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Trial of ZF automatic transmission on SWT Class 158s

For a large part of my formal career I was the principal focus for here in the UK for Voith’s rail activities and was heavily engaged in the original delivery of the T211rz transmissions for the Class 158/159 programme. I’m therefore interested to read your article regarding the trial of the ZF automatic transmission in the Class 158 DMUs currently running on SWT’s services and to offer my perception.

First I must say that I’m sure that Christian Roth would not have said that the existing gearbox design is hydrostatic, it is in fact a hydrodynamic transmission using a torque converter rather than the ball or piston pump arrangements typical of hydrostatic units.

At the outset of the 158/159 project back in the late 1980’s Voith had recognised the issues associated with transmission efficiency and the resulting impact on fuel efficiency. Indeed at the time it had led to us musing the benefits of introducing an hot-shift range change or even a little perhaps more outlandishly the concept of a solution that would involve the provision of a parallel power driveline in which an electric motor could be coupled with the Voith driveline to provide conventional drive from the dc traction that exists as I recall on the line from Waterloo to Basingstoke. Neither of these concepts got beyond the level of all but the most cursory of discussion, nevertheless this project was amongst those that provided aspiration for the development of Voith T312 fitted to First Group’s Transpennine Express Class 185, Deutsche Bundesbahn’s VT612 and T212, 3 stage (torque converter-coupling-coupling) transmissions.

I recall that Voith on re-entering the UK rail market in 1984/85  had a brief debate on the virtues of offering the rail derivative of the Diwa bus transmission starting with the two-speed D502 bus, which had a full reversing capacity. At that time this transmission was dropped due largely to a number of factors the most obvious being the torque capacity. As with the later 3 and 4-speed Voith transmission and the equivalent ZF bus transmissions torque capacity has now effectively been addressed, both ZF and Voith have built modern units capable of transmitting 2000 Nm and more. Second there is the question of ensuring that the transmission will not only take the engine torque, but also the torque arising when the vehicle transfer from traction to overrun – powered cars are more than 50% heavier than a coach on the road.

A third point when considering the then newer D851/D854 was its omission of a full reverse capability. First generation UK dmu’s employing the SCG transmissions of the time typically used a separate axle mounted reversing unit that was axle hung and pneumatically controlled. This unit was not entirely successful due to the environmental issues surrounding such devices when operating in environmental conditions that entail changes in temperature and humidity; large unsprung masses are also unattractive both to the vehicle dynamics and the track engineer. Voith had recognised the necessity for a reversing section in their latest DIWA rail-based offerings since the end of the 1990’s and I am sure that as a follow-on to the Class 172 solution ZF will have followed suit with their replacement for the Ecomat, the Ecolife. 6 AP 2000/2100.

How Vossloh intend to make use of the six speeds of a transmission that already provides an excessively wide ratio spread fascinates me. For comparison the ZF has a spread of about 13.1:1 while the Voith road transmission derivations are around 6:1 and Transpennine Express T312 torque converter-coupling-coupling transmission comes in at 6.2:1. Surely they won’t use more than 4 or 5 of the available ratios. Such ratio spreads are fine for road adhesion levels twice that of rail, they’re equally suited to the gradients encountered in road-going applications, but rail; no.

The last and essentially the most significant issue to be mentioned, the complexity of replacing the existing transmission with a torque converter/mechanical speed shift arrangement. In 1989 this was indeed as I’ve said a difficult subject, not least due the necessity of the transmission and the engine to communicate not only with one another but also with the power pack in each car talking to one another. Such an issue was amplified, especially if wheel turning is not optimised. It is apparent that with the possibility to run a train set with the wheel diameters cars varying by up to 10% or 80mm, one could be running one engine at 90% of full rated speed while another operates at full rated speed. Although in practice unlikely to happen such a variation could have implications due to the mismatch that would arise in the tractive effort delivered by each power pack.

I am sure that Vossloh will be more than aware of such obstacles, but I am equally sure that with the marked advances in control techniques in the last 15 years will mean that the need to control the gear changes and the desirability of introducing a suitable vehicle bus/CAN bus system to integrate and provide diagnostic tools can make this a seminal moment in the continued exploitation of diesel traction in the UK multiple unit fleets.