Is more always better when it comes to automatic transmission?
4 min read
Never has the automotive sector had to bring to market so many new technologies in such a short space of time.
The industry is full of apparently contradictory design drivers. For instance, the need to reduce the weight of structures is now matched by the need to integrate more safety features, which of course add weight. Equally, there is urgent need to improve recyclability, yet hard-to-dispose-of materials like carbon fibre, plastics and lithium batteries are not uncommon on modern models. With legislation set out as it is, the need to reduce tailpipe CO2 emissions is absolutely vital for the longevity of automotive OEMs. This need to reduce emissions is, however, at odds with performance. Consumers don't want less – or even the same – they want more: more performance, more acceleration and more fuel economy. These conflicting demands have made the job of engineers particularly difficult, with every conceivable area of the modern car being assessed and optimised for improvement. This has led to transmissions widely adopting increasing gear ratios. Standard manual transmissions are increasingly six-speed as standard, with some semi-automatic transmissions that dispense with clutch pedals looking at seven-speed. While manual gearboxes have a practical limit (i.e. the wish of the driver not to be continuously changing gears) there is much more scope for automatic gearboxes. This has seen German transmission expert, ZF Friedrichshafen, continue its trend with the announcement that it has teamed up with Land Rover to launch the 'world's first' nine-speed transmission known as the 9HP "Clearly there is a need to improve fuel economy and reduce CO2," says David Mitchell, chief programme engineer at Jaguar Land Rover. "But, this comes with opportunities. More gears give you a sharper response and the work we have done allows smoother shifts. "With the nine-speed gearbox we also introduced stop-start and combined this gives us about a 10% improvement." One of the downsides of more gears is that they usually add weight and require a larger space envelope. This challenge was made even greater by the design principle used leaning firmly toward passenger cars with front-transverse engines. This made transmission installation inherently limited, with little room for manoeuvre. However, a combination of smart engineering and clever use of materials has meant that the new unit weighs less than the outgoing six-speed transmission. The 9HP uses a combination of clever packaging and, despite the extra three gear ratios, is only 6mm longer. The small packaging space is achieved by a number of innovative design features including a new hydraulic vane-type pump which also contributes to improved efficiency and two patented dog clutches that replace multiple, and more bulky, clutch packs. "The introduction of the two dog clutches took some of the weight out against conventional clutch plates," says Mitchell. "Also the torque converter was sized slightly differently so the diameter is smaller. "Some of the electronics that control the transmission were able to be split and in doing so we made them slightly smaller – again taking off a bit of weight. So it wasn't just one area, it was a combination of a number of different areas that led to a 7.5kg reduction in total." The transmission also uses four individual gearsets and six shifting elements. For this reason, the gearsets have not been allocated one behind the other on the transmission's longitudinal axis. Instead these were 'Intelligently Nested' in to place. This concept was supplemented by using hydraulically-operated constant mesh elements. They can be integrated without major impact on the transmission length and feature a high level of efficiency. The result means the transmission improves efficiency by using a higher gear spread with closer gear stepping. The higher top gear reduces fuel consumption and also lowers engine revs. This improves comfort and reduces noise when cruising at higher speed. In ninth, the car can achieve around 70kph per 1,000 revs. The lowest ratio in the 9HP is also far lower than the existing six-speed transmission and is specifically designed for off-road use, towing and more extreme on-road conditions like gradients and altitude. As Land Rover was closely involved in development, it was also able to ensure that the 9HP is sufficiently robust for off-road driving. However, it is day-to-day road driving where the transmission will be expected to deliver the most improvement. Its torque converter incorporates a multi-stage damper system for smoother pulling away. Its adaptive shifting system can also match driver input when needed, sharpening up during brisk driving then slipping seamlessly back into a more economical regime. Existing six-speed transmission makes gear shifts sequentially, while the 9HP has a 'skip-shift' function for much swifter downshifting under rapid deceleration or from greater driver input demands. Interestingly ZF claims shift times are below 'the threshold of human perception'. This translates to shifts that happen in around a 150ms. This ability adds to the transmission's ability to respond quickly to driver inputs. "We have got a shift-down function with the gearbox that, in its extremes, can shift from ninth to fifth in one mode, or ninth to seventh to fifth to third," says Mitchell. "So we have a 'skip gear' shift-down function that really helps give a nice, sharp response when required." The nine-speed automatic transmission is not likely to be limited to JLR and can be used by any cars with front-transverse engines. JLR is the launch customer, however, and has been closely associated with its development. It will enter volume production with Land Rover in late 2013. ZF says the transmission's control unit can be changed and influenced by individual OEM and end customer requirements. Shifting points and shifting dynamics are highly variable and can be tuned with emphasis on comfort and optimised fuel consumption through to being 'extremely sporty'. ZF has prepared the nine-speed automatic transmission as a modular kit so that it can be used in as many vehicle applications as possible. With two model ranges, it covers a torque range between 200 and 480Nm. In addition, it is start-stop capable as standard without the need for an additional oil pump. It can even be hybridised on the basis of a parallel hybrid architecture where the torque converter is replaced with an electric motor. In addition ZF's 9HP has open software and an interface structure with a powerful electronic control unit. This, again, means that it will be possible to integrate it flexibly into the many different vehicle concepts. Land Rover was chosen by ZF to be the lead partner on the 9HP and Land Rover engineers have worked in partnership to develop the transmission. The Land Rover transmission engineering team is already extremely experienced in engineering ZF products to suit Land Rover vehicles, having integrated the 8HP with the Range Rover, Land Rover Discovery 4 and Range Rover Sport. "We have recently introduced the 8HP and that has been a very successful gearbox," says Mitchell. "The [current] six- and eight-speed automatic transmissions is what we are replacing across the board with the nine-speed." As to the possibility of yet more gears, however, Mitchell is dubious, saying: "The nine-speed is a very good optimisation. Personally, I can't see many more gears being of value."