Page 276 - Design for Six Sigma a Roadmap for Product Development

246 Chapter Eight

output of this project from the technical side. Of course, the process
should be conducted in the process domain via mapping the DPs to the
PVs. The output is the process structure.

8.4.2 High-level zigzagging
of automatic transmission
Automatic transmission is a power-transferring device that also can
allow the engine to act as a braking device and can change the direc-
tion of the drive wheels (Brejcha 1982). The automatic transmission
high-level FRs are to match engine speed and torque to the driver’s
needs and supply power to the driving wheels. There are many differ-
ent transmission designs, but all are intended to provide power to the
drive wheels at any engine speed without energy loss. Designing an
automatic transmission to meet desired driver needs depends on many
nontransmission aspects, including engine characteristics, which are
determined by engine displacement, torque output, and operating
specifications. A graphical curve, depicting torque versus engine speed,
at constant throttle openings, determines the engine’s useful torque.
The transmission shift schedule can be correlated to the engine
torque curve. It is also important to consider the type of trans-
mission that the vehicle requires. A high-performance model may
require lower ratios and firmer shifts to maximize the vehicle’s ability
to accelerate, meeting customer expectations. On the other hand, an
economy model requires a more aggressive overdrive gear ratio.
Luxury vehicles that carry heavy loads have still other requirements.
Noise, vibration, and harshness (NVH) are another important consid-
eration in automatic transmission design. The transmission itself
must operate as smoothly and as quietly as possible, and the shift
schedule should be designed to keep the engine from lugging or rac-
ing excessively.
Automatic transmission design may be approached from the
axiomatic perspective. The following high-level FRs and their mapped-
to DPs were identified by a group of transmission engineers:
■ High-level FRs
FR 1 launch performance
FR 2 fuel economy
FR 3 high gear gradability
FR 4 provide engine isolation
FR 5 torque requirements
FR 6 absence of torque disturbances
■ High-level DPs
DP 1 startup ratio