XTRAC DEVELOPS ADVANCED HYBRID TRANSMISSION FOR PREMIUM LUXURY SPORTSCARS
05 Dec 2012
Xtrac, the high performance sportscar and racecar transmission specialist, has designed a hybridised automated manual transmission (H-AMT), designated 1010, that will allow future luxury supercars to meet the 95g/km CO2 emission requirement being introduced by European legislators in 2020. Clive Woolmer general manager for Xtrac’s automotive and engineering business, together with principal engineer Chris Cholmeley, will present details of the new transmission this week at the 11th International CTi Symposium and Exhibition, which runs from 3-6 December in Berlin.
The prototype ready gearbox is aimed at vehicle manufacturers competing in the high growth sector of premium luxury road cars, which includes limousines as well as supercars. The 1010 H-AMT fits the same compact transmission envelope as the company’s ground-breaking 1007 gearbox with its high torque carrying capability and innovative transverse gear cluster orientation, as supplied to Pagani for the mid-engine Huayra supercar. Xtrac believes its distinctive transverse transmission configuration lends itself particularly well to hybridisation and offers a compelling solution to the supercar sector’s preference for retaining high power, multi cylinder engines while meeting 2020 emission targets. The transmission can handle engine torque capacities ranging from 800 to 1,000Nm dependent on vehicle gross weight and the particular application.
The electric vehicle speed range has been set just below 95mph (150km/h), covering most European maximum speed limits except for unrestricted German Autobahns, allowing torque infill up to 4th gear during high performance acceleration and in all gears up to the threshold speed during low speed driving when its desirable to keep the engine revs low, thus improving vehicle efficiency. The term ‘torque infill’ is the application of additional torque through the transmission to the road wheels during the period of a gearchange, where in a conventional transmission the torque is interrupted to allow the shift to complete.
Developed as an alternative to heavier and more complex dual clutch transmissions (DCTs) the Xtrac 1010 H-AMT has a lower total parts count and overall mass when compared to a hybrid DCT by focusing on optimised packaging, weight reduction and cost efficiency. It also offers increased functionality over other single electric motor hybrid systems and the modular design satisfies a number of sportscar driveline architectures including front engine longitudinal powertrain layouts. For increased efficiency the 7-speed gearbox offers multiple modes of internal combustion engine and electric motor-generator operation and an overall ratio spread of almost 6:1, which is higher than many other transmissions in this sector.
The overall gear ratio selected provides a balance between the amount of tractive torque available from the electric motor and the vehicle speed range available before the EV mode is disengaged at higher vehicle speeds. The size of the electric motor and the resulting gearing between the electric motor and the road wheels delivers a maximum torque infill during gearshifts of up to 60 per cent of the traction limit for a typical supercar, delivering a substantial improvement to an AMT’s particular weakness of shift comfort during urban driving.
The ability to provide torque to the road wheels independent of the engine also reduces the energy required to be absorbed by the main drive clutch during the vehicle launch pull away condition, potentially leading to clutch downsizing and a reduction in drive plate and flywheel inertia as an added benefit of hybridising an AMT.
During the design of the transmission, Xtrac set itself a number of objectives including developing an efficient assembly method for the gears and shafts. To minimise gearshift times the gear arrangement allows overlapping shifts, with gear ratio pairs arranged non-sequentially on the shafts. The number of additional gear sets for the hybrid connection has been minimised by exploring opportunities for multiple use of existing gears.
A key feature of the Xtrac 1010 transverse arrangement is to place the differential close to the rear face of the engine block, which keeps the overall package length as short as possible and the centre of mass of the gearbox as far forwards as possible. This design creates room rearward of the transmission to effectively package exhaust systems and rear diffuser and also allows for a short rear overhang for styling purposes.
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