Transmissions are as “Old as the Hills” and when one reviews some of the many designs experimented with and those used in production, he concludes that there are about as many varieties.
The use of an automotive transmission has always been, and still is, required to compensate for the shortcomings of an internal combustion engine. The internal combustion engine, as we know it today, cannot run below approximately 400 R. P. M. without stalling; it develops relatively little torque at low speed, where the torque demand is the greatest to obtain good car acceleration; and it cannot be made to run counterclockwise readily to reverse the motion of the vehicle.
Therefore, the primary purposes of a transmission (or a transmission and clutch combination) are to prevent the engine from stalling when the car speed is low or zero; to enable the engine to operate at a higher speed where it develops more power while the transmission multiplies the engine torque for better acceleration; and to reverse the rotation of the drive shaft, using a gear train to provide a means of reversing the vehicle motion.
These requirements can be obtained in many ways as one realizes when he sees the many types of transmissions which have been used in the past 50 years.
There are many secondary requirements which are expected of the transmissions of today; such as, ease of operation, smoothness, maximum car performance, best fuel economy, cost of manufacture, and weight. Everyone evaluates these items in a slightly different manner and no one combination has been arrived at which has the best of all items. Consequently, compromises must be made which result in different transmission designs and the perpetual argument, as to which is the best, goes on.
A few of the many transmission designs which have been used are as follows:
The Friction Drive which has a wide range of torque ratios, infinitely variable, was used in some very early vehicles. It has smoothness of ratio change and could be easily disengaged for neutral and shifted to obtain reverse. However, its short-coming was durability. Also, for the high powered
engines of today, materials have not been discovered which can endure the pressures required to prevent slippage. This type of drive is used today on some machines which do not require the trans-mission of much power and which can utilize the desirable characteristics of this design.
Inertia Drives have been designed and seem to be more of a novelty than a utility. They require a ratchet mechanism and consequently ‘free wheel’. The public more or less rejected ‘free wheeling’ cars back in the nineteen thirties and a reliable ratchet mechanism to drive a 4,000 pound car would be very bulky.
Electric or Magnetic Transmissions appeared on the American market years ago but did not stay long. Now they have re-appeared in the Diesel-Electric Locomotive field. The evaluation of factors; such as, initial cost, operating cost, performance, weight, and efficiency have proved that this type of drive for this application is very satisfactory.
Planetary Transmissions were used in many early vintage cars. These generally had one forward speed reduction, a direct drive, and a reverse. (As some of you can recall, the reverse was used too frequently to reduce the forward speed of the car when the brake was not very effective.) One forward speed reduction didn’t give much flexibility in performance and with the general trend of higher engine speeds and higher vehicle speeds, this type of manually operated transmission yielded to the Sliding Gear Transmission.
The Sliding Gear – 3-Speed – Transmission was used on many early cars and was used practically exclusively during the late 1920s and early 1930s. It was developed into a reasonably simple, durable device which covered a satisfactory ratio range and was manufactured for a relatively low cost. The introduction of helical gears to reduce noise and synchronizers to facilitate shifting were, outstanding advancements made in this transmis
sion. Many companies have spent many man-hours and large sums of money in developing this transmission into the smallest, lightest, and lowest cost unit being made today. Practically no development work is being carried on with this transmission, and its manufacturing cost is actually increasing because of the diminishing quantity being made. A Friction Clutch is used in conjunction with the Sliding Gear Transmission to enable the gear teeth to be meshed.
We, who have been driving automatic transmissions, realize that operating a synchromesh equipped car smoothly is an art in manipulating the clutch, throttle, and gear shift lever, while steering the car. (If you question that statement, try driving a synchromesh car, and you will find that that art of operation has become a bit rusty.)
To simplify the operating of a car and to obtain a wider ratio range coverage, the 4-speed ‘Self-Shifting Transmission’ was made by Buick Motor Division and offered as optional equipment on 1938 Buicks and Oldsmobiles. It required a Friction Clutch which had to be used when the vehicle was started or stopped. Approximately 25,000 units were built during that model year. This transmission was the forerunner of the Hy-dramatic Transmission which is being built by the Detroit Transmission Division.
