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Xtrac develops advanced hybrid transmission for premium sportscars

04 December 2012

Xtrac has developed a hybridised automated manual transmission that will allow future luxury supercars to meet the EU's 95g/km CO2 emission requirement being introduced in 2020.

The 1010 H-AMT
The 1010 H-AMT

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 December 3-6 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.  

Xtrac has been actively involved for almost a decade in developing transmissions for electric vehicles and hybrids and more specifically parallel hybrid electric vehicles (PHEV), which it believes can provide the best technology solution for satisfying the 2020 CO2 reduction targets.

“Xtrac sits in the enviable position of being able to work with leading race teams as well as high performance supercar manufacturers, so we can ensure that the technology transfer potential between the automotive and motorsport industries is fully exploited,” says Woolmer.

“The premium luxury car sector is predominately filled with high performance mid-engine sports cars, coupes, roadsters and luxury limousine derivatives.  The sector is enjoying strong sales growth especially in Asian markets, but the very nature of their high power output means they are currently the worst performing vehicles in terms of fuel consumption and emissions.  We believe we now have a viable and cost-effective transmission solution for these vehicles.”

Chris Cholmeley, who has taken the technical lead on the project, adds: “The product life cycles of premium luxury supercars are typically longer than mainstream cars due to significantly lower production volumes and the need to amortise high development costs.

With the 2020 CO2 requirement now very much on the horizon, the next generation of supercars must respond to the emissions challenge, but at the same time not lose any of their brand and product appeal.

The sector typically welcomes and embraces advanced and innovative engineering.  Consequently, it has the potential to lead the development of emission reducing technologies, initially as halo products, which can then be progressively introduced at lower price points for increasingly higher volume economies of scale.”

The wet weight of 168kg for the 1010 hybrid transmission comprises 120kg for the gearbox and 48kg for the motor and hybrid components.   This total mass offers up to 20 per cent weight saving over a comparably sized hybridised DCT.  Commenting on the requirement in the luxury vehicle sector to improve shift speed and refinement as well as fuel consumption, Cholmeley recognises that a dual clutch transmission is clearly one of the technical choices for hybridisation.  

“Their main strength is improved efficiency over a two pedal transmission and they offer comparable levels of refinement to a conventional automatic.   On the downside are the weight penalty and significant additional development and production costs.

"Conversely, an AMT is the best choice for efficiency and has the lowest cost base for development and production; two key attributes important for the development and electrification of hyper efficient supercars.   AMTs have the lowest mass and offer the best flexibility for packaging.

"Consequently, if the shift speed and quality of a dry clutch AMT can be significantly improved, without having to adopt an expensive and heavier wet clutch DCT, then this becomes a compelling solution for supercar hybridisation.”

Xtrac recognises that to deliver the full potential of a parallel hybrid electric vehicle (PHEV) propulsion system the torque path through the transmission must deliver multiple modes of operation for the engine and electric motor to function either independently or together.  The 1010 modes of operation include a pure EV efficiency mode, whereby the electric motor can drive the vehicle with minimal gear meshes, and torque infill to solve the inherent shift refinement weakness of AMTs particularly in urban driving.

Other modes include a power boost for overtaking effectively deploying the electric motor-generator and battery as an F1 style kinetic energy recovery system (KERS).    There is also a range extender mode for EV charging on the move and a plug in capability for EV charging whilst stationary.   Last but not least there is a high performance EV mode enabling the electric motor to drive through all gear ratios with a decoupled engine.

“To fulfil all modes of operation the Xtrac hybrid solution allows for connections to both the input and more importantly the output side of the transmission,” adds Cholmeley.   “It is through the output connected torque path that the pure EV and torque infill modes are realised.” 

Xtrac has worked with a number of manufacturers of electric motors for numerous automotive hybrid and EV applications.  For the purposes of its new high performance 1010 hybrid gearbox, the transmission technology specialist has partnered with Magneti Marelli for the bespoke electro-hydraulic actuation system, transmission control unit, electric motor and inverter, and system software.  The design includes individual actuators on each selector fork, and a 60kW continuous 120kW peak output electric motor.

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