Germany – To the layman, the inner workings of an automatic transmission are as bewildering as a foreign alphabet: gears of various shapes and sizes arranged in a row like typed letters on the line of a page. He or she can discern separate gear trains just as one might distinguish words in a sentence, but there is little understanding; and once the gears begin moving, they speak too fast for comprehension. One feels it would be a daunting proposition to learn how each one works, and how each is made to work together as a whole, just as it would be to learn a new vocabulary and grammar. Nowadays, the art of reading the insides of a transmission is threatened by the rise of a less esoteric machine, the electric motor. Many of the major cities in Europe and North America have committed to only purchasing buses powered by the simple conversion of electrical energy to mechanical energy. 

What does this mean for the DIWA, the line of automatic transmissions which Voith has supplied to the city bus market since the 1960s? The four-gear transmission with torque converter was designed to meet the needs of that niche, with its low speeds and frequent stops and the demands for comfort and a long service life. But Voith accepts that urban bus operators are gradually turning away from the diesel powertrain with its complicated array of gears. This does not mean, however, that there is no life left in diesel, Marc Osswald stressed. The world’s most prosperous cities have the infrastructure and the power supply to make the switch to electric feasible, but smaller, remoter settlements with fewer resources will rely on diesel for much longer. A significant part of the city bus market will remain with diesel over the next ten years, which more than warrants a new transmission concept which will “provide diesel at its best”. 

Acknowledging the shrinking need for diesel powertrains, however, Voith decided its next concept must be as “flexible” as possible. It should be suitable for a higher number of applications with deviating requirements, such as longer distance interurban transportation and even coaches, while continuing to meet the specific needs of the urban market. Moreover, it should incorporate aspects of electric powertrain technology in order to keep fuel consumption low. 

The result is the DIWA NXT, a seven-speed transmission which is designed to serve both city and intercity applications. To serve further applications without limiting the main application of a city bus, Voith has implemented a second overdrive, providing three gear modes that run in parallel to the transmission’s direct drive modes. When overdrive is engaged, the vehicle shifts between gear ratios seamlessly, allowing it to cruise at higher speeds more efficiently. With the changes made to the workings of the automatic transmission, Voith claims it can save up to 7% in fuel. With the addition of an optional “mild hybrid” system, the so-called Central Recuperation Unit (CRU) – which recovers energy from braking – the customer can save up to 16% of fuel and an according level of CO2. 

Comparison of DIWA NXT with and without CRU 

Innovations to the DIWA gearbox 

The Voith DIWA works by the “Differential-Wandler” principle, with a long first gear performed by a mix of the torque converter and a mechanical gear in parallel. The torque converter is positioned in the middle of the transmission, transferring power from the internal combustion engine to the load. It is a fluid coupling which uses hydromechanical power when starting up the vehicle. In former generations of the Voith DIWA, a retarder was integrated within the torque converter. However, in the NXT, the two are now separate, which means the torque converter has been optimised for the start-up procedure only. The secondary retarder can brake almost to standstill, with a maximum braking torque of 1,800 Nm. 

When the first gear is attached, the entry clutch is closed and the input power of the engine is converted mechanically through the “sun gear” or central rotating gear. Hydrodynamical torque is generated through the torque converter with oil coming from the sump. As the bus picks up speed, the transmission shifts to a purely mechanical operation with its internal gears engaged in direct drive. Such a system, Osswald claims, is optimal for city bus driving, with its frequent stops and starts, as it enables smoother and wear-free start-up and gear shifting.

Torque converter start-up procedure 

However, with the NXT, Voith also wanted to provide smooth and efficient driving at higher speeds. An intercity bus will travel at a considerable rate for a sustained period of time, without needing to make any stops, and this requires a different kind of transmission. Therefore, Voith brought in an overdrive mode which can be engaged with every mechanical gear, depending on the driving situation, at constant speeds automatically by the Voith DIWA ECU. “Once the second overdrive is applied, we try to keep it applied”, said Osswald. In other words, the driver can remain in overdrive mode while the transmission continues to switch between gear ratios. 

What this means is that the optimal shifting strategy for the DIWA NXT is not to move linearly through each mode (i.e., 1, 2, 3, 4, 5, 6, 7). Once the first mechanical gear mode is engaged (i.e., the second mode), the transmission will shift up through gear ratios in direct drive only (1, 2, 4, 6); if overdrive is engaged, the transmission will shift up through a different set of gear ratios (1, 2, 3, 5, 7; see diagram below). Both modes are suitable for high speeds, but when the vehicle needs more power, direct drive is optimal, whereas overdrive is preferable for maintaining speed. 

