Shorter braking distance with new research
Researchers at KTH have shown that the braking distance of an electrified vehicle can be shortened considerably by raising and lowering the chassis while braking. The research results were presented recently in a doctoral thesis by Johannes Edrén. The discovery represents a leap forward for the safety of future cars.
Johannes Edrén, PhD in vehicle dynamics, has had safety and energy efficiency as a basis for his research. He and his colleagues have made the discovery that the braking distance can be shortened considerably even at high speed, if the tire friction is increased by using a height-adjustable chassis. For a car driving at 100 km/h, the braking distance can be reduced by approximately one meter, shows Johannes Edrén in his doctoral thesis.
– When talking about shortening of braking distances, the improvements are often made in centimetres, so this really is a great step forward, says Johannes Edrén.
Height-adjustable chassis increases friction
The study was performed on electric cars with over-actuation, a technical solution where the vehicle has a larger number of actuators at each wheel than is needed to control a certain number of degrees of freedom. The technique is used for example for individual wheel motors and individual steering. Johannes Edrén and his colleagues have investigated the possibilities of raising and lowering the chassis when braking.
– During the time before the brakes fully engage, you lower the chassis. When the centre of gravity is lowered, the load on the front axle is reduced. Thus the load is distributed more evenly and the total friction is improved, explains Johannes Edrén. When the brakes are fully applied, the lowering of the chassis stops, which increases the load on the wheels even more so that you can brake extra hard. At the end of the deceleration, we use the car’s ability to raise the chassis again. At this point, the load on the wheels can be increased, so that the braking force is enhanced further.
A technique particularly suitable for electrified cars
Previous attempts to increase the friction force for braking have usually been made with ABS brakes. The potentials of active chassis and numerical optimisation now enabled the researchers to investigate whether it was possible to improve the braking performance further.
– With electrified powertrains you can have more compact actuators, and a greater number, for individual wheel steering and individual propulsion. Of course you can apply the same type of control on a standard vehicle with only a combustion engine as the power source. But the task becomes much more complicated. Over-actuation is a technical solution that is particularly well suited for electrified vehicles, says Johannes Edrén.
Several studies within the project
Within the scope of the research project, Johannes Edrén has also studied how over-actuation with individual propulsion and steering can be used to reduce the cornering resistance of electrified vehicles, and how much you can win in safety with active propulsion and steering. Studies on active camber control have also been included in the project. To investigate the effects of actuator limitations and response times, Johannes Edrén has constructed a scaled-down vehicle prototype.
The research results have recently been published in several scientific articles, as well as in Johannes Edréns doctoral thesi
Johannes Edrén defended his thesis Motion modelling and control strategies of over-actuated vehicles at KTH on 3 December 2014.
Phone: +46 70-4474685
Participants in the project: Johannes Edrén, Annika Stensson Trigell (project leader), Jenny Jerrelind, Lars Drugge, Mats Jonasson
The research project “Generic vehicle motion modeling and control for enhanced driving dynamics and energy management” has been part of the Swedish Hybrid Vehicle Centre.
Text and photo: Emilia Lundgren
Susanne Wilken’s research paves the way for safer large lithium-ion batteries
The expansion of consumer oriented lithium-ion batteries towards large-scale applications like vehicles is well under way. However, problems with safety, cycle life and capacity grow with the size of the batteries. In her doctoral thesis, Susanne Wilken emphasizes the need for a deeper understanding of the shortcomings of the lithium-ion batteries used today.
The electrolytes that are common in lithium-ion batteries for portable devices suffer from chemical and thermal instability, which may cause a number of failures. These could possibly be tolerated for small consumer batteries, but become unacceptable when the batteries are up-scaled for large applications such as vehicles. The overall aim of Susanne Wilken’s research has been to understand more about why the electrolyte fails, and to find ways to mitigate the failures.
Water or heat common causes for failure
The safety of lithium-ion batteries is related to the electrolyte and its interaction with other battery components. Most failures start due to impurities like water or because of thermal conditions such as heat.