Fluid Couplings were introduced on the American market in the 1930s. It is their characteristic to transfer very little torque at low speed and slip only a small amount at higher speed. This enables a vehicle to be brought to a stop without stalling the engine and thereby reduces or eliminates the use of the clutch pedal. It cannot multiply torque; therefore, it must be used in conjunction with gears to provide satisfactory car performance.
The transmission engineer is confronted with the problem of best utilizing the engine power and to do it in a manner which the car buying public finds most pleasing. This includes car performance, fuel economy, smoothness of operation, and a minimum manipulation of controls by the driver.
Hydrokinetic Torque Converters seem to fill this order in a satisfactory manner in view of their being used in all new automatic transmissions introduced on American made passenger cars since Buick’s introduction of the Dynaflow Transmission in 1948.
The Hydrokinetic Torque Converter is not new – it was invented some forty five years ago by Doctor Herrman Foettinger. It was first used as a speed reducer for marine steam turbines, which was before the time of present types of high speed reduction gear drives.
Torque Converter Transmissions have been used for some time in passenger busses. Their principal feature in this application is in reducing driver fatigue and in reducing the time to make a given run. This converter is used for starting purposes only and is locked-out after the bus is accelerated to some predetermined speed.
Torque Converter Transmissions were used extensively in World War II tanks because of the flexibility they provided and because they required a minimum of attention or manipulation of controls by the tank driver.
To explain how the Dynaflow Twin Turbine Converter functions, it is necessary to understand how a Hydrokinetic Drive works, that is, how a Fluid Coupling works and how a Torque Converter works. If the operating principles are reduced to comparisons with familiar mechanics, the Torque Converter is easy to understand.
The simple mechanics of a Hydro-kinetic Drive begin with the mechanics of a spinning flywheel. (Figure 1) A spinning flywheel has stored up energy and if it is stopped, it will exert a force on the mechanism stopping it. Conversely, a force must be exerted against it to get it up to speed again after it has been stopped. A fluid coupling with an engine driving the input member and the output member stalled or stationary, is a direct equivalent of this principle.
As an example, let us take a fluid coupling which has the input member running at 1000 R.P. M. and the output member stalled. The input member is nothing more than a centrifugal pump, picking up oil at a small diameter and making it spin along with the vanes in this member. Centrifugal force sends the oil out radially and the oil leaves the input member at a large diameter in the form of a spinning flywheel rim, a flywheel rim made of oil. The energy in this spinning flywheel rim of oil came from the engine. As noted before, the output member is stationary, so a force will be exerted on its vanes as this spinning flywheel of oil is stopped. The force exerted will depend on the rate at which it is stopped, its mean diameter, and its weight. The fluid then flows from the large diameter of the output member to its small diameter having an inward radial movement and no spinning movement. As it leaves the small diameter of the output member, it re-enters the input member to repeat this cycle.
If we examine the action of a fluid drive coupling running efficiently at a higher speed, we will recognize another direct comparison with flywheels. (Figure 2) Let us assume two flywheels running at 1000 R.P.M., each weighing the same but having different mean diameters. It is evident that the large diameter flywheel, while no heavier in total weight than the small one, is more of a flywheel. If the diameter of the larger flywheel is twice as great as that of the smaller flywheel, it will be four times the flywheel though no heavier in total weight. In other words, the energy contained in the larger flywheel is four times greater than that in the smaller one though both weigh the same and run at the same speed.
Let us record another basic fact – if both of these flywheels were to have the same energy, the smaller one would have to run twice as fast as the larger one.
Let us now look at a fluid coupling with both the input member and the output member turning at 1000 R.P.M. and assign a value A to the energy in the oil as it leaves the input member. The energy of this same oil as it leaves the output member will be one-fourth of A and the difference, or three-fourths of A , will be absorbed by the output member and used as the output driving force.
Actually, the speed of the driven member must lag the speed of the driving member so that the oil will flow and enable the transfer of power from the input member to the output member via the medium of oil. If the speed of the driving and driven members were equal, the counter centrifugal forces of the fluid in the driven member would balance the centrifugal forces of the fluid in the driving member and no fluid flow would result. The amount of lag will depend on the speed and torque being transferred.
