Automatic Transmission Ratios - when more becomes too many

Earlier this month, transmission specialist ZF Friedrichshafen AG released the world's first 9-speed automatic passenger car transmission, the 9HP. ZF did not specify which OEM will be adopting this transmission, but said that it is designed for front-transverse application (FWD / AWD). Industry rumour suggests that Chrysler will be one of the early adopters of the 9HP. It wasn't too long ago that we thought a 6-speed transmission is just about the most you will ever need. But in 2003, Mercedes-Benz launched its in-house developed 7G-Tronic transmission, the world's first 7-speed automatic. If a Formula One car has only 7-speed, why would you need anything more than a Formula One car, right? Or so we thought. In 2007, Lexus shocked the world by launching the LS600 limousine with the world's first 8-speed automatic transmission, the TL-80SN unit made by Toyota affiliate transmission maker Aisin. Just as we are getting used to a 8-speeder, we now have the 9-speed from ZF shoved into our consciousness. We are only 1 more ratio away from double digit transmission ratio count. The question many are asking is, is this really necessary? Surely car manufacturers are not expecting cars of the future to feature the same gear ratios as my mountain bike right? Right....?

At some point there must lie a point of diminishing return, where complexity and cost of the transmission will outweigh the benefits offered. So where is the practical limit? 10-speed? 11-speed? And here's the scary answer - nobody knows what is technical limit, and until we find limit, passenger cars transmission ratio count will continue increasing.

In an interview with industry journal Transmission Technology International (TTI), Dr Jürgen Greiner, executive of product development at ZF admits that he doesn't know when will this ratio growth stop. Dr Greiner pointed out that by looking at ZF's own history, passenger cars add one more gear to its automatic transmission every 10 years. ZF launched its 3-speed automatic in 1973, 4-speed automatic in 1982, 5-speed in 1990, 6-speed in 2001, 7-speed in 2003, 8-speed in 2007 and 9-speed in 2011. Of late, driven by an urgent quest for increased fuel efficiency, the pace of transmission development has upped considerably. Dr. Greiner did mention that the rapid growth transmission ratio is motivated by marketing, as a key differentiating factor. Market research indicate that consumers view a higher ratio count transmission unit as superior, never mind the fact ratio spread is more important than ratio count. Hence the one-upmanship that we are seeing now.

But is there really a clear benefit from having an 8-speed or 9-speed automatic, versus a regular 6-speed? The question was posed by TTI to ZF’s group executive of the car driveline technology division, Dr Gerhardt Wagner, whose answer was quite surprising. He said, “In our opinion, there is no advantage for the driver just by simply adding a gear. In terms of consumption and driving dynamics, the seven- and eight-speed transmissions currently on the market are in no way superior to our six-speed transmissions. Often, the call for ever more speeds is something that is driven more by marketing than by technical necessity.” His candid answer is very surprising because ZF supplies the 8-speed automatic unit currently used in BMW's flagship 7-series, on top of launching its new 9HP unit. Dr Wagner however later emphasized, “The objective at ZF was not to increase the number of speeds. Instead, we wanted a more efficient transmission design in terms of fuel consumption, performance, mechanical design and costs. Different levers were considered in the design study for a new transmission generation, including the number of gears. At the end of the optimization process, it emerged that the concept with eight speeds offered the best prerequisites for an optimal transmission system.”

BorgWarner is another transmission specialist, most famous for designing VW's famed DSG dual clutch transmission. BorgWarner's Chief Technology Officer Hans-Peter Schmalzl said that “While more transmission ratios may have some marketing appeal, the race for greater transmission efficiency is won with wider ratio spreads, quick and seamless ratio transitions, and the lowest possible parasitic losses. It is safe to say that more ratios beyond current levels will add complexity and cost, while the potential for incremental efficiency gains are approaching diminishing returns. There comes a point when the weight, size, and cost of adding another ratio outweighs the benefit it provides.”

Generally, the more ratios there are, the more likely for the transmission control electronics to find the most suitable ratio for the current driving environment, thereby optimizing fuel consumption and power delivery. But the converse also means that gear ratio selection logic is now a far more complicated task. Instead of being limited to just one choice, the transmission control unit (TCU) now has two or maybe three choices available. With more choices available, the chances of making an incorrect selection or unnecessary shifting / downshifting is increased. The Chevrolet Cruze, with its small 1.8-liter engine comes with a 6-speed automatic. Sounds good on paper, but some owners, especially those with a demanding taste and even media test drives reveal that the Cruze's transmission  is not one of the car's forte, as it sometimes finds itself either in the wrong gear or even shifting mid-corner! It would be stretching the truth to say that it drives better than a Honda Civic's 5-speeder. Plus, for such a small engine, why would you need a 6-speeder anyway? Previously 6-speeder automatics' mainstay are in high end luxury cars with powerful engines. Part of me suspects that manufacturers install these new generation high-ratio count automatic gearboxes to achieve a higher fuel economy rating under New European Driving Cycle test. In a predictable, lab environment, yes having an additional ratio will yield better fuel economy result.

Dr. Stephan Manger, senior manager for overall vehicle development of the Mercedes-Benz M-, R- and GL-Class once said that with "too many gears, the shifting numbers are too high. You come to the point where the car shifts too often." Which brings the additional cost of designing a complex shift map to keep the shifting numbers to a minimum, not to mention the added mechanical complexity. Mercedes-Benz's early generation 7G-Tronic transmission had an undisclosed issue that resulted in jerky shifts at low speeds as well as under braking. A quick Google will bring it up. However, a production running change have since solved the problem.

If you are the minority who don't suck up the marketing hype, and understands that what matters most is having a transmission ratio spread that matches the engine's torque curve, you have little choice but to cough up more to pay for a supposedly more advance transmission that you didn't asked for. Most probably, the added cost would have cancel out whatever savings gained from a marginal improvement in fuel economy.

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