Decomposition Analysis of an Automotive Powertrain Design ...

to be stalled (Rs = 0). Equation (33) defines a unique speed in terms of Ko. The stall speed,
N stall , is the speed at which the impeller rotates while the turbine is stalled. From Equation (33),
the stall speed depends on the input torque and the capacity factor at Rs = 0, K o , according to
N stall = K o T i 0.5 . (35)
The dominant design variables in converter design are Dc and the blade angle, αi. Using
functions in Eq. (33) and (34) as a base design, scaling laws can be developed. If similarity is
maintained in the circuit path, while D c is varied, the torque ratio curve remains virtually
unchanged, but K-factor changes according to a similarity law, such as,
D c ′/ D c = (Ko/Ko′ ) 0.4 . (36)
A smaller converter effects a higher K; a higher K implies a higher stall speed for a given input
torque. Blade angles cause substantial change in both the torque ratio R t and the K-factor, and a
simple scaling factor can be used to represent the effect based on data presented in Jandasek (op.
cit.), namely,
α t = (R to ′ / R to ) (37)
K o ′/K o = (α t ) 1.7 . (38)
The scaling factor, α t , can then be used as a variable to design a new converter. It defines
the K-factor, the torque ratio, and from Eq. (35) a new converter size can be determined according
to
D c ′/ D c = (α t ) -.68 (39)
Equations (32)-(34) and (36)-(39) serve to define the torque converter.