– There are two options for dealing with these problems, says Susanne Wilken. Either we stay with the standard electrolytes from the consumer supply chain and find good additives to deal with water or heat issues on the large-scale level, or we completely change the chemistry. In my research I have gone with the first of these options.
Surprising test results for common flame retardant
Susanne Wilken has focused on flame retardants, in particular phosphate based compounds and ionic liquids. A number of new approaches and experimental techniques have been employed in the search for further understanding of the failure mechanisms
Part of the research was done in collaboration with fellow doctoral students from KTH and Uppsala University, all members of the Swedish Hybrid Vehicle Centre (SHC). The researchers for example examined triphenylphosphate (TPP) as a flame retardant additive in high power applications.
– We built a battery with graphite and lithium iron phosphate electrodes and different concentrations of TPP in the carbonate based electrolyte, Susanne Wilken explains. To our surprise, we found that this additive is not at all suitable for high power applications, though it is very common for example in the space industry.
The flame retardant was found to participate in film formation on the electrode and increased the viscosity of the electrolyte, leading to decreased performance. Susanne Wilken concludes that the search for useful additives continues:
– We need to find an additive either less degrading or with higher flame-retarding performance.
Valuable collaboration paints the whole picture
Collaborating with other doctoral students has given the work additional strength, Susanne Wilken says
– The collaboration made it possible for us to paint the whole picture and present it in research papers, not only saying what happens but also why it happens.
The research has been financed by SHC and Susanne Wilken has been an active participant in the centre’s activities.
– The mingling and networking within SHC have been very valuable for my research, Susanne Wilken says. I have had unique opportunities to learn what others are doing and to understand the whole chain from battery to vehicle application.
As her work at Chalmers draws to a close, Susanne Wilken plans to work with industrial research.
Susanne Wilken will defend her thesis “Failure Mechanisms of Lithium-ion Battery Electrolytes: Detection and Mitigation” on 12 December, 10.00 at Lecture Hall PJ, 4th floor, Origo, Chalmers University of Technology, Göteborg
Prof. Patrik Johansson (project leader), Susanne Wilken (PhD student), Dr. Johan Scheers (co-supervisor), the Department of Applied Physics, Chalmers University of Technology
The project “Battery properties: designed, controlled and safer lithium ion cells – Electrolyte additives” has been part of the Swedish Hybrid Vehicle Centre.
Text and photo: Emilia Lundgren
Presentations from “Technology watch of Fuel Cells” conference
The annual conference for the project “Technology watch of fuel cells” was held in Stockholm on 27 November. Over 50 participants from academia and industry came to hear about the recent results of the technology watch. The presentations are now available at Elforsk >>
Good reviews for SHC’s PhD course
The PhD course “System Development of Hybrid Vehicles” that was given by SHC this the autumn for the second time, received very good reviews by the participants. The assessment of the course as a whole gave the whole 4,875 points out of a possible 5 in the course evaluation.
-It is a very good result, says Anders Grauers, Chalmers, who leads the course. There were also good ideas for improvements that can be included next time.
Where did this year’s participants come from?
-They came from KTH, Lund, Chalmers, Volvo Global Trucks Technology and Volvo Construction Equipment.
When will the course be given again?
-It’s not decided yet, but in one and half to two years is likely. Please register interest in attending the course to SHC!
What improvements do you plan for the next time?
I will try to develop a case study of a real vehicle to be discussed during the lectures and another video lecture, about simulation.
The goal of the course “System Development of Hybrid Vehicles” is to provide an understanding of hybrid powertrains from a holistic perspective. The participants work in groups to dimension a driveline for different vehicle types, in order to understand how the dimensioning is influenced by different requirements. The course is aimed at both students and people in the industry working with relevant parts of the driveline design.
Successful PhD network a hub for Swedish electric and hybrid vehicle research
The Doctoral Student Network for Swedish hybrid vehicle research brings together doctoral students from different areas and creates meetings with industry and research institutes. -The network plays an important role in providing future competence and opportunities for collaboration, says project leader Anders Nordelöf.
Read the article on the Swedish page.