That is how the fluid coupling works.
Now, we don’t have far to go from here to a simple three element torque converter. (Figure 3) Let us assume the circulation efficiency to be 100% and let us take the fluid drive coupling previously mentioned just as it was with the input member running at 1000 R.P.M., and the output member stalled. But let us replace the straight vanes in the output member by strongly curved vanes. We shall make the entrance of vanes so that the oil will be received without splash and strongly curve them backward so that the exit oil will be actually spinning backward – it is a backward spinning flywheel. The force felt by the converter output member, or turbine, is obviously much greater since it has not only absorbed the energy in stopping the spinning flywheel which entered it, but it has also reversed the direction of spinning.
An appreciable amount of energy would be required by the engine to stop this backward spinning flywheel if it were permitted to impinge directly on the vanes of the input, or pump member. To take care of this condition a stationary member, or stator, is interposed. This member has curved vanes also, so that it again reverses the direction of the spinning oil causing it to spin in the same direction as the pump it enters. The reaction force on this member establishes the amount of torque multiplication the unit is developing. The output torque must always equal the input torque plus the reaction torque on the stator support. The pump member picks up this oil, adds energy to it as it passes from the entrance diameter to the exit diameter, and this cycle continues to repeat itself.
The oil functions in the same manner when the turbine member is permitted to move. The rate of oil flow decreases as the turbine speed approaches the pump speed and the torque multiplication decreases at the same time. It is this characteristic which enables the engine together with the torque converter to supply maximum torque for starting and accelerating right where it is needed.
At cruising speeds the rate of oil flow through the openings between the vanes, or vortex flow, is small, but the rotary motion of both the pump and turbine is high. Under this condition the stationary member, or stator, would form a very serious obstruction to the flow of oil and cause the efficiency to drop as the speed increases. To prevent this from occurring the stator member is mounted on a free wheel clutch which enables it to change the direction of flow when needed and to free wheel out of the way, or along with the rotary flow of oil, when not needed. Under these conditions of cruising, this design of converter actually functions in a manner similar to a fluid drive.
As in any hydraulic device there are flow losses and shock losses in a Hydrokinetic Drive. To obtain no shock losses it is necessary to have the oil enter each member in a direction parallel to the vane entrance. The absolute direction of oil flow changes as the rotary speed changes in relation ‘to the vortex flow. Therefore, to avoid any shock losses it would be necessary to provide vanes with adjustable entrance angles. This is not practical to do. We approached this end in the original Dynaflow by adding a second stator and a second pump.
The torque ratio and efficiency curves for a Fluid Coupling show that the torque ratio is always 1:1 (in other words it does not multiply the input torque) and the efficiency curve is inversely proportional to the slip taking place. (Figure 4) As the relative speed of the elements approach 1:1, the efficiency again falls off to zero because no torque can be transferred, as previously mentioned.
Typical torque ratio and efficiency curves for a Torque Converter, superimposed on the fluid coupling curves, show that the torque ratio of more than 1:1 can be obtained (or in other words a Torque Converter can multiply torque) and the efficiency curve rises faster, peaks, and falls off until it reaches the fluid coupling curve. (Figure 5) At this point the stator element free wheels and the unit functions in a manner characteristic of a Fluid Coupling.
Different converter designs will produce different torque ratio and efficiency curves. The original Dynaflow Torque Converter which consisted of a Primary Pump, a Secondary Pump (which was mounted on an overrunning clutch), a turbine, and two Stators (which also were mounted on overrunning clutches) was, in a sense, 3 Torque Converters and a Fluid Coupling combined into a single unit. (Figure 6) This resulted in 4 sets of curves, the most desirable portions of which were utilized.
The break points 1,2, and 3 in the efficiency curve are the points at which the Secondary Pump stops overrunning the Primary Pump.the Secondary Stator starts to free wheel, and the Primary Stator starts to free wheel, respectively.
To improve the car performance and converter efficiency and to reduce the engine speed for a given car speed the Twin Turbine Dynaflow Transmission was introduced on the Buick 1953 models. (Figure. 7).
