45 Torque converter performance terminology

To understand the performance characteristics of a fluid drive (both coupling and converter), it is essential to identify and relate the following terms used in describing various relationships and conditions.

4.5.1 Fluid drive efficiency (Figs 4.11 and 4.12) A very convenient method of expressing the energy losses, due mainly to fluid circulation within a fluid drive at some given output speed or speed ratio, is

Vehicle Performance Characteristics
Fig. 4.11 Characteristic performance curves for a three element converted coupling
Fig. 4.12 Characteristic performance curves for a converter coupling plotted to a base of output (turbine speed) to input (impeller speed)

to measure its efficiency, that is, the percentage ratio of output to input work done.

Input work done

It is frequently necessary to compare the output and input speed differences at which certain events occur. This is normally defined in terms of a speed ratio of output (turbine) speed N2 to the input (impeller) speed N1.

The torque multiplication within a fluid drive is more conveniently expressed in terms of a torque ratio of output (turbine) torque T2 to the input (impeller) torque T1.

This is the maximum speed which the engine reaches when the accelerator pedal is fully down, the transmission in drive and the foot brake is fully applied. Under such conditions there is the greatest impeller to turbine speed variation, with the result that the vortex fluid circulation and correspondingly torque conversion are at a maximum, conversely converter efficiency is zero. Whilst these stall conditions prevail, torque conversion loading drags the engine speed down to something like 60-70% of the engine's maximum torque speed, i.e. 1500-2500 rev/min. A converter should only be held in the stall condition for the minimum of time to prevent the fluid being overworked.

Torque converters are so designed that their internal passages formed by the vanes are shaped so as to make the fluid circulate with the minimum of resistance as it passes from one member to another member at definite impeller to turbine speed ratio, known as the design point. A typical value might be 0.8:1.

Above or below this optimum speed ratio, the resultant angle and direction of fluid leaving one member to enter another will alter so that the flow from the exit of one member to the entry of another will no longer be parallel to the surfaces ofthe vanes, in fact it will strike the sides of the passage vanes entered. When the exit and entry angles of the vanes do not match the effective direction of fluid motion, some of its momentum will be used up in entrance losses and consequently the efficiency declines as the speed ratio moves further away on either side of the design point. Other causes of momentum losses are internal fabrication finish, surface roughness and inter-vane or blade thickness interference. If the design point is shifted to a lower speed ratio, say 0.6, the torque multiplication will be improved at stall and lower speed conditions at the expense of an earlier fall-off in efficiency at the high speed ratio such as 0.8. There will be a reduction in the torque ratio but high efficiency will be maintained in the upper speed ratio region.

4.5.6 Coupling point (Figs 4.11 and 4.12)

As the turbine speed approaches or exceeds that of the impeller, the effective direction of fluid entering the passages between the stator blades changes from pushing against the concave face to being redirected towards the convex (back) side of the blades. At this point, torque conversion due to fluid transfer from the fixed stator to the rotating impeller, ceases. The turbine speed when the direction of the stator reaction is reversed is known as the coupling point and is normally between 80 and 90% of the impeller speed. At this point the stator is released by the freewheel device and is then driven in the same direction as the impeller and turbine. At and above this speed the stator blades will spin with the impeller and turbine which then simply act as a fluid coupling, with the benefit of increasing efficiency as the turbine output speed approaches but never reaches the input impeller speed.

If the converter does not include a stator freewheel device or if the mechanism is jammed, then the direction of fluid leaving the stator would progressively change from transferring fluid energy to assist the impeller rotation to one of opposition as the turbine speed catches up with that of the impeller. Simultaneously, the vortex fluid circulation will be declining so that the resultant torque capacity of the converter rapidly approaches zero. Under these conditions, with the accelerator pedal fully down there is very little load to hold back the engine's speed so that it will tend to race or run-away. Theoretically racing or run-away should occur when both the impeller and turbine rotate at the same speed and the vortex circulation ceases, but due to the momentum losses caused by internal fluid resistance, racing will tend to begin slightly before a 1:1 speed ratio (a typical value might be 0.95:1).

4.5.8 Engine braking transmitted through converter or coupling on overrun

Torque converters are designed to maximize their torque multiplication from the impeller to the turbine in the forward direction by adopting backward swept rotating member circulating passage vanes. Unfortunately, in the reverse direction when the turbine is made to drive the impeller on transmission overrun, the exit and entry vane guide angles of the members are unsuitable for hydro-kinetic energy transference, so that only a limited amount of engine braking torque can be absorbed by the converter except at high output overrun vehicle speeds. Conversely, a fluid coupling with its flat radial vanes is able to transmit torque in either drive or overrun direction with equal effect.

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Responses

  • Aaron
    What medication torque converter torque multiplication?
    3 years ago

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