Text: Tito F. Hermoso / Photos: Nissan Press, Chrysler Press, Porsche Press, Ford Press | posted September 29, 2010 18:12
The evolution of the automatic transmission
Once derided as a sop to the weaker [read: feminine sex], automatic transmissions are now the rule in many of the world's car markets. Long a staple in the USA, where manual gearboxes are the options, Australia and the teeming city traffic of Asia have adopted the automatic as the part and parcel of motoring. Owing to its less dense population concentrations, excellent state subsidized multi-modal forms of public transport and highly taxed fuel, Europe took a while to embrace autos, but the leaps and bounds of its technical superiority is finally finding favor among technically minded European motorists.
Automatic transmissions are synonymous with 2-pedal motoring, i.e. gas and brake pedal driving as opposed to 3-pedal motoring which needs the pumping of the clutch pedal for changing gears on a manual gearbox. Though many are led to believe that manual transmissions have ruled since the dawn of the age of the automobile, the most numerous, most simple and most popular car of the early 20th century, the Ford Model T did not have a clutch pedal as we know it.
In those days, all the other brands of cars used unsynchronized manual gearboxes, which meant that the driver had to know the feel of what rpm and speed to shift, depress the clutch pedal while moving the throttle lever. If one did not practice, shifting caused the gears to 'crash' with a horrendous graunching noise.
The Ford Model T
The Ford Model T had 3-pedals but they functioned differently from today's manual gearbox cars. The right most pedal, the position of a modern car's gas pedal, was the brake. The middle pedal of the model T was for reverse gear. The left most pedal is akin to a clutch but functioned as a selector for low and high gear. The two levers on the steering wheel also had functions. The one on the left controls the timing of the spark plugs and is set before starting the engine. The lever on the right is the throttle, the equivalent of today's gas pedal. The lever on the floor acted as the brake and if shifted to the middle, put the gearbox in neutral.
The Model T's transmission was controlled by the three foot pedals and the lever that was mounted to the road side of the driver's seat. With the floor lever handbrake in either the mid position or fully forward and the left gear selector pedal pressed and held forward the car entered low gear. When held in an intermediate position the car was in neutral, which also can be engaged by pulling the floor-mounted lever to an upright position. If the lever was pushed forward and the driver took his foot off the left pedal, the Model T entered high gear, but only when the handbrake lever was fully forward. The car could thus cruise without the driver having to press any of the pedals. There was no separate clutch pedal. If this sounds like some clunky operating system with so many default options, it certainly was.
Was it an automatic?
Not in the sense of today's 2-pedal automatics. But the quest for simplified motoring was later on achieved and popularized in the 50's by the scooter, which had one gear, a centrifugal clutch and all the rider had to do was just twist the handlebar throttle to go and squeeze the brake handlebar levers to stop. In the Model T, the pedals actuated the transmission's friction elements (bands and clutches) to select the desired gear. In some respects, this type of transmission was less demanding of the driver's skills than the contemporary, unsynchronized manual transmission, but still required that the driver know when to make a shift, as well as how to get the car off to a smooth start. Overall, the Model-T's transmission may have avoided the need for special skills to prevent gear crunching in an unsynchronized manual gearbox, but the whole process of driving needed 4 hands and 3 feet.
Attempts to simplify the chore of changing gear had its pioneers when the Sturtevant brothers of Boston, Massachusetts experimented with an automatic horseless carriage in 1904 . The shift between the 2 forward ratios was effected by flyweights: at high speeds the higher gear was engaged, while descending RPM made the gearbox shift to low gear. Unfortunately, the metallurgy of the time wasn't up to what we now call as 'shift shock' as the resulting damage to the gears, considering the metallurgy available at that time, led to breakdowns.
The need arises
With such complicated shifting and clutching processes, its no wonder women were not trusted to do the driving. But thanks to this complication, man sought other means to simplify the process of gear selection. In the cause of easing the driving chores, the automatic transmission sought to deploy hydraulic power to replace left leg muscle power.
In 1934, both REO [Ransome E. Olds, founder of Oldsmobile] and General Motors developed semi-automatic transmissions that still used a clutch to engage the engine with the gears. The General Motors 'Automatic Safety Transmission,' used a power-shifting planetary gearbox that was hydraulically controlled.
