44Hydrokinetic three element torque converter

A three element torque converter coupling is comprised of an input impeller casing enclosing the

Fig. 4.6 Fluid friction coupling

Fig. 4.6 Fluid friction coupling

Fig. 4.7 Relationship of torque carrying capacity, efficiency and output speed for a fluid coupling

output turbine wheel. There are about 26 and 23 blades for the impeller and turbine elements respectively. Both of these elements and their blades are fabricated from low carbon steel pressings. The third element of the converter called the stator is usually an aluminium alloy casting which may have something in the order of 15 blades (Figs 4.8 and 4.9).

The working fluid within a converter when the engine is operating has two motions:

1 Fluid trapped in the impeller and turbine vane cells revolves bodily with these members about their axis of rotation.

2 Fluid trapped between the impeller and turbine vane cells and their central torus core rotates in a circular path in the section plane, this being known as its vortex motion.

When the impeller is rotated by the engine, it acts as a centrifugal pump drawing in fluid near the centre of rotation, forcing it radially outwards through the cell passages formed by the vanes to the impeller peripheral exit. Here it is ejected due to its momentum towards the turbine cell passages and in the process acts at an angle against the vanes, thus imparting torque to the turbine member (Fig. 4.8).

The fluid in the turbine cell passages moves inwards to the turbine exit. It is then compelled to flow between the fixed stator blades (Fig. 4.9). The reaction of the fluid's momentum as it glides over the curved surfaces of the blades is absorbed by the casing to which the stator is held and in the process it is redirected towards the impeller entrance. It enters the passages shaped by the impeller vanes. As it acts on the drive side of the vanes, it imparts a torque equal to the stator reaction in the direction of rotation (Fig. 4.8).

It therefore follows that the engine torque delivered to the impeller and the reaction torque transferred by the fluid to the impeller are both transmitted to the output turbine through the media of the fluid.

Engine torque

Reaction torque

Output turbine torque

4.4.1 Hydrokinetic three element torque converter principle ofoperation (Fig. 4.8) When the engine is running, the impeller acts as a centrifugal pump and forces fluid to flow radially around the vortex passage made by the vanes and core of the three element converter. The rotation of the impeller by the engine converts the engine power into hydrokinetic energy which is utilized in

Torque Transmission Element
Fig. 4.8 Three element torque converter action
Single Stage Torque Converter Fig

Fig. 4.9 Three element torque converter providing a smooth engine to transmission take-up and in producing torque multiplication if a third fixed stator member is included.

An appreciation of the principle of the converter can be obtained by following the movement and events of a fluid particle as it circulates the vortex passage (Fig. 4.8).

Consider a fluid particle initially at the small diameter entrance point A in the impeller. As the impeller is rotated by the engine, centrifugal force will push the fluid particle outwards to the impeller's largest exit diameter, point B. Since the particle's circumferential distance moved every revolution will be increased, its linear velocity will be greater and hence it will have gained kinetic energy.

Pressure caused by successive particles arriving at the impeller outermost cell exit will compel the particle to be flung across the impeller-turbine junction where it acts against the side of cell vane it has entered at point C and thereby transfers some of its kinetic energy to the turbine wheel. Because the turbine wheel rotates at a lower speed relative to the impeller, the pressure generated in the impeller will be far greater than in the turbine. Subsequently the fluid particle in the turbine curved passage will be forced inwards to the exit point D and in doing so will give up more of its kinetic energy to the turbine wheel.

The fluid particle, still possessing kinetic energy at the turbine exit, now moves to the stator blade's entrance side to point E. Here it is guided by the curvature of the blades to the exit point F.

From the fixed stator (reactor) blades the fluid path is again directed to the impeller entrance point A where it imparts its hydrokinetic energy to the impeller, this being quite separate to the kinetic energy produced by the engine rotating the impeller. Note that with the fluid coupling, the transfer of fluid from the turbine exit to the impeller entrance is direct. Thus the kinetic energy gained by the input impeller is that lost by the output turbine and there is no additional gain in output turning effort, as is the case when a fixed intermediate stator is incorporated.

4.4.2 Hydrokinetic three element torque converter velocity diagrams (Figs 4.9 and 4.10) The direction of fluid leaving the turbine to enter the stator blades is influenced by the tangential exit velocity which is itself determined by the vortex circulating speed and the linear velocity due to the rotating turbine member (Fig. 4.10).

When the turbine is in the stalled condition and the impeller is being driven by the engine, the direction of the fluid leaving the impeller will be determined entirely by the curvature and shape of the turbine vanes. Under these conditions, the fluid's direction of motion, ©j, will make it move deep into the concave side of the stator blades where it reacts and is

Fig Torque Converter
Fig. 4.10 Principle of the single stage torque converter

made to flow towards the entrance of the impeller in a direction which provides the maximum thrust.

Once the turbine begins to rotate, the fluid will acquire a linear velocity so that the resultant effective fluid velocity direction will now be 02. A reduced backward reaction to the stator will be produced so that the direction of the fluid's momentum will not be so effective.

As the turbine speed of rotation rises, the fluid's linear forward velocity will also increase and, assuming that the turbine's tangential exit velocity does not alter, the resultant direction of the fluid will have changed to 03 where it now acts on the convex (back) side of the stator blades.

Above the critical speed, when the fluid's thrust changes from the concave to the convex side of the blades, the stator reaction torque will now act in the opposite sense and redirect the fluid. Thus its resultant direction towards the impeller entry passages will hinder instead of assist the impeller motion. The result of this would be in effect to cancel out some of the engine's input torque with further speed increases.

The inherent speed limitation of a hydrokinetic converter is overcome by building into the stator hub a one way clutch (freewheel) device (Fig. 4.9). Therefore, when the direction of fluid flow changes sufficiently to impinge onto the back of the blades, the stator hub is released, allowing it to spin freely between the input and output members. The freewheeling of the stator causes very little fluid interference, thus the three element converter now becomes a two element coupling. This condition prevents the decrease in torque for high output speeds and produces a sharp rise in efficiency at output speeds above the coupling point.

4.4.3 Hydrokinetic torque converter characteristics (Figs. 4.11 and 4.12) Maximum torque multiplication occurs when there is the largest speed difference between the impeller and turbine. A torque output to input ratio of about 2:1 normally occurs with a three element converter when the turbine is stationary. Under such conditions, the vortex rate of fluid circulation will be at a peak. Subsequently the maximum hydrokinetic energy transfer from the impeller to turbine then stator to impeller again takes place (Figs 4.11 and 4.12). As the turbine output speed increases relative to the impeller speed, the efficiency rises and the vortex velocity decreases and so does the output to input torque ratio until eventually the circulation rate of fluid is so low that it can only support a 1:1 output to input torque ratio. At this point the reaction torque will be zero. Above this speed the stator is freewheeled. This offers less resistance to the circulating fluid and therefore produces an improvement in coupling efficiency (Figs 4.11 and 4.12).

This description of the operating conditions of the converter coupling shows that if the transmission is suddenly loaded the output turbine speed will automatically drop, causing an increase in fluid circulation and correspondingly a rise in torque multiplication, but conversely a lowering of efficiency due to the increased slip between input and output members. When the output conditions have changed and a reduction in load or an increase in turbine speed follows the reverse happens; the efficiency increases and the output to input torque ratio is reduced.

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