. Now I understand GTO referring to BMW's ZF8 tuning as wizardery.

Originally Posted by arunpools

I don’t think Honda would have put a DCT or a performance AT under the hood of their Accord just for India, they love their CVTs too much.

Sadly, Honda won't probably be making an accord available in India, much less a sporty one (hybrid/ anything beyond the usual i-vtec) from the looks of things. Even the Civic (whose earlier iterations are still cherished by owners) seems to have left for good.

[Sumedik]

Originally Posted by arunpools

In effect, the CVT ratio limits are divided into, let us say six, discrete ratios and the driver can select each one by using the paddle shifter.

Excellent analysis !

I think the essence of CVTs (no feel of shifts and more mileage) would still be retained in the stepped setup. Otherwise considering maintenance complexity and replace cost vs traditional AT, it does not make sense to go for Honda CVTs.

Also since you brought it up, in terms of maintenance complexity, how would you rate DSG vs 6 speed ATs in the Indian context?