Even while the world waits for the first eight-speed
transmission, let’s not forget that ZF found
little interest for its
seven-speed auto.
Why? Maybe because at this point it can be difficult to make arguments (other than marketing) to go with more
than 5 or 6 forward ratios – but they do exist.
Transmission Wars, Part I
Decreasing the ?step? between each ratio reaches the point of diminishing returns, as engine technology continues to
yield wider powerbands, not narrower ones. This could change, of course, if much smaller engines are desired
for economy reasons. With the 6:1 ratio spread of the superb ZF 6HP26 gearbox, not to mention the torque converter?s
contribution (typical stall torque ratios are 2:1 or greater), adding additional gears on either end makes little
sense.
Ah, but what if the convertor is removed and replaced with a multi-plate wet clutch? Then it?s desirable to increase
the ratio spread (remember that we just lost the converter?s considerable effects), and that?s where additional ranges
might come in handy. The question then comes down to whether eliminating the torque converter is worth the complexity
of additional ratios. Considering that modern torque converters are no longer the source of inefficiency that they once
were, it?s largely an issue of packaging. Torque converters are most efficient with a circular cross section, but that
increases their bulk which in turn makes the overall transmission package longer. That?s particularly problematic in
transverse applications. Eliminate the converter, and some space magically reappears. A bit of money can be saved,
since the multiplate clutch is already present in many modern converters.
The picture I?ve tried to paint above is basically this – additional speeds make most sense on cars where an attempt
is made at increasing fuel efficiency via a smaller engine, and where packaging space is at a premium in a FWD
application. That doesn?t sound like the average luxury car, does it? It just might be that the extreme opposite end of
the auto spectrum provides the natural starting point for the next generation of automatic gearboxes.
There?s one more application as well, and that?s in hybrids. CVTs are useful for allowing the IC engine to run at its
most efficient speed for both charging the batteries and providing forward thrust, but of course they have torque
limitations. An engine such as a high-performance turbodiesel might provide enough torque to exceed the limits of
available CVTs, but might also be too peaky to be used with current ratio steps. Add more steps, and this combination
potentially becomes more efficient.