Vehicle brakes are air-cooled – until now. This is because the electrification of the drivetrain is leading to a fundamental shift towards emission-free brakes. Prof. Dr. Katharina Völkel, former head of department at the Research Center for Gears and Transmission Systems (FZG) at TU Munich and professor at Friedrich-Alexander-Universität Erlangen-Nürnberg since 1 May 2025, will be speaking on this topic at the VDI Congress DRITEV 2025 on 9 and 10 July 2025 in Baden-Baden. In the run-up to the event, she will shed light on research approaches, potential and possible challenges.
Prof. Dr. Völkel, from your point of view, what are the advantages of wet vehicle brakes?
Prof. Dr. Katharina Völkel: Electromobility stands for zero local emissions. However, this must not be limited to the avoidance of CO2 emissions, but must also include tire abrasion and brake dust in the long term. Particularly in view of the Euro 7 regulations, solutions are needed for respirable brake dust, which is a health hazard. The idea of switching to an encapsulated wet braking system is therefore gaining increasing attention.
What are the technical advantages of wet braking systems compared to conventional dry brakes?
Prof. Dr. Katharina Völkel: A key advantage is the encapsulation: wear particles are bound in the oil and do not enter the environment, effectively preventing brake dust emissions. At the same time, the system is permanently protected against environmental influences such as moisture and ice. Corrosion and its consequences, which are often observed with dry disc brakes, are also avoided, as the wet vehicle brake runs in oil. Finally, in electromobility, braking is done by recuperation, so the mechanical vehicle brake is rarely used. A wet brake is also very resistant to wear. It is therefore conceivable that a design as a lifetime component is achievable – combined with significantly lower service costs.
What are the requirements for this?
Prof. Dr. Katharina Völkel: In order to achieve the desired operational safety and at the same time a high level of functional and comfort behavior, stable and optimum friction behavior must be ensured in all operating conditions – even if the brake is activated infrequently. These requirements can be reliably met with specially defined high-performance lubricants.
What about possible drag torques and their effect on efficiency?
Prof. Dr. Katharina Völkel: In this context, I would like to refer to the long-proven solutions in dual-clutch and torque converter automatic transmissions and the associated wealth of experience from which the development of wet vehicle brakes can benefit. A supply of lubricant is only required when braking, which results in only low cumulative drag and flow losses over the cycle due to the very small time components. Today, valve technology is fast enough to control this reliably.
What about the thermal load capacity – for example during emergency braking on the highway?
Prof. Dr. Katharina Völkel: Here we see great potential for wet brakes, as well. The thermal load capacity of wet clutches and brakes is high. The heat generated can be reliably dissipated via the housing and by means of oil cooling.
At the same time, there is even a positive additional effect: instead of releasing the resulting braking heat into the environment, it can be used in an integrated thermal management system to heat drive components such as the battery or transmission oil. This can have a positive effect on the efficiency of the entire electric drivetrain, especially in cold temperatures.
Where will the wet vehicle brake be located – surely no longer directly on the wheel?
Prof. Dr. Katharina Völkel: Precisely, due to the oil cooling, the wet vehicle brake can be positioned on the inside of the axle or even - in the form of a monofluid system - in the transmission. This relocation reduces the unsprung masses in the wheel and opens up further potential for aerodynamically optimized wheels.
How far along is the development stage – are we still talking about pure theory here or are there already concrete applications?
Prof. Dr. Katharina Völkel: FZG has been working with wet clutches and brakes for almost 40 years. We believe that this expertise can be reliably transferred to vehicle brakes. A key requirement is to develop appropriate drivetrain topologies and, of course, to obtain approval for wet vehicle brakes for use in road traffic.
What role do simulations and AI-supported tools play in the development of the systems?
Prof. Dr. Katharina Völkel: A very large one. We are able to describe thermal and thermo-mechanical processes as well as the drag torque behavior and oil distribution using simulations. We have also developed an AI-supported drag torque prediction system at the chair, which can be used to make early statements about energy efficiency. Digital condition monitoring will also play an important role – similar to what is already used today in modern gearboxes or differentials.
Are there also challenges that currently speak against the development of wet vehicle brakes?
Prof. Dr. Katharina Völkel: Technically speaking, I don't think so. The biggest hurdles lie more in existing structures. After all, the switch from air-cooled brakes to oil-cooled brakes is a fundamental change in technology with a profound impact on the vehicle topology – from the wheel brake to a system in the axle or directly in the electric drive. Legal requirements could help to drive this change forward in the coming years.
What is your forecast – when could wet braking systems go into series production?
Prof. Dr. Katharina Völkel: From a technical point of view, I assume that we will see the first prototype vehicles with wet brakes on the roads in the next few years – the subsequent readiness for series production depends largely on legislation, among other things.
I am reckoning with a realistic time horizon of eight to ten years – i.e. around 2033 to 2035.
Last question: In your opinion, will the “wet brake” become the new standard – at least for electric vehicles?
Prof. Dr. Katharina Völkel: Yes, I think the concept is clearly a system linked to electric vehicles. Many braking processes in electric vehicles are carried out with recuperation anyway, so the wet brake can take over the other braking processes – efficiently, emission-free and low-maintenance. From a technological point of view, the wet brake is the next logical step.
