41 Hydrokinetic fluid couplings

The hydrokinetic coupling, sometimes referred to as a fluid flywheel, consists of two saucer-shaped discs, an input impeller (pump) and an output turbine (runner) which are cast with a number of flat radial vanes (blades) for directing the flow path of the fluid (Fig. 4.1).

Owing to the inherent principle of the hydro-kinetic coupling, there must be relative slip between the input and output member cells exposed to each

Fig. 4.1 Fluid coupling action other, and the vortex flow path created by pairs of adjacent cells will be continuously aligned and misaligned with different cells.

With equal numbers of cells in the two half members, the relative cell alignment of all the cells occurs together. Consequently, this would cause a jerky transfer of torque from the input to the output drive. By having differing numbers of cells within the impeller and turbine, the alignment of each pair of cells at any one instant will be slightly different so that the impingement of fluid from one member to the other will take place in various stages of circulation, with the result that the coupling torque transfer will be progressive and relatively smooth.

The two half-members are put together so that the fluid can rotate as a vortex. Originally it was common practice to insert at the centre of rotation a hollow core or guide ring (sometimes referred to as the torus) within both half-members to assist in establishing fluid circulation at the earliest moment of relative rotation of the members. These couplings had the disadvantage that they produced considerable drag torque whilst idling, this being due mainly to the effectiveness of the core guide in circulating fluid at low speeds. As coupling development progressed, it was found that turbine drag was reduced at low speeds by using only a core guide on the impeller member (Fig. 4.2). With the latest design

Fluid Flywheel Simple Diagram

these cores are eliminated altogether as this also reduces fluid interference in the higher speed range and consequently reduces the degree of slip for a given amount of transmitted torque (Fig. 4.6).

4.1.1 Hydrokinetic fluid coupling principle of operation (Figs 4.1 and 4.3)

When the engine is started, the rotation of the impeller (pump) causes the working fluid trapped in its cells to rotate with it. Accordingly, the fluid is subjected to centrifugal force and is pressurized so that it flows radially outwards.

To understand the principle of the hydrokinetic coupling it is best to consider a small particle of fluid circulating between one set of impeller and turbine vanes at various points A, B, C and D as shown in Figs 4.1 and 4.3.

Initially a particle of fluid at point A, when the engine is started and the impeller is rotated, will experience a centrifugal force due to its mass and radius of rotation, r. It will also have acquired some kinetic energy. This particle of fluid will be forced to move outwards to point B, and in the process of increasing its radius of rotation from r to R, will now be subjected to considerably more centrifugal force and it will also possess a greater amount of kinetic energy. The magnitude of the kinetic energy at this outermost position forces it to be ejected from the mouth of the impeller cell, its flow path making it enter one of the outer turbine cells at point C. In doing so it reacts against one side of the turbine vanes and so imparts some of its kinetic energy to the turbine wheel. The repetition of fluid particles being flung across the junction between the impeller and turbine cells will force the first fluid particle in the slower moving turbine member (having reduced centrifugal force) to move inwards to point D. Hence in the process of moving inwards from R to r, the fluid particle gives up most of its kinetic energy to the turbine wheel and subsequently this is converted into propelling effort and motion.

The creation and conversion of the kinetic energy of fluid into driving torque can be visualized in the following manner: when the vehicle is at rest the turbine is stationary and there is no centrifugal force acting on the fluid in its cells. However, when the engine rotates the impeller, the working fluid in its cells flows radially outwards and enters the turbine at the outer edges of its cells. It therefore causes a displacement of fluid from the inner edges of the turbine cells into the inner edges of the impeller cells, thus a circulation of the fluid will be established between the two half cell members. The fluid has two motions; firstly it is circulated by the impeller around its axis and secondly it circulates round the cells in a vortex motion.

This circulation of fluid only continues as long as there is a difference in the angular speeds of the impeller and turbine, because only then is the centrifugal force experienced by the fluid in the faster moving impeller greater than the counter centrifugal force acting on the fluid in the slower moving turbine member. The velocity of the fluid around the couplings' axis of rotation increases while it flows radially outwards in the impeller cells due to the increased distance it has moved from the centre of rotation. Conversely, the fluid velocity decreases when it flows inwards in the turbine cells. It therefore follows that the fluid is given kinetic energy by the impeller and gives up its kinetic energy to the turbine. Hence there is a transference of energy from the input impeller to the output turbine, but there is no torque multiplication in the process.