Chrysler's aerodynamic Airflow series
During the early 30s, the era of streamlining, Chrysler attempted to upgrade the fluidic shaped Airflow models with a fluid coupling that would avoid stalling the engine when the transmission was stopped and in gear. Called 'Fluid Drive', quite appropriate for the Airflow's look and image, the fluid coupling was used to help the driver 'take off' from a dead stop without using the clutch pedal, but the pedal still had to be use to switch gears. The other automatic function was that the electric overdrive kicked in when you reach top speed. In essence, Fluid Drive and the other variant, Hy-drive [hybrid drive] were semi-automatics.
The Torque Converter
The key component of replacing the clutch pedal and the clutch is the torque converter. Its a further development of the fluid coupling, the torque converter multiplies the turning power or torque produced by the engine. It is connected to the engine an input shaft but is not in direct contact with the engine crankshaft. The link between transmission input shaft and the engine output shaft [crankshaft] is achieved by the hydraulic fluid, what we now know as ATF.
The torque converter looks like a doughnut and it contains an impeller (or pump), a turbine, and the stator (or guide wheel). The impeller and the turbine are like 2 electric fans that blow air, in the case of the automatic, push fluid, to one or the other. The impeller is positioned on the engine side, while the turbine is located where a clutch would normally reside in a manual gearbox. Both of these have vanes that sweeps transmission fluid, making them spin.
As one impeller spins, the other turbine mimics and follows. Through centrifugal force, the fluid is pushed to the outside of the vanes, where a third fan redirects the fluid to the turbine side. This continual flow of fluid is what causes the torque to multiply.
The Lockup Torque Converter
Since the fluid is not a direct mechanical link, there is always a degree of slippage resulting in power loss, running from about 2-8%. The modern post fuel crisis era introduced something called as the lockup clutch (aka, torque converter clutch). At speeds over 60km/h, the pressurized ATF flows to an orifice that activates a clutch piston. This causes a metal pin to lock the impeller directly to the turbine, bypassing the torque converter. This mechanical lock pin disengages at speeds lower than 60km/h.
Automatic gearboxes have different-sized gears that orbit circularly and revolve around a central gear known as a sun gear. These gears are composed of a sun gear, planet carrier, drum, pistons, ring gear and drum. In neutral, there is no motion within the transmission.
Clutches, Bands, and Servo Pistons
Gear shifting in the early automatic transmission is controlled by a series of valves, sensors, and other components that have been gradually being taken over by the modern car's ECU. Automatic transmissions do use a clutch pack, which looks like a regular clutch. Others use transmission bands which is a flexible metal ring that fits around the outside of the clutch housing. It tightens to engage the gears, and loosens to release them.
In1939, General Motors introduced the Hydra-Matic, the world's first mass-produced automatic transmission. Initially available in the model year 1940 Oldsmobiles and later Cadillacs, the Hydra-Matic combined a fluid coupling with three hydraulically-controlled planetary gearsets to produce four forward speeds [including overdrive] and reverse. The Hydra-Matic was developed to be sensitive to engine throttle position and road speed, producing responsive and appropriate fully automatic up- and down-shifting according to varying operating conditions.
The Hydra-Matic was sold to various other automakers, including Bentley, Hudson, Kaiser, Nash, and Rolls-Royce. Many military vehicles used in World War II had them. Even rival Ford used Hydra-Matics for their upmarket 1950-1954, Lincoln cars. Fiercely independent Mercedes-Benz engineers designed their own four-speed fluid coupling transmission that was similar in principle to the Hydra-Matic, after a long spell using Hydrac, a semi-automatic that had a conventional manual gearbox clutch actuated by an electro-magnetic solenoid switch on the column mounted shift lever knob.
In 1956, GM introduced the 'Jetaway' Hydra-Matic, which improved the shift quality of the original Hydra-Matic, by using two fluid couplings, one linked to the transmission to the engine, and a secondary replaced the clutch assembly of the planetary gearset. The result was much smoother shifting, especially from first to second gear, but with a loss in efficiency as the shifts 'dragged' longer. This was also the first automatic with a 'P' position as the original Hydra-Matic used reverse gear to hold the transmission in park.
