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.
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
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.