02.03.2020 | Author/ Editor: Claus-Peter Köth / Maximiliane Reichhardt
Lightweight construction measures are not an end in themselves, but increase the range of electrified vehicles. What needs to happen for all developers to finally realize this.
The statement that the weight of electrically powered vehicles is of secondary importance persists. Heavier e-cars would recover more energy when braking, which is why lightweight construction is negligible, according to proponents of this theory. But they are wrong: recuperation can only recover part of the energy used in electric vehicles, namely the energy used during acceleration or when driving uphill. The thermal losses, on the other hand, are wasted, as is the additional energy required to actively cool the electric drive, for example. In terms of vehicle weight and lightweight potential, this means that the less mass that needs to be accelerated or brought to a different height potential, the less energy is lost.
“At first glance, it may be true that recuperation compensates for the weight disadvantage to some extent. But not in its entirety and it also depends heavily on the driving profile. Overall, we need to take a holistic view, from development to vehicle production,” says VW manager Ludger Lührmann in an interview.
Lighter equals more efficient
In 2018, the engineering service provider EDAG used real-world tests to prove that lightweight construction in urban electric cars makes sense from an energy perspective: according to the tests, a vehicle that is 100 kilograms lighter saves around four percent of drive energy, which in turn enables the installation of a smaller and lighter battery – without reducing the range. Specifically, in the city driving scenario, a constant speed of 50 km/h and 100 kilograms less weight resulted in energy savings of 2.8 percent.
At a reduced speed of 20 km/h, the percentage of weight-dependent rolling resistance power increased and the energy saving amounted to 3.3 percent. When a gradient was added, the savings in the simulation increased to 4.3 percent, or 3.6 percent when recuperation was taken into account. If the engineers chose an acceleration of two meters per second squared instead of the incline, which corresponds to a normal acceleration process, they achieved an energy saving of 5.4 percent, or 5.0 percent if recuperation is taken into account.
“It is clearly recognizable that acceleration processes have a major influence on the potential of lightweight construction, even when recuperation is taken into account. This is helped by the fact that in urban areas, such as densely built-up cities, many acceleration processes can be assumed. The average savings in real operation are over four percent,” comments Martin Hillebrecht, Head of Innovation and Head of the Lightweight Design Competence Center at EDAG, on the results.
Tests by Fraunhofer and KIT
Scientists at the Fraunhofer Institute for Chemical Technologies (ICT) and the KIT Institute of Automotive Systems Technology (FAST) came to the same conclusion with a model test. The initial scenario was: What impact does a weight reduction of 100 kilograms have on the fuel consumption of an electric car? This was determined for a VW E-Golf in the WLTP and RDE test procedures – once with and once without the 100 kilograms as a payload. “We then fed this data into an overall vehicle simulation, which also took into account the efficiency of various drivetrain components,” explains Prof. Frank Henning, Deputy Director of Fraunhofer ICT.
“The power electronics and the electric motor only have an average efficiency of around 80 percent, the battery 95 percent. There are losses there, for example due to switching, waste heat or friction.” Energy is therefore inevitably lost when operating an electric vehicle. Logically, therefore, not all of the energy can be recovered during braking. In addition, recuperation itself only has an efficiency of around two thirds.
The simulations and calculations carried out by the Karlsruhe researchers revealed that Reducing the vehicle weight by 100 kilograms reduces the energy requirement of the selected E-Golf by around four percent. “The rolling, acceleration and gradient resistance are significantly reduced with a lighter vehicle,” says Henning. Another side aspect: the mass not only affects the efficiency of the vehicle. “The driving dynamics are impaired with a higher weight and tire wear increases,” says Henning.
Other secondary effects
In relation to EDAG’s real-life test, the four percent increase in efficiency means that the battery capacity can be reduced by four percent or 1.2 kWh – without reducing the range. The cost savings and reduction in installation space can be derived directly from this.
The lower weight also has other secondary effects: For example, the lighter battery further increases the efficiency of the vehicle. The charging time is reduced accordingly. Volkswagen is also turning the weight spiral further downwards in the form of secondary effects. “Our greatest leverage lies in the fact that we already have the resulting permissible total weight in mind and limit it at the vehicle design stage,” explains Ludger Lührmann. Measures for the body and battery structure are experiencing a renaissance, which has very different reasons – including limitations within the production facilities. With all these arguments, the legend that ” lightweight construction is expensive per se and therefore not worthwhile” can be debunked.
