Industry insight: Continuously variable TransAxle

Continuously variable fun

Ford and Fiat have brought automatic transmission within the grasp of Fiesta and Uno buyers with a joint development of the steel belt/variable diameter pulley CTX (Continuously variable TransaXle). In Britain the system costs £673 on 1.1 Fiestas, whilst Fiat is expected to charge £550 for it when it becomes available this summer.

Those with a regard for the Model T may feel Ford has always been in the automatic business, but in contemporary Britain automatic gear changing is a small, but growing, sub-sector of Ford business. In 1984 Ford of Britain sold 26,067 automatic Escorts, Orions, Sierras and Granadas; last year that figure was 38,454, some 7.5% of all sales.

Ford, Fiat and Variomatic producers Van Doorne in Holland have worked upon the CTX since 1976, consuming $120 million. Even now, after trying to produce large numbers in what became a protracted exercise in corporate frustration, the CTX systems can only be built on a relatively small scale at Van Doorne. Following an £80 million expenditure at Ford’s Bordeaux transmission factories, CTX transaxles will be fitted to “model 1988 year cars”, according to sales director Ernie Thompson.

Thus the restriction to 1.1 Fiesta for CTX is a result of production, not technical constraint. Ford development engineers say “there is no durability reason why the CTX should not work alongside 1.6 litre CVH Ford engines, but we already have the ATX (Automatic TransaXle) made by Ford USA and Mazda for the Escort line.”

Technically speaking, the triumvirate which produced the alloy-cased transaxle feel that it offers low weight, mechanical efficiency and a better spread of ratios than a five-speed manual transmission. Ford feels it is equivalent to a six-speed manual, having a lower initial ratio (14.08 versus 13.75) and higher top gear (2.41 versus 2.90) than the present Fiesta five-speed 1.1 ratios offer. Top gear is extraordinarily tall for a tiddler at 25.7 mph per 1000 rpm— 2700 rpm for 70 mph.

Ford says the CTX Fiesta should reach a maximum of 87 mph, and 60 mph 15.4 seconds after leaving rest. Not startling figures, but it is not that long since such performance from the 997cc Mini-Cooper was considered sporting.

Inside the CTX are two multi-plate wet clutches, a planetary gear set, a multi-link steel drive belt and two pulleys (driven from engine to belt to second pulley, one which ultimately turns the final drive). Ratio changes are enacted by the fixed-length belt containing 320 vee links mounted on two sets of composite steel bands.

The belt has to accommodate variable pulley diameters to provide stepless ratios on the principles you see (but over variable width pulleys rather than toothed gears) in a double-chain-wheel bicycle with rear-wheel derailleur gears.

An epicyclic gear set operates between engine flywheel and the variable ratio system. It provides forward motion when clutch one locks the sunwheel and planet carrier; for reverse clutch two locks the annular gear, planet gears (connected to the input shaft via the planet carrier) turning the sun gear upon the output shaft.

Primary pulley widths alter via hydraulic pressure bearing on a servo cylinder and an axial force upon the second pulley’s independent hydraulic servo cylinder. Pressure hydraulics are supplied by a recognisable cousin to the conventional torque convertor and an engine-driven oil pump.

How does it feel to drive? We chose the worst of London’s traffic, from the vicinity of Oxford Street out to Epsom Downs and back, to explore its characteristics.

First impression is how wise it was to develop a transmission without fixed ratios, the Fiesta accelerates without perceptible pause, but further refinement is still needed. On idle with the Ghia’s sunroof open there was an unholy vibration (fading away after five city miles) whilst the over-run caught out two drivers concentrating on smooth progress with just the sort of jerkiness that this unit’s principles should avoid. Transmission snatch was pronounced just before halting and on acceleration following a period on over-run. There is a low-ratio hold for particularly hilly or curvy terrain, but we did not feel the need to recourse to it frequently. However, the effort saved in suburban use without the usual automatic creep away from standstill makes this unit a worthwInle investment. Lower performance and economy losses (the worst comparison over the urban cycle shows a 4.9 mpg deficit in CTX vis-a-vis five speeds) than are usual on automatics promise a new era in small car transmissions, for Ford expects the cooperative development to be purchased by manufacturers outside the original trio. JW