This design utilizes the torque multiplying characteristics of a planetary gear set in conjunction with the torque multiplying ability of a hydrokinetic torque converter and retains the smooth, uninterrupted power flow, characteristic of a Fluid Torque Converter. The arrangement of the Twin Turbine Dynaflow Torque Converter is unique in that all the power transferred is through the gear set and first turbine at low speeds, and gradually and smoothly diminishes as the power transferred through the second turbine increases until it does all the work at higher speeds.
Let us take time out to go over the nomenclature of the Twin Turbine parts. (Figure 8) The Converter Pump is driven by the engine and energizes the Converter Fluid. The First Turbine receives the Fluid leaving the Pump and drives the Planetary Ring Gear through a diaphragm member which is riveted to the Ring Gear. The Second Turbine is doweled and bolted to the Planet Carrier which in turn is splined to the Converter Output Shaft. The Stator is mounted on an overrunning clutch which is common to it and to the Planetary Sun Gear.
To explain the operation of this unit let us first review the driving and reaction torques and
forces. (Figure 9) At stall, the output shaft and turbines will be stationary – by definition. The engine is driving the converter pump which produces the driving force to the fluid as shown by Arrow P. The fluid circulates through the first turbine which is stationary and in so doing infposes a force which is resisted as shown; namely, force T1.
The fluid passes through the second turbine with no appreciable change and on through the stator which redirects it and consequently has a reaction force, as shown by Arrow S. The driving force, P, plus the reaction force, S, must always equal the driving forces, T1 plus T2. This relationship must always exist. (When it doesn’t, one has better take another look at his analysis and calculations.)
The driving force T1 is fed into the planetary gear set where it is increased by the ratio of 1.6:1. At the gear set – the ring gear, which is connected to the first turbine, is driving; the sun gear is held stationary by the overrunning clutch; and the planet carrier, which is splined to the converter output shaft, is driven by the planet pinion. This arrangement has increased the stall torque ratio from 2.25:1 in the original Dynaflow to 2.45:1 in the present Twin Turbine Dynaflow and raised the efficiency curve. (Figure 7).
As the car speed increases the driving force (T1) on the first turbine gradually decreases as the driving force (T2) on the second turbine increases. (Figure 10) The latter force on the second turbine is delivered directly to the converter output shaft because the second turbine is secured to the planet carrier which in turn is splined to the main shaft.
As the car speed continues to increase, the driving force on the second turbine (T2) exceeds that on the first turbine (T1) and the reaction force on the stator (S) continues to decrease. (Figure 11)
At cruising the first turbine is transferring no power and the second turbine is transferring all the power. (Figure 12) The stator is taking no reaction and is free wheeling in a clockwise direction as permitted by its overrunning clutch mounting. At this stage the converter is no longer multiplying torque and is operating in a manner similar to a fluid coupling.
So much for the driving forces. Now to describe how these changes take place, a few vector diagrams should be of some assistance. (Figure 13).
With a low member speed, as shown at S1, and a high vortex flow, as shown at F1., the vector sum will be O-A. With a high member speed, S3, and a low vortex flow, F3, the vector sum will be O-C.
The length and direction of the vector sum, or absolute velocity of the oil, will change as the conditions change. The vectors shown are for purposes of illustration and do not necessarily represent true values. The absolute velocity of a particle of oil will be the vector sum of the velocity of the converter member it is leaving, which will be a tangential vector, plus the vortex velocity of the oil in relation to the blade it is leaving. The latter will have a direction parallel to the exit end of the blade. If the member is stationary, the absolute velocity will be the same as the vortex velocity.
As previously mentioned, the pump operates like a simple centrifugal pump, and the absolute velocity of the fluid will be the vector sum of the circumferential speed S of the pump blade which drives the fluid plus the linear speed F of the fluid in relation to the blade. As the pump speed increases and the fluid flow decreases, the vector sum will change in direction and magnitude as shown by vectors A, B, and C. Because of its mass and velocity, this fluid contains energy, which it obtained from the engine.
When the direction of this flow is changed, a force is imparted to the member causing it to change in direction.
A vector diagram showing the oil leaving the first turbine blades will show the amount of directional change of the oil flow (between the pump exit and the first turbine exit) and will consequently indicate the force imparted to this member (Figure 14).