The first torque converter automatic, Buick's Dynaflow, was introduced for the 1948 model year, followed by Packard's Ultramatic in mid-1949 and Chevrolet's Powerglide in 1950. This represented the other path in automatic transmission development. While Hydra-Matic worked like a smooth self shifting 3-speed transmission, Dyna flow was trying to make the existence of gears imperceptible and inaudible, hence the reduction to just 2 forward speeds. The technology trusted in the powers of the torque converter to multiply torque, eliminating the need for more gears than 2. Shifting of gears have now become a function of the torque converters rather than a conventional clutch actuated by valves. Chrysler distinguished their automatics by naming the 2-speed PowerFlite and the 3-speed ones TorqueFlite. Borg Warner made automatic transmissions for American Motors, Ford, Studebaker and several other car makes as Chrysler and GM made their own. In time, stricter emission controls and State imposed minimum fuel consumption standards were to lead the American car makers to add more gears, mostly overdrive ratio types, to improve the Freeway cruising consumption of the big V-8s.
With America and its cheap gasoline pounding the beaten path with bigger engines, heavier cars and less gears in the transmission, Europe, with its heavily taxed fuel, were going the opposite direction. Europeans were making cars smaller, lighter and with smaller engines. To maximize the output of small engines, more and more European cars of the late 60s were ditching 3-speed manual gearbox in favor of 4-speeds. The company leading the charge for ever more gears was sporty Alfa Romeo, which began supplying 5-speed manuals for most of its sports range. As Europe's population was not as highly concentrated in cities, pumping a clutch was not that much of a chore as traffic jams were still bearable. In time, increasing number of gears, regardless of the size of car and engine will catch on with automatics.
As computerized engine control units (ECUs) became more intelligent, much of the function of the the transmission's valve body was taken over by the ECU. Here, solenoids turned on and off by the computer control shift patterns and gear ratios, rather than by the spring-loaded valves in the valve body. This resulted in more precise shift points, smoother shift quality, shorter shift times, and manual override control, where the driver decides by shift lever or steering wheel paddle when to shift.
No clutch? More gears
ZF Friedrichshafen and BMW introduced the first six-speed ZF 6HP26 in the 2002 BMW E65 7-Series. Mercedes-Benz's 7G-Tronic was the first seven-speed in 2003, with Toyota introducing an eight-speed in 2007 on the Lexus LS 460. Derived from the 7G-Tronic, Mercedes-Benz unveiled a semi-automatic transmission with the torque converter replaced with a dual-clutch called the AMG SPEEDSHIFT MCT.
Out of orbit
Not all automatic transmissions use a planetary gear set. The exception is the Hondamatic which uses sliding gears on parallel axes like a manual transmission without any planetary gearsets. The early Hondamatics tended to shift abruptly.
Automated Manual transmissions
An example of this is the clutch pedal less Sequential Manual Gearbox or SMG made by BMW. A computer controlled clutch took the effort out of pumping a clutch pedal and timing a shift. These tended to jerky at urban crawls and jerky on mid range rpm shift points. It works best in a track where shifting at optimum RPM brings out power deliver as smooth as a speed shifting expert race car driver. Essentially, it is a semi-automatic like the Mercedes Benz Hydrac as it retains the clutch like a manual gearbox but activates the clutch through electro-hydraulic means.
Ford, BMW, VW and Mitsubishi's automatic twin clutch system is a sophisticated evolution of the computer controlled clutch, but with 2 alternating clutches, shift speed and losses are even further reduced making such automatic clutches useful and advantageous even in F1 Grand Prix racing cars.
Continuously variable transmission
A CVT is known to smoothly and steplessly change its gear ratio by varying the diameter of a pair of belt or chain-linked pulleys, wheels or cones. Some use a hydrostatic drive - consisting of a variable displacement pump and a hydraulic motor - to transmit power without gears. CVT designs are usually as fuel efficient as manual transmissions in city driving, but early designs are bedeviled by the constant 'slipping clutch' sound and lose efficiency as engine speed increases.
Some current hybrid vehicles, notably those of Toyota, Lexus and Ford Motor Company, have an 'electronically-controlled CVT' (E-CVT). In this system, the transmission has fixed gears, but the ratio of wheel-speed to engine-speed is continuously varied by controlling the speed to a differential using an electric motor-generator.
