Germany – At last year’s Busworld, Voith Turbo celebrated the 70th anniversary of its ‘DIWA’ line of automatic transmissions for urban and suburban buses. The German manufacturer had just brought out the latest addition to the DIWA range, the ‘DIWA NXT’. This reimagining of the city bus transmission, incorporating a mild hybrid system (optional), appeared on the scene when it still seemed that fully electric was the only way to go. Markus Bischoff – who is responsible for the DIWA product line – believes the company made a “brave decision” in supporting the development of the mild-hybrid transmission; today, he feels vindicated by the resurgence of interest in hybrids. “The main request we get from our customers,” Bischoff told Truck & Bus Builder, “is affordability.” The DIWA NXT is a technology that not only fits seamlessly into a conventional powertrain, but also provides significant fuel savings: up to 7% with the transmission alone (compared to the ‘DIWA 6’) and up to 16% with the addition of the ‘central recuperation unit’, which recaptures braking energy and provides stop-start and boosting support.

Putting the “hybrid revival” to one side, the fact remains that the DIWA’s core market – the city bus segment – is “jeopardised” by electrification. “It will happen very fast in Europe,” said Bischoff: by 2030, at least 90% of all new city buses sold in the EU must be zero emission, and after 2035, the traditional combustion engine bus will virtually disappear from European cities. With this eventuality in mind, Voith decided to redesign the DIWA to expand its capabilities and to make it into a more flexible system. By making some changes to the core design – including the implementation of a second overdrive – Voith produced a transmission which is as well suited to intercity applications and longer distances as it is to urban driving with its frequent stops and starts.

When it first launched the DIWA NXT, Voith described it as a “bridge technology” between diesel and electric, an economic means of slashing fuel consumption and emissions which can be achieved with the technology of today. For the city bus segment, the DIWA NXT is likely to be the last transmission which the European OEMs will buy before going all in with electric. However, in developing markets, the demand for transmissions is likely to continue for much longer; and for interurban applications, which are more difficult to decarbonise, the combustion engine, in one form or another, will probably be around for much longer than we originally expected, even in Europe. “A few years ago,” said Bischoff,

no one expected that engines and transmissions would survive beyond this decade. Nowadays, the end of combustion engine is no longer visible on the horizon – certainly not on a global scale.

As such, Voith is “very optimistic” that there is still a future for its automatic transmissions in the bus market. Naturally, the company expects that the interurban segment will take over from the urban segment as the primary market for the DIWA NXT, starting in Europe and other places where the rate of city bus electrification is fastest. Bischoff was prepared to say this much, but he added: “the only thing that is certain, is that nothing is certain!” He expects there to be a mixture of driveline technologies and fuels, even in the bus market, and especially in the interurban segment. Happily, the DIWA NXT will pair with any engine, regardless of the fuel it uses. “There’s a good share of CNG in Europe,” Bischoff said, “HVO is an upcoming technology. And we have had some good initial experience with hydrogen combustion, though, in the end, its success will very much depend on the availability of green hydrogen.” Ultimately, cost is king: every option must pay off in order to compete, “otherwise it’s not beneficial for the customer to invest in it”.

For the interurban segment – with its greater demands on range and flexibility – the traditional powertrain retains a clear advantage over electric. With the DIWA NXT, coupled with its mild hybrid functionality, operators can invest in a system that helps them to reduce their emissions and enjoy what Bischoff calls that “good feeling” one gets from driving a greener vehicle, all while making their money back in fuel savings. This proposition is offered in addition to, not instead of, the pure electrification option, which is well suited to the needs of the urban segment. Those OEMs and suppliers who are able to support various driveline types “have a good market perspective, because they can react to all the market trends and all the requests from the market, and also be very fast in offering solutions,” said Bischoff. Voith, of course, offers not only the DIWA NXT, but also the VEDS – its electric drive system for commercial vehicles. “So we are very confident about the future,” said Bischoff.

