N&O JOERNAAL APRIL 1992 B c c*2t The hydrodynamic modelling of torque converters by P. J. Strachan,* F. P. Reynaud,** and T. W. yon Backstriim,*** (Recelved ln flnal form November 1991) List of Symbols factor used in calculation of Y* blade chord length (m) tangential component of absolute fluid velocity cor- responding to F (m/s) d minimum opening between blades (m) D NACA diffusion factor Dh hydraulic diameter (m) H blade height (m) i incidence angle (') i, stall incidence angle (') k casing friction loss coefficient, or Vt/V, k. tip clearance (m) k, height of surface roughness asperities (m) rh mass flow rate (kg/s) r blade radius (m) p, static pressure (Pa) pt total pressure (Pa) Re Reynold's number Re. casing Reynold's number s blade inlet pitch (m) t blade thickness (m) U blade tip speed (m/s) V relative fluid velocity (m/s) V* tangential component of relative fluid velocity (m/s) Yk tip clearance loss coefficient Ye primary or profile loss coefficient Y, secondary loss coefficient Zt blade loading Zn number of blades o absolute inlet flow angle (') od blade inlet angle (') om mean flow angle (") p absolute outlet flow angle (') 9o blade outlet angle (") 6 shock loss coefficient AC* C*z C,"r (m/s) AV* difference between actual and design V* (m/s) e half blade width, see fig. 9, (m) €ri, limiting radius ratio p fluid density (kg/mt) o slip factor A impeller friction factor Subscript Notation inlet outlet + Senior Lecturer,rf M.Eng. Student,rt* Professor, Member Department of Mechanical Engineering, University of StellenboschSummary One method of predicting the performance characteris- tics of torque converters is by means of a hydrodynamic model in which the geometry of the torque converter and the properties of the fluid are given, and the angular mo- mentum flux over the members is calculated at specific operating points. A number of such models has been de- veloped in the literature, all of which rely on empirical input data for determination of the losses and slip factor. This paper describes a hydrodynamic model in which the empiricism has been removed from the input data and built into the program in the form of empirical equations and loss models. The program can be applied to torque converters having profile and thin blades, and in addition a new shock (incidence) loss model is introduced which is employed in the hydrodynamic model to calculate the shock losses of the different members of a torque con- verter. Prediction of torque converter performance by means of this model agreed well with published exper- imental data for a wide range of torque converter ge- ometries. Introduction Torque converters are widely used in automotive applica- tions, from passenger cars to heavy commercial and mil- itary vehicles. The primary function of the torque con- verter is to provide torque multiplication. This is a maximum at stall and decreases without step to a value of unity at the coupling point. The configuration of a typical torque converter is shown in fig. I []. The torque con- verter is similar to a fluid coupling, but with the addition of a stator or reactor. In a fluid coupling, power is trans- mitted from the pump or impeller to the turbine without change in torque, but with the insertion of the stator in the circuit, the angular momentum of the fluid is changed between the turbine exit and impeller entrance, resulting in torque multiplication between impeller and turbine. AsDesign path lmpeller One-way element (f reewheel) Output shaft Figure 1 Torque Gonverter Gonfiguration I 222 its name implies, the stator is norrnally stationary but in most modern torque converters it is usually fitted with a one-way clutch. The characteristic performance curves of torque con- verters and fluid couplings are shown in fig. 2. The ab- scissa is speed ratio, the ratio of output (or turbine) speed to input (or impeller) speed. The ordinates are torque ratio i.e. output torque to input torque on the one side and efficiency on the o