Antonov's six-speed auto comes of age

Roger Bishop explains the technical advances that have led to a fully featured six-speed automatic gearbox for front-wheel-drive small cars

Antonov Automotive Technologies, the Paris-based geartrain development company, appears to have found a niche in the market with a six-speed automatic box that is lightweight, simple and extremely compact. It is being aimed at front-wheel-drive vehicles below two litres engine capacity, directly challenging continously variable transmissions (CVTs) and current 'robotised manual' developments-including the latest double-clutch versions-on issues of efficiency, complexity, cost and smoothness of operation. The system uses manufacturing technologies and components familiar to transmission specialists and offers six forward and two reverse speeds from two simple planetary gears arranged on two parallel shafts. Once fully optimised and engineered for production, the unit cost is predicted to be $650 and the mass as low as 45kg for a 250Nm capacity box. Most importantly, the length of the prototype gearbox is just 250mm, allowing it to be used in the smallest of front-wheel-drive powertrains. Axial pressure operation Like earlier Antonov designs, the six-speed box uses axial pressures to operate internal clutches, instead of the usual high pressure hydraulics-so there is no need for a torque converter-but has three hydraulically actuated valves to control the process more closely and provide consumer features such as pushbutton control and shift management strategies. And the company has just revealed that it will be showing a fully mechatronic version of the six-speed box, in which the valves will be replaced by linear actuators under electronic control, in time for the IAA Show in Frankfurt during September. This will facilitate the integration of the gearbox with powertrain and dynamic control functions. European Automotive Design test drove an Opel Corsa fitted with the prototype gearbox, alongside two Corsa reference vehicles-an Easytronic five-speed automated manual and a conventional four-speed automatic with lock-up. Given that it was using the launch mechanism borrowed from the Corsa and lacked the refinement that extensive mapping against the engine would provide, the Antonov gearbox performed smoothly through the gears, both up and down. There was none of the uncomfortable surging evident in the automated manual and the kickdown was smooth and precise. Engine braking was good at accelerator lift-off and the gearbox's design enables it to miss out gears during deceleration (unlike a conventional auto). So how does the technology differ from the six-speed designs being introduced by European and Japanese specialists? These gearboxes rely on the Lepelletier kinematic to reduce componentry and weight over conventional five-speed designs. They require one simple and one Ravigneaux gear train in combination with five clutch elements. Although compact compared with a conventional five-speed design, three shafts are needed between the two modules, which prevents them from being situated in parallel. In all other respects, they are like a conventional automatics, requiring high pressure hydraulics to shift between ratios. The Antonov gearbox uses a two-speed planetary module arranged in parallel with a three-speed module to deliver six forward and two reverse ratios. In its initial state, there is no speed difference between input and output within the two-speed module. Oil pressure holds the brake locked, the clutch is open and axial force is zero. At the start of the shift process, hydraulic force on the brake is released and applied to the other side of the same piston. The sun gear begins fractionally to move to the left and the clutch plates begin the drag. This generates the initial axial force which, in turn, increases drag further. As axial force increases, the one-way clutch unlocks, setting up a speed difference between input and output. Axial force progressively closes the clutch, changing ratio until the clutch is fully locked by the axial force. The total stroke length is just 2 to 4mm Two clutch packs, two lightweight brakes Input to the three-speed module has two clutch packs, one linked to the sun gear and the other attached to the ring gear. The planetary carrier is the output. Two lightweight brakes (just one or two friction elements) are each linked to a one-way clutch; the left one is linked to the ring gear and the right to the sun gear. As with the two-speed module, the forces controlling clutch opening and closure are axial and hydraulic. Hydraulic pressure (5 bar maximum) generates forces in the clutches that overcome the axial force. When pressure is applied to the other side of the pistons, it triggers axial force to control clutch operation. Any change in direction of hydraulic pressure ensures that engine braking will be engaged via the relevant one-way clutch-an interesting and very noticeable feature during the test drive. During downshifting, the direction of hydraulic pressure is reversed, overcoming the axial force and creating a ratio change downwards. Hydraulic pressure holds the engine brake locked in the new gear ratio. The illustration shows how the six forward speeds are achieved by multiplying the ratios above and below one of the three-speed module and the two speed module. An animation of the shift interactions is on Antonov's web site Antonov Transmission . Antonov is has sold production licences to Honda and three specialist transmission suppliers and is seeking further partners.