At stall the turbine speed is zero; therefore, the direction of oil flow will be tangent to the exit end of this blade as shown at F. This results in the largest change in direction between the entering oil and the leaving oil; consequently, it produces the greatest driving force on this member.
The difference in directional change decreases as the first turbine speed increases as shown by the resultant vectors A, B, C, and D. At D the high rotational speed of the first turbine causes the oil to flow between its blades without being changed in direction. At this point it is delivering no drive and the second turbine blades do all the driving.
The second turbine also has curved blades and has a smaller diameter at its exit end than its entrance end which further enables it to extract energy from the flowing oil. (Figure 15) This member functions in the same manner as the turbine in the previously mentioned 3 element torque converter. The exit oil flow is shown by vector sums, A, B, C and D.
By virtue of the converter planetary gear set which multiplies the first turbine torque by a ratio of 1.6:1, the speed of this member will be 1.6:1 faster than that of the second turbine. It is this relationship which enables the first turbine to reduce its load carrying ability while the second turbine increases its load carrying ability. The difference in their speeds is zero at stall and increases as the output shaft speed increases. This feature enables the first turbine to move ahead of the second turbine so that the latter can take the driving force of the fluid and deliver it directly to the converter output shaft.
The fluid leaving the second turbine and entering the stator changes in direction A, B, C, and D as the car speed increases, thereby imposing a diminishing torque reaction on the stator blades. (Figure 16) When the oil impinges on the back side of the stator blades, as shown at D, the stator free wheels along with the oil so that it will not obstruct the fluid flow.
The Twin Turbine Converter has eliminated the ‘Slip’ of ‘buzz-up’ sensation and has greatly increased the low-end performance. For example, test results show that a 1953 Buick Roadmaster equipped with a Twin Turbine Dynaflow, checked against a Buick companion car of the same engine power and car weight, outdistanced the latter by 35 feet, or 2 car lengths, when accelerated, full throttle, from a standing start to 50 miles per hour in 11.3 seconds. This is the result of a more efficient converter design which has retained the smooth uninterrupted power flow to the rear wheels that has been characteristic of all Dynaflow Transmissions built. Today, it is the only automatic transmission that has this smoothness of operation, free of automatic shifts, in the Driving Range from start to cruising speeds.
The complete Dynaflow Transmission consists of a torque converter together with a hydraulically operated transmission to obtain the various ranges – Park, Neutral, Drive, Low, and Reverse. (Figure 17) The torque converter drives the car through a compound planetary gear cluster which is locked-up by a multiple disc clutch for all normal operation. When the range selector lever at the steering wheel is set to Low Range, oil pressure applies the low band, thereby holding one of the planetary reaction members producing a geared reduction of 1.82:1. Low range is used for emergency power and for increased engine braking while descending long steep grades. Reverse Range is obtained in a similar manner by applying the reverse band which locks a different reaction member of the planetary.
To supply oil under pressure to operate the high clutch piston, low and reverse servo pistons, and to circulate oil through the converter and lubrication system, two pumps are used. One is driven at engine speed and the other at propeller shaft speed. The latter is a small pump which supplies all the oil required when the car speed is high enough for it to do so. The oil from the larger pump is then relieved to the oil suction line so that a minimum amount of power is needed for pumping. The rear pump which is run at propeller shaft speed provides oil pressure to engage the low band to obtain a drive through the transmission when a car needs to be push started, as in the case of a weak battery.
At the rear of the transmission is a parking lock which will rigidly lock the propeller shaft when the car is parked. This takes the place of parking a synchromesh equipped car ‘in gear’. In addition this enables the engine to be started while the car is held from moving.
A torque converter cannot transmit torque when the pump and turbine members are stationary. Therefore, a car so equipped cannot be held or parked against the engine friction and compression as can be done with a Synchromesh Transmission equipped car.
Since 1948 Buick has built over 1,800,000 torque converter type transmissions which is more than any other passenger car manufacturer has built. Five other makes of automatic transmissions have appeared on the market since that time — all of which have Incorporated a torque converter. From this it certainly appears quite evident that a torque converter has characteristics which meet the fancy of the car buying public.