With the ongoing turnover of emissions regulations for heavy vehicles, the focus for innovation often seems to be the engine, with transmissions taking a back seat. Many can trace their origins back several decades, while Torotrak’s innovative CVT (continuously-variable transmission) languishes perpetually on the cusp of production, despite the long-held licensing agreement with Allison.
But incremental and sometimes substantial improvements are taking place not just to improve efficiency but also driveability – particularly centred on starting, stopping and low-speed manoeuvring.
The battle between manual gearboxes and AMTs (automated manual transmissions) is over, with only a few diehards sticking with the manual option. Indeed, ZF says that for medium-duty trucks manual transmissions will have disappeared “by the year 2025”. And there isn’t the race to add more ratios, as seen at lighter weights (Aisin now offers a 10-speed AMT for cars): truck transmissions are staying at 12 and 16 speeds.
However, Volvo recently introduced two-speed crawler gears for its I-Shift AMT (TE, July 2016, page 16), with an extra-low option, down to 32:1 for precise manoeuvring in off-road driving, even at extreme weights – impressively, using the standard clutch. Meanwhile, hydrodynamic or torque-converter transmissions (TCTs) are incorporating better control electronics and even being combined with conventional gearbox mechanisms to improve efficiency.
ZF AS-Tronic was the first of the AMTs to make a splash, selling a million units to date. Now its long-awaited TraXon successor, announced in 2012, has finally entered service on Iveco’s latest Stralis XP tractor: Iveco calls it the Hi-Tronix. The TraXon’s novel modular construction allows different clutch, PTO and hybrid drive units to be added to the basic 12- or 16-speed AMT. Options include conventional single- and dual-plate dry clutches, or a double-clutch for more seamless shifting. The latter, dubbed TraXon Dual, can make eight of its 11 shifts under load.
Other options include a PTO module and the WSK torque converter clutch, which adds 300kg but enables zero-wear take-off and includes a retarder. Meanwhile, a 120kW electric hybrid module incorporates energy recovery and electric stop-start, as well as the ability to boost engine torque.
TraXon’s maximum torque rating is an impressive 3,500Nm at up to 940bhp; the ratio range is from 12.92-0.77:1 for the 12-speed, while ZF claims the highest overall transmission efficiency in the industry, at 99.7%. Its design incorporates GPS-assisted predictive shifting and a ‘rock-free’ mode for ice. ZF has also demonstrated its optional ‘crawl’ function – which drives the vehicle slowly with the brake pedal released, as per a torque convertor – on a DAF-powered VDL coach. Finally, ‘manoeuvring mode’ limits clutch engagement for precise forward or reverse movement.
Software is improving existing boxes, too. MAN’s TipMatic 2 (rebranded AS-Tronic) features ‘idle speed driving’, designed to make low-speed manoeuvring easier and let the truck flow in traffic jams. It allows the driver to proceed without touching the accelerator, the engine idling at around 600rpm. The box downshifts if it sees insufficient torque, and a firm touch of the brake pops the clutch.
Scania, too, has put a lot of thought into its clutch actuation and control. Its two-pedal Opticruise AMT has an electro-hydraulically actuated unit said to offer more precision (TE, December 2014). The clutch engages fully at low engine speeds, using data from load and inclination sensors, while its ‘manoeuvring mode’ gives more sensitivity at low speeds, disengaging automatically in high range.
Scania’s manual gearboxes also have ‘launch control’, which maintains revs during takeoff, and a couple of optional clutch protection features. ‘Clutch wear protection’ limits engine speed to 900rpm when taking off (but can be overridden) while a clutch overload warning alerts the driver if the clutch is being abused.
In North America, Allison is promoting its TC10 transmission as an alternative to manuals or conventional AMTs. Its ‘blended architecture’ combines a torque converter with a 10-speed twin-countershaft box with multiple internal wet clutches to deliver constant drive. The torque-multiplying effect of the TC lets it start with a relatively tall first gear (7.40:1) while maintaining a 0.86:1 overdrive top. Allison says fleets have documented 5% fuel economy improvement with TC10-equipped tractors over their current manual or AMT-equipped tractors. The figures for retail distribution fleets are said to be even better.
The TC10 is suitable for engines up to 600bhp and 2,305Nm, and Allison is confident enough to give a five-year/750,000-mile warranty. It is currently offered in Navistar’s ProStar and TranStar trucks, and Paccar intends to release the transmission in Kenworth and Peterbilt ranges. “TC10’s advantages related to fuel consumption and drivability have been gaining interest from European manufacturers, but ongoing discussions cannot be disclosed,” comments Allison marketing director Manlio Alvaro.
Nigel Marson, OE account manager at ZF, suggests that some of the push towards torque-converters is coming from operators. “In heavy city operation, where the stops per km are high, we always go with TCT”. The spectrum of driver skill level is also a factor, but he cites one home-delivery firm that recently moved from AMTs in its 3.5-tonne fleet to ZF 8HP automatics.
“Today’s adaptive shift control means we can run the engine in the green band more consistently. It’s coming to the point that TCTs are saving clutch life and now saving fuel – and this is coming from the car side.” ZF’s new PowerLine eight-speed torque-converter unit is related to the 8HP but designed for commercial use, with PTO provision. Despite a dry weight of just 150kg, it is rated at 1,400Nm for vehicles up to 26 tonnes gcw.
Marson makes the point that on intercity routes an AMT performs well on the motorway, but in traffic congestion “you are significantly reducing your clutch life”. ZF’s EcoLife six-speed TCT, largely used in coaches and city buses, will soon be available with stop-start, said to deliver fuel savings of up to 10% in urban operation. The torque converter and lock-up clutch have been reinforced to handle the extra load (there should be no effect on service life).
More significantly, the converter design has also been changed to sustain oil pressure for longer when the engine stops, in order to speed up gear engagement on start-up. The start-stop option will first be available for torque ratings up to 1,700Nm, but a planned increase to 2,300Nm may make it attractive for heavy coaches, tippers and larger distribution trucks.
That said, Alvaro is bullish about the prospects for traditional Allison transmissions. “Due to CLOCS [Construction Logistics, Cyclist Safety], many builders’ merchants and contractors that previously operated HGVs with AMTs in inner-city delivery are now experiencing the advantages of fully-automatic transmissions.” He cites Mercedes’ Econic and Dennis Eagle’s Elite. “Drivers have reported an increase in productivity, particularly in stop-start driving and progressing in queues of traffic.”