4.1.2 Hydrokinetic fluid coupling velocity diagrams (Fig. 4.3)

The resultant magnitude of direction of the fluid leaving the impeller vane cells, VR, is dependent upon the exit velocity, VE, this being a measure of the vortex circulation flow rate and the relative linear velocity between the impeller and turbine, Vl.

The working principle of the fluid coupling may be explained for various operating conditions assuming a constant circulation flow rate by means of velocity vector diagrams (Fig. 4.3).

When the vehicle is about to pull away, the engine drives the impeller with the turbine held stationary. Because the stalled turbine has no motion, the relative forward (linear) velocity Vl between the two members will be large and consequently so will the resultant entry velocity VR. The direction of fluid flow from the impeller exit to turbine entrance will make a small angle 01, relative to the forward direction of motion, which therefore produces considerable drive thrust to the turbine vanes.

As the turbine begins to rotate and catch up to the impeller speed the relative linear speed is reduced. This changes the resultant fluid flow direction to 02 and decreases its velocity. The net output thrust, and hence torque carrying capacity, will be less, but with the vehicle gaining speed there is a rapid decline in driving torque requirements.

At high turbine speeds, that is, when the output to input speed ratio is approaching unity, there will be only a small relative linear velocity and resultant entrance velocity, but the angle 03 will be large. This implies that the magnitude of the fluid thrust will be very small and its direction ineffective in

Vorrex velocity

Vorrex velocity

Stalled turbine

Impeller Turbine

V[ = Impeller exit velocity

VL = Relative linear velocity of both pump and turbine Vr = Resultant effective velocity and direction of fluid x w

- speed rat io -input VnV

Fig. 4.4 Relationship of torque capacity efficiency and speed ratio for fluid couplings

- speed rat io -input VnV

Fig. 4.4 Relationship of torque capacity efficiency and speed ratio for fluid couplings

Medium speed

High speed

Vt

Si

*

Fig. 4.3 Principle of the fluid coupling p o a o

100

so

60

3 J

Slip %

i

—100% torque

40

s-50% torque

ZO

1 ^

0 1000 2000 3000 4000 WOO

0 1000 2000 3000 4000 WOO

Engine speed (rev/mjn)

Fig. 4.5 Relationship of engine speed, torque and slip for a fluid coupling rotating the turbine. Thus the output member will slip until sufficient circulating fluid flow imparts enough energy to the turbine again.

It can be seen that at high rotational speeds the cycle of events is a continuous process of output speed almost, but never quite, catching up to input speed, the exception being when the drive changes from engine driven to overrun transmission driven when the operating conditions will be reversed.

4.2 Hydrokinetic fluid coupling efficiency and torque capacity (Figs 4.4 and 4.5) Coupling efficiency is the ratio of the power available at the turbine to the amount of power supplied to the impeller. The difference between input and output power, besides the power lost by fluid shock, friction and heat, is due mainly to the relative slip between the two members (Fig. 4.4). A more useful term is the percentage slip, which is defined as the ratio of the difference in input and output speeds divided by the input speed and multiplied by 100.

The percentage slip will be greatly influenced by the engine speed and output turbine load conditions (Fig. 4.5). A percentage of slip must always exist to create a sufficient rate of vortex circulation which is essential to impart energy from the impeller to the turbine. The coupling efficiency is at best about 98% under light load high rotational speed conditions, but this will be considerably reduced as turbine output load is increased or impeller speed is lowered. If the output torque demand increases, more slip will occur and this will increase the vortex circulation velocity which will correspondingly impart more kinetic energy to the output turbine member, thus raising the torque capacity of the coupling. An additional feature of such couplings is that if the engine should tend to stall due to overloading when the vehicle is accelerated from rest, the vortex circulation will immediately slow down, preventing further torque transfer until the engine's speed has recovered.

Fluid coupling torque transmitting capacity for a given slip varies as the fifth power of the impeller internal diameter and as the square of its speed.

where D = impeller diameter

N = impeller speed (rev/min)

Thus it can be seen that only a very small increase in impeller diameter, or a slight increase in impeller speed, considerably raises the coupling torque carrying capacity. A further controlling factor which affects the torque transmitted is the quantity of fluid circulating between the impeller and turbine. Raising or lowering the fluid level in the coupling increases or decreases the torque which can be transmitted to the turbine (Fig. 4.4).

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