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01/01/2011
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Transmission systems are increasingly coming under the development spotlight as manufacturers strive to eke as much efficiency as possible from their products. Ian Adcock and Tony Lewin take a look at some of the latest offerings.
 When selecting a transmission, it used to be easy for both the vehicle manufacturer and their customers: it was down to a choice of manual or automatic. In many cases, there wasn't even a choice; it was manual.
But, as with everything, choice has extended as manufacturers seek different solutions, not just to attract customers, but also to differentiate themselves from rivals. And, of course, those great drivers that underline all powertrain development today – emissions and fuel economy – are putting even more emphasis on technologies that will improve an OEM's emissions rating.
That challenge is about to become even more difficult when new cars are labelled and bracketed for their CO2 efficiency, in the same manner that domestic white goods are. Although the parameters have yet to be set, it's clear that clever use of gear ratios and transmissions could prevent a car slipping from a low band to a higher one, penalising potential sales to more CO2 friendly rivals.
The biggest breakthrough in recent years has been the emergence of dual clutch transmissions (DCT). Although the concept predates the Second World War, it wasn't until Porsche developed one for its 956 endurance race cars in 1983 that the technology started to emerge. Even then it would be 20 years before VW introduced a mass-produced version in its Mark Four Golf.
Now Fiat Powertrain Technologies (FPT) has entered the fray with not one, but two new transmissions.
Fiat 500 DCT hybrid
Fiat has important assets at its disposal in the tricky task of applying a hybrid drivetrain to the small 500 city car. The normally acute packaging constraints are less onerous, thanks to the company's novel two-cylinder Twin-air engine, half the length of a conventional four-cylinder unit. Additionally, Fiat Powertrain Technologies' (FPT) experience in dual clutch transmissions provides a versatile means of blending the torque flows from the two power sources.
The short two-cylinder engine has enabled a two-shaft gearbox layout that is not only more efficient in minimising mechanical losses, but also allows all the synchronisers to be on the same shaft and permits the use of a simpler electromagnetic drum-type sequential gear selector mechanism.
Five speeds suffice for the A-segment applications of the hybrid, reasons Dr-Ing Constantinos Vafidis, director of transmissions and hybrids at FPT. The model's primarily urban use pattern means that a high top speed (and thus a very tall cruising top gear ratio) is unimportant and the traction motor's torque assist at low speeds removes the need for a low first gear for launch.
The 3 kW electric traction motor is mounted on a parallel axis, above the gearbox. This offers the major advantage of an idler gear, permitting torque multiplication, and the fitting of a lighter and cheaper motor. One consequence of this architecture is that the motor is permanently geared to the second gear pinion on the transmission's layshaft. This means there is no clutch between the motor and gearbox, and that the motor is always spinning: for this reason, it was important to select a motor type with no parasitic losses.
The e-motor does more than just provide hybrid operation: it has an important role in improving driveability and smoothing out the responses of the drivetrain. The motor cranks the engine at each restart and adds extra torque, if it has to restart the engine, while also powering the vehicle – so the driver does not feel any reduction in tractive effort. The motor even spins up or slows down the gearbox layshaft, so that its inertia does not affect the smoothness or speed of the gearshifts.
The mechanical air conditioning compressor is driven through an electromagnetic clutch off the traction motor, maintaining climate control during engine stop; this, says FPT, avoids the need for a costly electric compressor.
The development team is confident that the targeted CO2 reductions are feasible. The base vehicle, with its naturally aspirated twin-cylinder engine, already has low overall CO2 emissions of 95 g/km on the European cycle: the team is aiming for a 12 % reduction in this figure – to just 85 g/km – in the combined cycle, with an even bigger cut of 24 % in the urban portion of the cycle.
