Turbo-Retarder Clutch: When Starting and Braking Converge in One System
A technical solution for heavy loads, slow maneuvering, and challenging downhill grades
In heavy-duty and construction site traffic, situations that are already demanding for a normal truck have become part of everyday life. Vehicles must start with high total weights, maneuver millimeter-precisely on difficult terrain, and safely brake heavy loads even at low speeds. The Turbo-Retarder Clutch was developed specifically for these requirements.
At Mercedes-Benz Trucks, the so-called Turbo-Retarder Clutch (TRK) is primarily associated with the heavy-duty variants of the Arocs and Actros SLT. However, a small chronological correction is necessary here: Contrary to the common association with the year 2018, the TÜV Nutzfahrzeug-Report states that the Turbo-Retarder Clutch was introduced for Arocs and Actros SLT in 2016. In 2018, however, the technology was already part of the Mercedes-Benz program and was demonstrated in detail at various events and vehicle presentations.
Two Functions in One Unit
The basic principle of the TRK is remarkable: it combines a hydrodynamic starting clutch with a primary retarder in a single assembly.
When starting conventionally via a dry friction clutch, the clutch must allow slip when transmitting torque. This slip generates heat and leads to long-term wear. In contrast, with the Turbo-Retarder Clutch, the force during starting is transmitted hydrodynamically via a fluid circuit.
Voith describes the system as a fill-controlled hydrodynamic torque converter. During starting, engine power is transferred from the pump wheel to the turbine wheel and then to the transmission input shaft. A conventional friction clutch is arranged in parallel as an auxiliary clutch and takes over mechanical power transmission during normal driving operation.
This creates an effect that is crucial for heavy vehicles: Starting and maneuvering can be performed without the clutch wear associated with a conventional friction clutch.
This is particularly relevant when a vehicle needs to maneuver or move under high load for extended periods or at very low speeds.
Precise Starting Under High Load
One of the special characteristics of the TRK does not necessarily lie in the fact that a truck can start with it at all—other drive concepts can also achieve this.
The crucial point is rather the controllability at very low speeds and high loads.
Mercedes-Benz describes the Turbo-Retarder Clutch as a solution for precise starting and maneuvering even at the lowest speeds. For heavy-duty applications, this is particularly interesting when high tractive forces are required while the vehicle must move only very slowly.
Voith specifies an input torque of over 3,000 Nm for the system and describes the ability to convert engine torque up to the traction limit. According to the manufacturer, the technology is designed for loads or total tow weights of up to 250 tonnes.
This represents a significant difference from normal road operations, especially in heavy-duty transport. There, it may be necessary to start with high tractive force while simultaneously maintaining a very low speed precisely.
The Second Task: Braking
The uniqueness of the Turbo-Retarder Clutch is already evident in its name. The system is not just a starting clutch.
During braking, the hydrodynamic circuit is switched accordingly. The turbine wheel is locked, causing the unit to operate as a primary retarder. Thus, the braking effect is also generated hydrodynamically.
Mercedes-Benz specifies a continuous braking power of 350 kW or 476 hp for the TRK. This braking power is available independently of the actual service brake. In combination with the powerful engine brake, very high wear-free braking performance can be achieved in the corresponding heavy-duty vehicles.
For example, on the 500th Mercedes-Benz Actros SLT, the 350 kW from the TRK were combined with up to 475 kW of engine braking power. Mercedes-Benz calculated the resulting total wear-free braking performance at 720 kW or 980 hp.
This is particularly important on long downhill grades with high tow weights. The service brake does not have to perform all the braking work alone.
Why a Primary Retarder?
A major advantage of the design lies in the fact that the retarder function is already integrated into the drivetrain.
Voith points out that the primary retarder function enables high braking performance even at low vehicle speeds. Furthermore, according to the manufacturer, the retarder function is also available in reverse gear.
For a normal long-haul truck, this feature might initially seem unremarkable. The situation looks different in heavy-duty and construction site traffic. There, a vehicle with very high mass can still generate considerable forces even at walking speed.
The combination of low speed, high load, and controllable braking effect is therefore precisely the area where the design aims to demonstrate its strengths.
Wear-Free Starting – What Does That Actually Mean?
The term “wear-free” should, however, be technically classified correctly.
It does not mean that the entire transmission or the complete drivetrain operates without wear. Rather, torque transmission during starting and maneuvering occurs hydrodynamically, so that the friction clutch used for this purpose does not have to operate continuously with slip as a conventional clutch would.
Voith therefore describes a significantly extended service life for the friction clutch and wear-free hydrodynamic torque transmission.
In normal driving operation, however, the hydrodynamic transmission is bypassed. The mechanical friction clutch then takes over the power transmission. In this way, the system combines the characteristics of hydrodynamic starting with the efficiency of a mechanical drivetrain.
Particularly interesting for heavy-duty and off-road applications
The Turbo-Retarder Clutch is therefore not technology primarily developed for classic long-haul transport.
Its field of application lies where high towing weights, low speeds, frequent shunting, and difficult terrain conditions converge.
Mercedes-Benz used this technology, among others, on Arocs vehicles. For the Arocs, the TRK was described as a solution for demanding operations, while the SLT series is designed for heavy-duty and large-volume transport with total towing weights up to 250 tonnes.
Even the practical tests from that time show why this technology is particularly interesting off-road. For example, a test of an Arocs equipped with a Turbo-Retarder Clutch described shunting and driving in extreme gradients as a key area of application for the system.
A technical detail with great impact
What is interesting here is that the Turbo-Retarder Clutch does not simply replace a classic torque converter.
The system operates with a filling-regulated hydrodynamic clutch. By filling and emptying the hydrodynamic circuit, the characteristic of power transmission can be changed steplessly. During starting and shunting, the turbo-clutch operates; during normal driving, power is transmitted mechanically, and during braking, the system takes over the function of the primary retarder.
In essence, three different operating states are thus integrated into a single technical solution:
- Starting and Shunting: hydrodynamic turbo-clutch
- Normal Driving: mechanical power transmission via the friction clutch
- Braking: hydrodynamic primary retarder
It is precisely this combination that makes the technology interesting.
Conclusion
The Turbo-Retarder Clutch is an example that innovation in commercial vehicles does not always consist of making an engine more powerful or a vehicle faster.
Here, something else is at stake: applying force to the road in a controlled manner and dissipating it equally controllably.
During heavy starting, the hydrodynamic power transmission reduces wear on the friction clutch. During shunting, it enables sensitive power transmission even at very low speeds. During braking, an integrated primary retarder provides a powerful, wear-free continuous brake.
For classic long-haul transport, such technology is not strictly necessary. However, for heavy transport, construction vehicles with heavy loads, and operations with extreme towing weights, it can represent a significant technical advantage.
And that is exactly where the true significance of the Turbo-Retarder Clutch lies: It is less a spectacular individual innovation and rather a solution consistently tailored to extreme operating conditions within the drivetrain of heavy commercial vehicles.