Recap of the DIWA NXT’s design

The basic principle of the DIWA NXT’s design – using a “Differential-Wandler” or torque converter in the first gear – is retained from previous iterations in the range. However, the way in which the torque converter operates was changed. In the DIWA NXT, the retarder is separate from the torque converter: the latter is used purely for traction, whereas the former works as a pure retarder on its own, “still strong but much easier to control”. Moreover, Voith added a dog clutch in order to provide a second overdrive mode, giving a wider gear spread that allows the vehicle to be driven at high speeds without sacrificing fuel efficiency.

When the dog clutch is engaged, the transmission enters second overdrive and remains in this mode until the vehicle slows down. The system is optimised to shift gears as efficiently as possible to match the application. Thus, in an interurban bus travelling at a constant high speed, the transmission will shift up from the second gear ratio to the third, the fifth and then the seventh, skipping the gears in between and reducing energy losses.

The central recuperation unit can be used in two different ways: it can be used either to “boost” the drivetrain and provide more mechanical power, or to supply auxiliary power to electrical subsystems. The preferred option is to use the electrical energy directly; however, depending on the application, the boost-support function may also be appropriate, providing 0.9 kWh of mechanical energy (from 1kWh of recuperated electrical energy, after energy losses are taken into account), equivalent to 2.36 kWh of engine power derived from diesel combustion.

Part of the DIWA NXT’s ingenuity is that the same system can be used for either urban or interurban operations, across vehicles optimised for different applications. The driver does not need to drive the bus in a different way: the DIWA NXT automatically engages or disengages overdrive depending on the situation. The shifting strategy is completely controlled by the software, which is protected according to the latest cybersecurity requirements for automotive systems. Bischoff added that OEMs have different “performance strategies”, while operators may wish to adjust the rear axle ratio to suit different operations. Otherwise, the DIWA NXT is the same piece of kit, whether it is installed in a city bus or an interurban bus; the flexibility is provided by the unique gear-shifting strategy and the software which controls it.¹

¹ For further details on how the DIWA NXT works, see our previous feature on the subject: https://truckandbusbuilder.com/article/2022/12/06/the-right-transmission-for-our-time-the-diwa-nxt-and-voiths-mild-hybrid-system

Markets for the DIWA NXT in interurban applications

The world’s markets, said Bischoff, “are changing at different speeds”. The European city bus market has changed very quickly, moving to a majority share of pure electric vehicles in half a decade. In North America, the move to electric is being pushed by financial incentives. The picture in South America is more mixed. “We have a long history in South America,” said Bischoff, “and I would say that you will see each country moving at different speeds towards electrification.” Even in Europe, however, the situation becomes more complicated as one moves beyond the city bus segment. Many operators

are still hesitant to buy electric. Of course, when they receive subsidies, it’s an easier case to make on the financial side, but there’s always the issue of availability. But most importantly, they need a technology which is affordable. And this is where the combustion engine – provided it is efficient, and maybe also driven by alternative fuels – still has a future in the bus market, especially in the intercity and long-distance applications.

Voith expects that demand for its transmissions will remain high among Chinese customers as well, though not because the domestic market wants them. Rather, Chinese OEMs will continue to build traditional buses in large numbers for export to markets such as Latin America, Central Asia, or Africa.

In parallel with the shift to electric, Bischoff expects the shift from manual to automatic transmissions will continue, particularly as developing countries mature and start demanding greater comfort and driveability. In China, there is still “a big share of manual transmissions”, so the rate of change towards automatic will depend on what export customers ask for. As for India, Bischoff admitted it is a “difficult” market for Voith.

The overall cost of vehicles [in India] is very low, so the share of the cost for the transmission is very, very high when you export it from Europe to an Indian customer. Therefore, it is still not a typical market for a supplier of automatic transmissions.

Nevertheless, Bischoff is confident that “the trend is our friend”: whether that trend is towards hybrids, towards automatic transmission, or even towards decarbonisation in general. Bischoff feels that Voith’s DIWA NXT is still the right transmission for the current moment, and he is certain that it will seamlessly shift to meet any demands the market will throw at it.

Markus Bischoff is Vice President (DIWA, Damper) at Voith Turbo, where he has worked for over twenty years. He began at the company in 2001 and was involved in product areas such as retarders and mechatronics before, in 2016, he was put into a lead engineering role for the DIWA range. Bischoff took up his current post in 2023.