19 February 2013
The free-piston linear generator (FKLG) – a new kind of range extender for electrically powered vehicles
DLR researchers in Stuttgart have become the first team in the world to demonstrate the feasibility of the free-piston linear generator, which they accomplished using a test bench developed specifically for this purpose.
DLR (CC-BY 3.0).
The control room at the FKLG Laboratory at the DLR Institute of Vehicle Concepts
A particularly powerful mechanism in combination with a highly dynamic feedback control system controls the individual components of the free-piston linear generator (FKLG) – the internal combustion component, linear generator and gas spring.
Battery flat? – The free-piston linear generator acts as a range extender, supplying the electrical power needed by electric vehicles
Different fuels can be used with the free-piston linear generator, from petrol, diesel and natural gas through to bio-fuels and hydrogen.
Scientists at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) have developed an entirely new kind of drive concept to extend the range of electrically powered vehicles. The free-piston linear generator (Freikolbenlineargenerator; FKLG) is a combustion engine that generates electricity. This electrical power drives the electric car when its battery is flat. In contrast to conventional range extenders, different fuels can be used in the free-piston linear generator.
Researchers at the DLR Institute of Vehicle Concepts in Stuttgart have demonstrated the feasibility of this technology on a test bench specifically developed for this purpose. This makes them the first in the world to succeed in commissioning this kind of energy converter, comprising an internal combustion component, a linear generator and a gas spring. "Innovative solutions like the free-piston linear generator will help to make electrically powered mobility an everyday reality and demonstrate the scientific strengths of Baden-Württemberg as a location," said Rolf Schumacher, the Ministerial Director at the Ministry of Finance and Economics in Baden-Württemberg, at the official inauguration ceremony for this project.
New structural approach with powerful feedback control
Engineers have been aware of the principle of this drive unit for some time. Through the installation of a gas spring, DLR researchers have now succeeded, for the first time, in operating this system in a stable manner. The challenge here was to develop a particularly powerful mechanism with a highly dynamic control unit that regulates the complex interactions between the individual components," said Ulrich Wagner, DLR Director of Energy and Transport, as he described this innovation.
The free-piston linear generator works in a similar manner to a conventional combustion engine. But instead of converting the linear movement of the piston into the rotational movement of the crankshaft, it generates electricity directly. A fuel-air mix is ignited in the combustion chamber. This expands and pushes the piston towards the gas springs. These springs decelerate the piston movement and push it back. The linear generator converts the kinetic energy of the piston into electricity and this in turn powers the electric motor. The control system devised by the DLR engineers is able, for example, to control piston movement accurately to within one tenth of a millimetre. At the same time, it recognises fluctuations in the combustion process and compensates for them.
Optimum operating strategy through variable properties
In contrast to conventional drive technologies, the free-piston linear generator enables the compression ratio, piston speed and cubic capacity to be adjusted flexibly. For this reason, different fuels can be used – from petrol, diesel and natural gas through to ethanol or hydrogen. By virtue of its versatile properties and depending on vehicle speed and driving characteristics, the settings of the DLR range extender can always be adapted to deliver the optimum operating strategy. "We can therefore set the operating point of the engine when driving to ensure that we can drive as efficiently as possible and in the most environment-friendly manner," summarised the Director of the DLR Institute of Vehicle Concepts, Horst E. Friedrich. At the same time, the free-piston linear generator functions with fewer components. For example, certain crankshaft and camshaft components normally essential in a conventional combustion engine can be dispensed with altogether.
Making electric cars more flexible
Exceptionally efficient range extenders such as the free-piston linear generator are more than just emergency power units. The free-piston linear generator makes it possible to equip electric vehicles with a much smaller battery while still deriving optimum benefit from electrically powered mobility. Short distances of up to 50 kilometres, in town for example, can be covered using only electrical power; for longer distances, the range extender takes over. It provides the accustomed peace of mind and autonomy of a combustion engine. As a bridging technology, it makes electric vehicles an attractive option for the general public.
Technology transfer into industry
"With our functional demonstrator, we have shown for the first time that our free-piston linear generator principle can be implemented. In the next step, we need to work with industry to develop this technology and build a prototype," explained Friedrich. To accomplish this, DLR has concluded a technology transfer contract with Universal Motor Corporation GmbH and will provide scientific support during further work. One of the tasks ahead is to optimise the weight and size of the free-piston linear generator in such a way that one or more of the assemblies can be located in the underbody area of a vehicle. In this way, initial estimates suggest that an additional range of about 600 kilometres could be achieved without increasing the weight of the car.
Last modified:20/02/2013 14:21:16
I can think of a variation of this engine if anyone from DLR would like to engage in a friendly interesting chat... Also... How much consideration and work has gone into using a Sterling engine as a range extender..
Dear Paul,thanks for your comment. Prof Friedrich is the person in charge, see above. You may contact him directly via mail form by clicking on his name. Best, Marco
Exellent research, gratulations! This is a great unit to run in my basement. It's working quet and it is powerfull enough to care for all my home energy needs. But many parts are still concept. Hurry up pls.
Congratulations !This is a great research, a bridge from the two parallel world that, in normal opinions, will never cross each other.Ad MAioraAntonioItaly
I wonder how this engine compares with fuel cells in terms of efficiency and reliability.I would like to test this engine in one of our trucks as a range extender, running on hydrogen.www.visionmotorcorp.com
This is an exciting development of opposed cylinder design. I assume that you will need one generator per cylinder? You can certainly minimize the valve train with this. Any thoughts of a direct injected two stroke? It seems that this is optimized for a two stroke cycle- installing central valve would greatly weaken the engine, and increase the complexity.Assuming that it is two stroke, part of the gas shock charge could be used for supercharging, especially useful for that cycle. Congratulations!
The question that has yet to be answered is: what is the overall efficiency of the linear generator? In other words, what is the actual electrical energy generated for the potential fuel energy put in?
Similar concept to the hydrostatic INGOCAR.
Congrats to the whole team. Really interesting.Regards,S.Seetharaman
Congratulations on the great achievement to have a functional and controllable free piston engine. To get the efficiency of the linear generators to a acceptable level may prove to be a very difficult job but if you succeed, you might have a potential winner.Good luck in your development and all the best wishes from an other opposed piston engine Range Extender company.Best regards from R. Smallegange (www.peecpower,com)
Very impressive concept. It begs two questions?Can it be scaled up? I'm thinking ~100kW per unit instead of 35kW. For trucks, buses and maybe marine applications?How is it cooled? The animation video doesn't reveal the cooling principle.
It is intresting concept, how about the induction of engine, is it super or turbo chrged?. How much energy does it produce, and what about the cooling of it?.
Thank you for your comments. I am going to provide some more information concerning your questions:- Upscaling: Upscaling to a greater power output is possible in two different ways. Firstly, the swept volume per cylinder can be increased. Secondly, it is possible to use several units. This is the equivalent to today’s multi-cylinder engines – but in contrast to those, some of the cylinders / units of the FPLG can be deactivated completely in case they are not needed currently.- Cooling: Water cooling for combustion section, linear generator and gas spring with different temperature levels for the 3 subsystems.- Gas exchanged: Indeed, it is necessary to turbo- or supercharge the engine making sure the scavenging process works satisfactorily.- Power output: Our current function demonstrator generates up to 12 kW from a single piston module operated at a frequency of 20 Hz. For a production version the frequency will be increased and a layout with two pistons will be used. We expect an electrical power output between 20 and 35 kW per module to be most beneficial.