7-speed shifting strategy 

It is a system which Osswald claims is a combination of the “best of both worlds”. The bus can seamlessly shift between drive modes, taking advantage of the high power and quick start-up of direct drive and the consistent performance of overdrive, and all without sacrificing the comfort of the passengers and the fuel efficiency of the vehicle. 

The DIWA NXT “mild hybrid” option 

The principle of energy recuperation from braking is now widely adopted in various applications. Electric trailer axles, for example, can convert kinetic energy from braking – which would otherwise be wasted – into electrical energy that can be stored in a battery and later used for supporting functions. The Central Recuperation Unit (CRU) in the DIWA NXT works in exactly this way. It is an electric motor directly mounted between the transmission entry and the flywheel housing of the engine, adding only a very little extra length (approximately 40mm) to the transmission. With a continuous power rating of 25 kW and peak power of 35 kW, the CRU recovers energy from braking and stores it in a battery mounted on the roof of the bus. 

The battery uses lithium-titanate-oxide (LTO) chemistry, storing 1 kWh of energy. It was designed in concert a partner according to Voith’s own specification. Depending on the application, the stored energy can be reused in two support functions: to “boost” the drivetrain and provide more mechanical power; or to supply auxiliary power for the vehicle’s electrical systems. 

CRU energy management 

The first option employs the CRU to convert the electrical energy back into mechanical energy, giving the vehicle a “boost” in power. When energy losses are taken into account, 1 kWh of stored energy would provide 0.9 kWh of mechanical energy. To achieve the same output, 2.36 kWh of engine power derived from diesel would be required. 

The second option employs a DC/DC converter which steps the voltage down from the battery’s 48V to 28V, used to power the vehicle electronics. The energy wastage is less, with 1 kWh of stored energy providing 0.98 kWh after conversion. To achieve the same output, 3.44 kWh of engine power derived from diesel would be required. Therefore, the operator stands to save more diesel – up to 9% – when the CRU is engaged in providing auxiliary power. 

The LTO battery is preferable to a supercapacitor, Osswald argues, because the latter is not suitable for storing energy for a longer time. LTO batteries may have lower power density, but there is sufficient to serve the mild hybrid system which Voith has designed. 

With a different driveline – for instance, a hydrogen engine – the demands on the system change. A hydrogen engine, Osswald says, is relatively weak at start-up, and stands to benefit more from “boosting” than a diesel engine would. In a prototype bus which Voith developed with Keyou, the mild hybrid system was optimised to favour greater mechanical energy. This kind of tweak, tailored to the needs of each vehicle specification, requires no changes to the CRU itself, but only to the software that controls the system – meaning that it can be easily adapted to a variety of uses. 

The DIWA NXT’s future 

In all forms of work, specialisation is essential to maximising performance in any particular function. The Voith DIWA series was tailored to perform well in a specific application – urban and suburban buses. Voith therefore risks losing the DIWA’s specialised advantages in making the NXT more broadly applicable to a wider range of uses. But the company finds itself in unusual circumstances, where demand for diesel drivelines is falling unevenly across the bus segments, and dropping faster in some places (e.g., Europe) than in others (e.g., South America). It wants a concept that bridges the gap between diesel and electric, following rather than obstructing the prevailing direction of the city bus market; and a concept that, at the same time, can seamlessly move to segments that are harder to electrify (e.g., interurban) and to applications which are growing (e.g., natural gas and hydrogen). 

The result is the DIWA NXT, a product which incorporates aspects of city bus design, truck design, and hybrid technology in a single system. A machine like the DIWA NXT, with its mild hybrid system attached, probably could not have been made without “smart” digital technology, and the control units which manage gear shifts, engine RPM, and energy use. These are software innovations which Voith has gradually introduced since the DIWA 6 was launched in 2012. They will help Voith to keep the DIWA NXT relevant well into the future, even as the traditional applications for vehicle transmissions fall away. Osswald is “absolutely sure” that hydrogen engines combined with automatic transmissions will play a role in decarbonising transport; it is much easier to continue making use of the knowhow and supply chains that serve the internal combustion engine than to switch to fuel cells, though they will, of course, have their part to play as well. When it is still unclear which alternative driveline will take precedence over the others, it is probably best to be flexible. 

Dr Marc Osswald is currently Vice President Product Management DIWA & Damper, a role which he has held at Voith since 2018. He began working for the company in 2016 as an Executive Assistant. He received his doctorate in Business & Economics at Ulm University. As well as working for Voith, Osswald has also worked as a consultant for iTOP.PARTNERS.