Manually controlled automatic transmissions
Thanks to more powerful computer chips, most automatic transmissions offer the driver a certain amount of manual control over the transmission's shifts. This can be in the form of preset shifting programs. For example, 'Economy mode' saves fuel by upshifting at lower engine speeds, while 'Sport mode' or Power or Performance delays shifting for maximum acceleration. The modes also change how the computer responds to throttle input.
Manual +/- override
Some transmissions have a mode which allow the driver full control of ratio changes (either by moving the selector, or through the use of buttons or paddles), completely overriding the automated function of the hydraulic controller. This is particularly useful in cornering, to avoid unwanted upshifts or downshifts that could unbalance the vehicle traction. 'Manumatic' shifters, first popularized by Porsche Tiptronic in the 1990s have become a popular option on sports cars and even mainstream Honda Civics and INVECs equipped Mitsubishis. The amount of true manual control is highly variable: some systems will override the driver's selections under certain conditions in the interest of preventing engine damage.
'W' for 2nd gear takeoff
Some automatics fitted to larger capacity or high torque engines, either when '2' is manually selected, or by engaging a 'winter mode', will start off in second gear instead of first, and then not shift into a higher gear until returned to D. This reduces torque multiplication when proceeding forward from a standstill in poor traction condition. In some Mercedes-Benz, BMW and Opel models, a 'W' or 'Winter mode' can be engaged so that second gear is selected instead of first to reduce the likelihood of loss of traction due to wheelspin on snow or ice. Many Korean and Japanese brands have followed suit.
OverDrive (D, OD)
This mode is used in some transmissions to allow early computer-controlled transmissions to engage the Automatic Overdrive. In these transmissions, Drive (D) locks the Automatic Overdrive off. OD (Overdrive) in these cars is engaged under steady speeds or low acceleration at approximately 50-70 km/h). Under hard acceleration or below 50-70 km/h, the transmission will automatically downshift.
In non-hybrid Toyota cars, this mode lets the engine do compression braking, also known as engine braking, when going down a steep hill. Instead of engaging the brakes, the engine in a non-hybrid car switches to a lower gear and slows down the spinning tires. For hybrid Toyotas, this mode converts the electric motor into a generator for the battery. It is not the same as downshifting in a non-hybrid car, but it has the same effect in slowing the car without using the brakes.
Hydraulic automatic transmissions are almost always less energy efficient than manual transmissions due mainly to viscous and pumping losses; both in the torque converter and the hydraulic actuators.
Manual transmissions use a mechanical clutch to transmit torque, rather than a torque converter, thus avoiding the primary source of loss in an automatic transmission. Manual transmissions also avoid the power requirement of the hydraulic control system, by relying on the human muscle power of the vehicle operator to disengage the clutch and actuate the gear levers, and the mental power of the operator to make appropriate gear ratio selections. Thus the manual transmission requires very little engine power.
Energy efficiency today
The energy efficiency of automatic transmission has increased with the introduction of the torque converter lock-up clutch, which eliminates fluid losses. Modern automatic transmission also minimize energy usage and complexity, by minimizing the amount of shifting logic that is done hydraulically. Control of the transmission has been transferred to computerized control systems which do not use fluid pressure for shift or clutch actuation.
The on road acceleration of an automatic transmission can occasionally exceed that of an otherwise identical vehicle equipped with a manual transmission in turbocharged diesel applications. Turbo-boost is normally lost between gear changes in a manual whereas in an automatic the accelerator pedal can remain fully depressed. This however is still largely dependent upon the number and optimal spacing of gear ratios for each unit, and whether or not the elimination of accelerator lift off represent a significant enough gain to counter the slightly higher power consumption of the automatic transmission itself.
In the end, large portions of motoring society made a choice to pay for the slight penalty in efficiency that automatics incur versus a manual. But today's modern automatics are consistently faster and thriftier than a manual gearbox, especially when the driver turns sloppy, tired from all that clutching in and out in stop and go traffic. One of the few occasions left where a manual is absolutely superior to an automatic is when one is already a skilled driver, behind the wheel of a powerful car with competent suspension and with a wide open road, with no traffic to reduce you to stop and go driving.