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PSA evaluating Aquarius Engines’ free-piston linear generator for range-extender

PSA Groupe is evaluating a free-piston engine linear-generator under development by Israel-based start-up Aquarius Engines for use as a range-extender in its electric vehicles. According to a report from Reuters, PSA Research and Development Director Gilles Le Borgne said that while the company is evaluating the technology, “Nothing has been decided yet.” According to Aquarius, several prototype vehicles using its free-piston generator will be road-tested by 2017.

A number of groups have recently or are currently investigating the use of free piston engine power generation applications with a focus on automotive, including academic work at West Virginia University (WVU), Sandia National Laboratory (SNL), Chalmers University of Technology (Sweden), and Shanghai Jiaotong University. (Earlier post.)

A team from Toyota Central R&D Labs is developing a prototype 10 kW Free Piston Engine Linear Generator (FPEG) featuring a thin and compact build, high efficiency and high fuel flexibility. Toyota envisions that a pair of such units (20 kW) would enable B/C-segment electric drive vehicles to cruise at 120 km/h (75 mph). (Earlier post.)

In the UK, startup Libertine LPE is developing a free piston linear power generator technology that it says overcomes the key challenges faced by earlier free piston engine schemes of motion control, emissions and power conversion efficiency. (Earlier post.)

Aquarius, founded in 2014, has already filed filed three patents; the company raised $8 million in five days in the first funding round. As outlined in one of its patent documents, the company is proposing a new cycle process—the Aquarius cycle—as well as the new free-piston engine.

The basic engine unit consists of a double-sided cylinder, the cylinder bounded by an engine head at each end; an exhaust unit positioned at each side of the cylinder; and a freely-sliding piston with the cylinder. Two piston rods are aligned with the longitudinal axis, with each piston rod connected at a different side of the piston. Each of the piston rods has a cavity extending to an exhaust opening. I.e., the exhaust openings constitute exhaust valves that are an integral part of the piston rods; each of the piston rods constitutes a sliding valve.

Cross-section of one embodiment of the engine. Click to enlarge.

The continuous flow of air scavenges exhaust gases, cools the cylinder wall and the piston, and enriches the burned gases without depending on the position of the piston.

Stationary sealing rings provide the seals between the piston rod and the engine head and between the piston rod and the exhaust unit.

For a generator application—instead of an application for converting linear to rotational movement—aligner rods are directed parallel to the longitudinal axis and are connected to the piston rod through connecting arms. The aligner rods comprise coil windings; stator coils are positioned around the peripheral envelope and distanced from the longitudinal axis. The engine thus constitutes an electric motor that generates electric power by means of the stator coils that are energized by the linear back and forth movement of the aligner rods.

Top: The piston alignment system. Bottom: The engine as linear generator. Click to enlarge.

The Aquarius cycle comprises the following steps:

  1. Work
  2. Exhaust
  3. Scavenging
  4. Gas Boost
  5. Compression

When one side of the cylinder goes under a given step of the cycle, the opposite side of the cylinder goes also a different step of the cycle. The entire cycle takes place inside the cylinder every time the piston completes its stroke from one end of the cylinder to the other end and simultaneously.




I fail to see this as any more/less efficient than a two stroke engine. Would seem to have the same emission problems as a two stroke. Further, there would seem to be sealing and/or lubricationissues w/ the hollow piston rod. It is an astonishing small package.


The thing about a free-piston engine is the piston motion is not restricted by a crankshaft - leading to the potentially valuable feature of variable compression ratio.

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