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Why ‘Free Piston’ engines could power your next plug-in hybrid

The free-piston linear generator (FKLG) - a new kind of range extender for electrically powered vehicles (Source: DLR)
An engine with no crankshaft, where the pistons float on air? It’s not a dream; it’s a free piston engine and it might be the next major advance in car tech.

When we think of automotive technology of the future, we usually think of alternative fuels, whether its hydrogen or electric power from futuristic batteries. It turns out, the future might actually be a new form of internal combustion: the ‘free piston’ engine.

This technology radically reshapes engines, replacing the crankshaft that normally transfers power from the pistons with a compression chamber and a turbo impeller. Free piston engines hold out the potential of radically improved efficiency. Prototype free piston engines are already 50 to 60 percent more efficient than any engine currently in production – and there is the potential for much more.

How does it work?

A free piston has most of the same components as any internal combustion engine. The key difference is how it captures the power of combustion.

In a standard engine, fuel and air are injected into the compression chamber and ignited. This combustion pushes down on the piston, which is connected to the crankshaft. In cars, the energy of the rotating crankshaft is run through a transmission and out to the drive wheels.

In a free piston engine, combustion happens the same way, but there is no crankshaft. On the face of it, this seems crazy, as the pistons are just left rattling around in their cylinders. But this is actually a stroke – yes, I intended that pun – of genius. By horizontally opposing two cylinders and detonating them simultaneously, the pistons compress the air between them. This air is piped through a compressor that looks a great deal like a turbocharger.

While it is technically possible to drive an axle directly from this turbine, in practice this turbine is put to its best use driving an electrical generator.

The idea for such an engine has been around since 1807, but the challenges of getting the pistons to fire and move simultaneously have prevented designers from taking full advantage of the free piston engine … until now.

The advantages

So, with those challenges, why even bother? Well, it all has to do with efficiency. In a standard internal combustion engine, a great deal of the power is lost as heat and vibration. Modern gasoline engines can convert only 18 to 25 percent of the potential energy of the fuel they burn into usable energy. The most efficient gasoline engines can momentarily achieve as much as 35 percent, but even then only under ideal conditions.

In a standard internal combustion engine, a great deal of the power is lost as heat and vibration.

The lack of a crankshaft in the free piston system means there are none of the constant momentum changes that create the vibration and friction that foul efficiency. These advantages have been apparent for two centuries, but with the precision of modern technology there is another: variable compression ratios.

With a crankshaft the size of the combustion chamber, the length of the piston’s movement is fixed. In modern, experimental free piston engines, the size of the chamber and length of the power stroke can be varied on the fly, changing from revolution to revolution, with the engine management unit tailoring the size of the combustion chamber to the ideal fuel air mixture.

Additionally, this variability should allow free piston engines to run on anything from hydrogen to natural gas. In fact, I wouldn’t be surprised if one of these engines would run on single malt scotch.

The control room at the FKLG Laboratory at the DLR Institute of Vehicle Concepts (Source: DLR)
Battery flat? - The free-piston linear generator acts as a range extender, supplying the electrical power needed by electric vehicles (Source: DLR)

All of these qualities make the free piston engine ideal as an onboard generator, the role in which it may revolutionize powertrain design.

Seriously, when can I get one?

So far, two prototype free piston engines are in the works. The first, unsurprisingly, comes from German scientists/dark magicians at the German Aerospace Center (DLR).

Their prototype, dubbed the “Freikolbenlineargenerator,” which I am not sure is proof that Germans have a sense of humor or not, is still in the early stages of development. Yet despite this, it promises to deliver the sort of performance other powertrain engineers can only dream of.

Toyota’s engine would be capable of generating 15 horsepower from a package eight inches around and two feet long.

The Freikolbenlineargenerator uses cylinders that face each other. This piston orientation allows it to be even more compact than typical free piston engines. This makes it perfect for use as a range extender.

Take for example, the Chevy Volt. It has batteries and is capable of running solely on electricity. But when the batteries go flat, it has a 1.4-liter gas engine that can recharge them and propel the car itself above 65 mph. This works well, but unfortunately even small displacement gas engines are bulky, heavy, and expensive.

The Freikolbenlineargenerator can accomplish the same job, while being small enough to mount beneath the floor of a car, which saves space for other things like kale, let’s say.

But a miracle engine on the workbench of a Teutonic mad scientist is a long way from actual production, right?

Well, Toyota is on board, too. This summer, the massive Japanese automaker announced its own free piston engine or ‘linear generator.’ The Toyota version uses a single cylinder rather than an opposed pair, and is capable of 42 percent thermal efficiency under constant operating conditions. That’s better than any production engine is capable of achieving – even momentarily, under optimal conditions.

Toyota’s engine would be capable of generating 15 horsepower from a package eight inches around and two feet long. Two of these would likely be more than enough to recharge the batteries in a modern range-extended vehicle with huge space and energy savings. The German technology is amazing, but Toyota’s engine could be in the Prius before long.

Conclusion

Pure electric – or possibly hydrogen – is still almost certainly the long-term future energy source for cars. However, neither of those technologies is quite ready. Electric comes close, but is still far too expensive for most American consumers, to say nothing of literally billions of Chinese and Indian consumers who aspire to car ownership.

Free piston engines may be a critical bridge technology that can dramatically improve efficiency of internal combustion. They may also have a longer lifespan than traditional forms of internal combustion, thanks to their ability to burn nearly anything flammable … I’m looking at you, oily rags.

If industry giants like Toyota decide to throw their weight behind the technology, it could be in production cars very soon. Let’s hope.

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