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Advancing the auto

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Advancing  the autoAllison has written another chapter in the long history of advanced automatic gearboxes, says FRANK BEETON

Driving city buses before the 1930s must have been incredibly challenging. Using unassisted steering, clutch, gearbox and brake controls to navigate a vehicle weighing upward of ten tonnes through increasingly heavy traffic, almost defies understanding in these days of unbridled technology.

Fortunately, vehicle manufacturers soon recognised the physical effort demanded of bus drivers, and sought to develop technical solutions to ease the load. One of the earliest was the pre-selector epicyclic gearbox, which employed a miniature gear selector and a gear-change pedal to replace the usual clutch pedal. This took a great deal of the physical effort out of gear shifting. This transmission type became widely used in Great Britain before the Second World War, and continued to be used well into the 1950s.

As that decade progressed, the epicyclic transmission evolved into the direct-acting type, which eliminated the third control pedal, and provided almost instantaneous gear changes.

With the contemporary adoption of full-air braking and low-geared manual steering, British buses of the 1950s became even more driver-friendly. Just as well, as one-man operation became increasingly prevalent; the driver also performing the fare collection duties previously entrusted to a conductor.

This essentially British easy-change transmission type was also fitted to bus chassis of German and Japanese origin into the 1970s, when their manufacturers sought to compete more strongly with the likes of Leyland, Dennis, Foden and ERF in their traditional market strongholds.

While it was possible to add fully automatic functionality to epicyclic transmissions, the vast majority operated as semi-automatic units, requiring the driver to make gear changes under manual control.

In America, Allison introduced a fully-automatic bus transmission in 1947, but it took some time before its recipe of torque converter and planetary gearbox was accepted in markets traditionally supplied by British manufacturers.

By the late 1970s, however, operators were looking increasingly for efficient fully automatic functionality, and, as the technology evolved, an increasing number of front-engined ladderframe bus chassis, equipped with Allison full automatics, found their way into fleets in “emerging markets”.

The subsequent swing to rear-mounted engines for more sophisticated bus operations brought fully automatic transmissions of German origin into play, but Allison has retained its position as a preferred supplier of bus transmissions in many parts of the world.

During 2015, Allison introduced its B3400 xFE bus automatic transmission to the United States market. It has subsequently followed up with the T3280 xFE, T3325 xFE and T3375 xFE models for sale elsewhere.

In essence, the xFE line-up has advanced the capabilities of the Allison Torqmatic range, with its software and electronic controls, by the application of first-range lockup, optimised gear ratios, lower shift points, and automatic engagement of neutral when the vehicle is at rest.

Allison claims fuel consumption improvements of around seven percent over the previous generation, and some examples have exceeded this in real-world tests.

Interestingly, we have not yet seen any large-scale adoption of automated manual transmissions (or AMTs) for city or transit bus use, although they are definitely a factor in limited-stop coach applications. However, a trend has emerged where AMTs have been chosen as viable alternatives to traditional torque converter/planetary gear automatics in stop-start truck operations.

It will be interesting to see if any manufacturers decide to use similar transmissions for frequent city stop-start bus operations.


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