This is also the conclusion of a study by the Baden-Württemberg State Agency for Lightweight Construction, Leichtbau BW for short. The study examined the largely untapped potential of lightweight construction concepts in the mobility sector. “We show how lightweight construction can be used to reduce costs and be more competitive,” emphasizes Managing Director Dr. Wolfgang Seeliger.
He is convinced that there can be no mobility of tomorrow without lightweight construction. This is because the increasing demand for sensors/electronics for automated driving, the desire for more comfort and, last but not least, the fact that a heavier vehicle also requires increased measures in terms of crash safety, means that there is no way around reducing vehicle weight elsewhere.
Lightweight construction for climate protection
The Federal Ministry for Economic Affairs and Energy is taking the same line. Es sieht den Leichtbau als Schlüsseltechnologie für die Mobilität der Zukunft sowie branchenübergreifend für den Umwelt- und Klimaschutz. With the “Lightweight Construction Technology Transfer Program”, the BMWi wants to give its activities a new boost from the beginning of 2020. Approximately 280 million euros in public funds (as of December 2019) have been earmarked to promote lightweight construction for the period from 2020 to 2023. New materials with the lowest possible CO2 footprint, innovative manufacturing processes such as additive manufacturing and digital development tools are just some of the keywords here.
Elisabeth Winkelmeier-Becker, Parliamentary State Secretary at the BMWi, and Werner Loscheider, Head of the “Construction Industry, Lightweight Construction/New Materials, Resource Efficiency” department, will explain how and what is specifically being funded at the “Automotive Industry Lightweight Construction Summit” on 24/25 March 2020 in Würzburg.
Prime example battery box
Martin Hillebrecht cites the structural integration of the battery boxas a prime example of cost-efficient lightweight construction in electric vehicles. EDAG has developed the “ScaleBat” demonstrator in a joint project with Siemens Industry Software, Baomarc Automotive Solutions and Carl Cloos Schweißtechnik. This is a scalable, multi-variant battery housing that can also be produced economically in large quantities. The concept proves that structural, crash-relevant measures can be reduced in other areas and that any additional costs of lightweight construction can at least be compensated for.
Tesla can ‘t necessarily do it better, but simply dares to do more.
Mark Gevers, Director Business Development at Tecosim
Mark Gevers, Director Business Development at engineering service provider Tecosim, goes one step further: he even describes the intelligent integration of the battery box into the vehicle structure as a very important lever in terms of lightweight construction and therefore the efficiency of electric vehicles. Tesla is ahead of traditional OEMs in this respect, for example because the US company binds the battery box firmly to the vehicle structure at the front and allows it to be torn off at the rear in the event of a crash.
In the worst-case load scenario, the so-called side impact pile, the vehicle structure can deform and thus dissipate energy while the battery box underneath is pushed away. This approach saves many kilograms of material as a safety reserve and makes it possible to make better use of the installation space in the battery box, says Gevers.
Tesla ‘s head start
” Tesla ‘s lead in vehicle structure is particularly painful for the German automotive industry. Tesla is not necessarily better at it, but simply dares to do more,” says Gevers , referring to his benchmark analysis, which he will present at the “Automotive Industry Lightweight Construction Summit” in Würzburg at the end of March.
Overall, he believes that better communication and cooperation is needed among traditional manufacturers across all trades. After all , crash safety is an integral issue. “The ideas are there. But the German automotive industry needs to become bolder again and push its limits more,” concludes Gevers. Martin Hillebrecht adds: “We shouldn ‘t leave the future of mobility and electrification to business economists alone , but rather to engineers again. Technical facts should again play a decisive role in the decision-making process. Otherwise, I am certain that we will miss out on a competitive advantage if we continue to focus on lightweight construction.”
Ludger Lührmann is optimistic. “As engineers, we have to keep demonstrating the positive characteristics of lightweight construction to our customers in our companies. Then we will find many supporters for this key technology in industry, universities, politics and the media.”
Source: automobil-industrie.vogel.de