C635 Dual Clutch
According to chief engineer Vittorio Doria, the C635 Dual Dry Clutch Transmission (DDCT) has benefited from technological advances since 2003 that have allowed FPT to increase its torque input capacity to 350Nm and output torque to 4200Nm. The latter is particularly important, as it future protects the transmission for all-wheel drive applications from both Fiat and Chrysler, although that will demand a new clutch housing to accommodate the PTU, as well as transverse installations.
FPT claims that its new 'box – weighing only 81Kgs – is lighter than comparable wet DCTs from VAG, as it doesn't require two kilos for the oil.
FPT has been particularly adept at engineering 30% commonality between this transmission and its six-speed manual, including synchronisers, meaning both transmissions can be assembled on a common assembly line at Verrone, northern Italy. Annual capacity is 400,000.
Likewise, the micro controller belongs to the same family as those used in the recent Multijet and Multiair FPT developments. In effect, all these applications share the same chipset and Base.
Application Software
As well as weight saving, there is also improved packaging, as a result of the three-shaft transmission architecture being housed in a two-piece aluminium housing, with an intermediate support plate for the
shaft bearings. This has allowed the positioning of the differential group to be closer to the engine Rear Face
Of Block, making the transmission compatible with the packaging requirements in the B-segment vehicles.
Doria has also made it clear that FPT will make the transmission and its controllers available to other manufacturers.
Fastest shift times
Italian supercar manufacturer Lamborghini claims that its new in-house developed robotised transmission has shifts speeds of less than 80 milliseconds, making it 50% faster than a DCT, according to VP of research and design and chief technical officer Maurizio Reggiani.
The secret to the seven-speed Independent Shifting Rods (ISR) transmission is that, unlike a manual, the gear wheels from the second and third gears are separate from each other and the shifting sleeves are actuated by independent shifting rods.
Now the shifting process can run virtually in parallel: while one shifting rod is disengaging one gear, the second shifting rod can already engage the next gear. As these movements partially overlap and the mechanical distances are considerably shorter, this facilitates a significant saving in shift time. Overall, the Lamborghini ISR transmission shifts around 40% faster than the e.gear transmission in the Gallardo, which is one of the world's fastest automated manual gearboxes.
The new transmission, assembled in co-operation with Graziano, has four of these independent shifting rods, with sensors constantly monitoring their exact positions. They are operated via hydraulic actuators, with a system pressure of 60 bar ensuring the necessary operating speed. The system incorporates seven hydraulic valves, with pressure supplied by an electric pump. The double-plate clutch is also hydraulically actuated. All system components are contained within one casing, with no external piping. The total weight of the transmission is only 70Kgs, due to advanced CAD work, resulting in reduced casting thicknesses and the use of carbon fibre synchroniser rings.
Xtrac offers instant shift
Competition transmission manufacturer Xtrac has developed its Instantaneous Gearshift System (IGS), a race car-like automated manual using dog rings and gear-face dogs, instead of synchromesh. The mechanism achieves seamless, torque-interrupt free gearshifts by engaging the next gear an instant before the current one disengages.
This is enabled by pawls fitted between each gear and its shaft, the ratcheting effect taking care of any overspeeding between gear pairs. As soon as the first gear is disengaged, the pawl on the second ratio re-engages, taking up the drive and enabling the shift to be as near instantaneous as possible.
The system has been tested in several race series with some success and telemetry tests, comparing IGS with a conventional transmission cluster, show a spike in acceleration (as opposed to a drop) during each upshift. On the track, this corresponds to a speed advantage of 3 km/h over a 450-metre straight.
Xtrac technical director Adrian Moore says IGS has significant advantages over DCT for road vehicles, including lower weight and an architecture with much more in common with a conventional transmission. With suitable calibration of the engine and transmission, it could be as refined as DCT, too, he claims.
Italian supercar manufacturer Lamborghini claims that its new in-house developed robotised transmission has shifts speeds of less than 80 milliseconds, making it 50% faster than a DCT, according to vp of research and design and chief technical officer Maurizio Reggiani.
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Author
Ian Adcock/Tony Lewin
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