Free Piston Linear Generators

Free Piston Linear Generators could be the “missing link” our ancestors will trace back to the transition from fossil fuels to clean energy, but what are they?

They’re kind of like internal combustion engines

You’re probably familiar with internal combustion engines to some extent if you’re reading this. Air and fuel mix inside a metal cylinder where they become compressed before a contained explosion pushes a piston outward.  All of this moving around generates mechanical power. Combustion engines usually come in sets of 2-8 cylinders connected to a common rotating crankshaft. We are omitting a great deal of complexity in valves, ignition systems, lubrication, controls, etc. for the sake of simplicity.

Compressed Fuel/Air ⇒ BOOM! ⇒ Spinning Shaft.

free piston linear generators are similar to traditional internal combustion engines

They’re kind of like Electrical Motors

You are also likely somewhat familiar with electric motors and generators.

An electric charge will make a motor spin.  Likewise, you can produce electricity by spinning the shaft of a motor.  That’s called a generator.

There’s an electromagnetic coupling between conductive copper wires and permanent magnets.  They’re arranged in a circular fashion so that the attracting and repelling fields translate to rotating motion.  Again, we’re taking a liberty and skipping a great deal of complexity here.

Free piston linear generators (FPLGs) combine internal combustion engines and electrical generators together in a single oscillating line to go directly from moving piston to electricity generation.

This skips a lot of the complexity typically found in internal combustion engines.  It takes out many losses from heat and friction in interconnected parts.  It’s kind of like connecting a DC generator directly to the output of a small 2-cycle lawnmower engine, but still less moving parts.

What exactly ARE Free Piston Linear Generators?

Cutaway View of a Simple Free Piston Linear Generator

Here’s a basic section of an FPLG.

On the right of the diagram, a piston oscillates within a cylinder while exchanging exhaust gas for fresh air.  It compresses the air with a fuel mixture as it moves to the right. Then finally ignites the mixture for an explosion that forces the piston back to the left.

On the extreme left of the diagram is some sort of spring mechanism.  It slows the piston down before shoving it back to the right. Since mechanical metal springs can wear out, FPLGs usually keep it simple and use a trapped volume of air as a cushion or air spring – essentially works the same.

Located between the piston and air cushion is a set of high strength magnets.

free piston linear engines generate electricity through electromagnetic induction

Copper wire is coiled around that center section just outside the magnets. When the magnets move through the coils, an electrical current is induced in the wire. This happens in both directions, though the current flow is reversed. An alternating current of electricity (AC) is generated when the unit cycles.

How are Free Piston Linear Generators Designed?

The simple setup shown above works, but it causes a tremendous amount of vibration.  The large mass of the piston and magnets is constantly moving from side to side.  Maybe vibration is too subtle a word.  It’s not something small like feeling your phone buzzing in your pocket – this would be more like Godzilla’s iPhone!  

The monster-sized vibration is taken care of by putting two of these systems back-to-back. They can be arranged as two separated units set end-to-end, or combined in one cylinder.  Combined units can share either a common air cushion or combustion zone in the middle.

Cutaway View of a Simple Balanced Free Piston Linear Generator

How are Free Piston Linear Generators Useful?

What makes this generator concept so interesting today is its ability to be size and weight efficient range-extenders for electric vehicles. The electric vehicle charging infrastructure is not fully established and battery technology hasn’t reached the point where instantaneous charging is possible.  There’s still a need for hybrids in the current transitional era. Not in the sense of a full hybrid powertrain as early vehicles were designed, but a way to recharge batteries from fuel on the go. As we move further toward all-electric vehicles, FPLGs have a lot of potential.  They’re mechanically simple and highly efficient.  They can also be packed away within the chassis somewhere since they’re so compact.  FPLGs are not just meant to go under-the-hood.

low battery warning on electric vehicle
electric vehicle charging while parked

They are also more easily adapted to different fuels.

gasoline • diesel • biodiesel • natural gas • methane • ethanol • hydrogen

Since the compression ratio and valve timing is all independently controlled electronically, the computer can instantaneously adapt between fuels. Displacement volume can vary since there isn’t a crankshaft that sets a fixed top dead center or stroke length. Intake and exhaust valves are controlled by solenoids rather than a timing chain and cam shaft. Combustion factors can be altered on the fly for a wide range of fuels, temperatures, and load conditions.

free piston linear generators can be compactly integrated into an electric car chassis as a range extender

Where can I get one?

Designs for FPLGs are just on the cusp of starting commercial use now. Development work has accelerated over the last decade or so. Research institutions and industry leaders like Aquarius Engines (Israel – recently acquired by Hyundai), Mainspring Energy (USA), and Toyota R&D Labs (Japan) seem to be heading the pack in development and regularly come out with interesting advances.

Current development efforts seem to be most challenged by controlling valve timing and increasing compression ratios as well as dealing with vibration and lubrication needs. Some proposed commercial units offer 24kW of electrical power and take up just 8” x 8” x 20” of space. That’s something about the size of two toasters putting out enough electrical power for 1-2 homes!

Watch for these “range extender” features to be offered on new electric vehicles in the coming years. 

Chances are they’re using free piston linear generators and you can be the one to explain them to the sales reps at the car lot.

CDN Inc. is a product design and engineering firm that can adapt easily to your project needs; engineering, industrial design, prototyping & manufacturing.

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