Free-piston IC generator developed for range-extender hybrids

  • 03-Jul-2013 10:36 EDT

The DLR's free-piston engine on test. The linear generator EV range-extender concept could be efficient, quiet, and multifuel-capable.

Despite the best efforts of industry, the market for electric vehicles is still hampered by inadequate battery range and a dearth of recharging services. To get around "range anxiety" and infrastructure issues, automakers resorted to adding a gasoline engine range-assist, first building parallel hybrids such as the Toyota Prius, which drive the wheels with both power plants. Increasingly, however, series hybrids such as the Chevrolet Volt, Cadillac ELR, and BMW i3 are hitting the road. These range extender-type hybrids power an electric tractor motor solely with a gasoline-fueled generator set.

A research team working at the German Aerospace Center (DLR, or Deutsches Zentrum für Luft- und Raumfahrt) in Stuttgart has developed a new kind of EV range-extender concept that could be efficient, quiet, and compatible with many different fuels. If successful, the device could also be used as an auxiliary power unit in aircraft or in decentralized combined heat and power plants (CHP).

The DLR’s 8-kW (10.7-hp) prototype is a free-piston linear (or "crankless") generator. It comprises three main components: an internal combustion chamber, a linear alternator, and an adjustable gas spring. After a fuel-air mixture is ignited in the combustion chamber, the expansion pushes a piston-driven magnet through linear-generator coils inducing electricity. Then a spring decelerates and rebounds the piston for the compression stroke, which generates more electricity, explained Florian Kock, Research Associate at the DLR’s Institute of Vehicle Concepts. (

Shuttlecock generator

Free-piston engines have variable compression ratios, which enable optimized operations under all conditions as well as multifuel capability—from gasoline, diesel, and natural gas through to ethanol or hydrogen, he said. The DLR engine's simple design has few moving parts, providing for a compact unit with low maintenance costs and reduced frictional losses.

In particular, the free-piston design avoids the extra complexity of the traditional rotating crankshaft, which Kock said provides additional operational flexibility such as the ability to adjust stroke length and compression ratio.

“Engineers have been aware of the principle of this drive unit for some time,” noted Ulrich Wagner, DLR Director of Energy and Transport. The concept of combining the free-piston engine and the linear generator, which dates back to the late 1950s, has been revived by several research groups in recent years for use in hybrids.

“Through the installation of a gas spring,” he said, “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.”

Although linear motors aren't new, “the technology has never been a great success,” Kock explained. One reason for this, he said, “is that control is very difficult because one combustion event is not as good as the next one; there are always fluctuations in the process.”

When a combustion event does not burn to full completion, it cannot provide sufficient compression for the next burn. This cycle-to-cycle variation propagates through the system and is difficult to maintain in a steady and stable condition.

Single chamber plus smarts

Work on the linear generator concept at DLR started in 2002 with simulation studies. “Our team brought two new things to the problem,” Kock said, “much more computing power—10,000 calculations per second—and a different design. Instead of two combustion chambers, we use one, which simplifies things.”

The explosion of a fuel-air mix in a center combustion chamber drives the pistons on either side toward gas springs, which decelerate the pistons and push them back. The opposing synchronized pistons minimize noise and vibration.

In the single-piston lab bench unit, adaptive control is provided by a powerful software algorithm that does an energy calculation for each combustion cycle—from beginning of cycle to the end of combustion—in real time. “Its goal is to extract the same amount of energy from every cycle,” he noted.

Sensors monitor piston travel, pressure, and temperature, which enables the controller to control and balance the chemical energy stored in the fuel, the kinetic energy of the piston, the induction energy of the coils, and the potential energy in the springs to provide steady operation. Managing the three interdependent subsystems provides the biggest challenge.

Control flexibility

The system created can accurately control the piston movement to within a tenth of a millimeter, while recognizing fluctuations in the combustion process and compensating for them. It also allows flexible adjustment of the compression ratio, piston speed, and cubic capacity. The engine is thought to be highly suitable for HCCI (homogeneous charge combustion ignition) operation as well.

“We adjust the physics of the combustion event for dynamic downsizing,” Kock said. To operate at part load, for instance, the controls alter the compression ratio or shorten the piston stroke. “If you need to reduce power rapidly, you increase the force in the linear generator,” he noted. “If you want a slower adjustment, you increase the pressure in the gas spring.”

The current demonstrator is rated at 8 kW, but “we’re now working to get it to 12 kW [16 hp],” he said. It now operates at about 20 Hz. The DLR group believes that a 35-kW (47-hp) unit would make sense for a car. The device would run at a frequency of 40 to 50 Hz. Its flat low-profile design could allow one or more units to be installed in the underbody area of a vehicle, potentially providing an additional range of as much as 600 km (373 mi) without raising the vehicle weight. One 26-kW (35 hp) design, for example, has nominal dimensions of 97 x 23 x 14 cm (38 x 9 x 5.5 in).

Remaining hurdles

Kock said that a list of unresolved issues remains to be addressed.

“First, it’s too large and needs redesign and downsizing,” he stated. In addition, with the single chamber, “there is no space for a cylinder head with valves so we’re using scavenger ports—small holes in cylinder like a cheap two-stroke engine which have poor emissions.”

The team, he said, is working on alternative delivery systems and zero lubrication operation: “Maybe a carbon piston and piston rings, or some other approach.” They expect that the additional degrees of freedom with compression ratio and stroke will allow them to compensate for the scavenging process.

Besides developing the general design of the system, the team aims to increase the operating frequency from 20 to 50 Hz on the production version, because power density goes up with frequency. "But gas exchange is easy at 20 Hz and a lot tougher at 50 Hz. Plus, we’ll need the controls to be able to act faster," Kock observed. 

The DLR researchers are partnering with the lab’s first commercial spin-off firm, Universal Motor Corp. GmbH, to develop the technology and build a prototype vehicle unit. To accomplish this, DLR has concluded a technology transfer contract with Universal Motor and will provide scientific support during further work. (

They hope that the free-piston linear range extender will be a bridging technology for car buyers who want to buy an EV but have been hesitant to plug in all the way.

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