3 of the World's Most Unusual Engines

From pistonless engines to bacteria-driven cartwheels, humanity has some interesting ideas in motion.
Christopher McFadden

Getting from one place to another fast and with minimal effort is a feat exclusive to humans. Generations of automotive innovation and the human desire to do things faster and more efficiently have led to the transportation technologies which humans continue to rely upon.

There are thousands of different types of vehicles designed to take someone or something from point A to point B, and they all rely on a critical component, an engine. As new technologies emerge and the will to move away from fossil fuels grow stronger, engineers continue to adapt by modifying and redesigning engines.


unusual engines engine
Source: Darryl Braaten/Flickr

What are some of the most unusual engines ever built?

And so, without further ado, here are some of the most unusual engines on the planet. This list is far from exhaustive and is in no particular order.

1. The free-piston engine range extender technology is an interesting take on the engine

unusual engines free piston
A free-piston engine. Source: DLR

In principle, electric cars are fantastic. However, it is difficult to manufacture battery stores large enough to support transportation over long distances. Many electric vehicles are hybrids, which allows them to extend their range by using a combustion engine to add extra power and to charge the battery.

The term "range extender" is typically used to describe gasoline-fueled generators that are used to charge an electric vehicle’s battery pack, but aren’t used to directly power the wheels. This type of range extender is used in 'series' or 'inline' hybrids like the Chevy Volt. These differ from 'parallel' hybrids like the Toyota Prius, where the wheels can be driven by either the electric motor or the internal combustion engine, depending on conditions. 

Now, engineers are looking at ways to disconnect the engine from the driveshaft entirely. Instead of the power being directed to a generator, the piston engine will be the generator itself.

In one design, the crank in between the pistons would hold a strong magnet, which would pass through a series of coils, creating an electric current. The electricity would then be used to charge the batteries.

Ideally, this engine will reduce the number of batteries required. Fewer batteries will need to be recycled making it an economical and environmentally sound solution.

Unfortunately, the design of the engine produces a lot of vibrations as the pistons oscillate from side to side. While the principle of the engine remains sound, designers will need to significantly reduce the vibrations before these engines can be integrated into real motor vehicles.

Another design, the free-piston linear generator, uses an internal combustion chamber, a linear generator, and a gas spring. The internal combustion engine drives the pistons. However, rather than converting the linear movement of the piston into the rotational movement of the crankshaft like in a conventional engine, the device converts the piston’s kinetic energy directly into electricity which is used to charge the batteries.

2. The SABRE Skylon is another unusual engine design

unusual engines SABRE
Source: MDx media/YouTube

The SABRE, or Synergetic Air-Breathing Rocket Engine, which is currently in development, is intended for use in propelling both high-speed aircraft and spacecraft.

Conventional rockets are propelled by a fuel (liquid hydrogen, kerosene, or methane) and an oxidizer (liquid oxygen). When the fuel and oxidizer combust, mass is projected out of the back of the rocket, creating thrust. However, this type of engine limits the size of the payload, because carrying more mass translates to a higher fuel requirement to lift the craft off the ground.  

The SABRE, in contrast, is a hybrid air-breathing rocket engine capable of reaching Mach 5. While in the atmosphere, it would take oxygen from the atmosphere, and switch to onboard oxygen only after leaving the atmosphere. This would give it a lower mass to payload ratio than a conventional rocket.

The reduction in mass could be used to add features such as wings and thermal-protection systems which would make the craft reusable.

One key component of SABRE is the precooler. To create thrust from an air-breathing engine, you have to increase the speed of the air that passes through it. However, once you reach high speeds, the air needs to be slowed down to allow the internal machinery to operate.

When the fast-moving air slows down, it also heats up as kinetic energy is converted into thermal energy. So, at speeds of about Mach 5, the air entering the engine is about 1832 degrees Fahrenheit (1000 degrees Celsius), much too hot for the jet to function.

The SABRE uses a precooler to slow the air down. Fast-moving air is first slowed down by the intake by a series of shockwaves created by the geometry of the engine’s components. The air heats up as it slows, but this hot air then passes into the precooler, where heat is transferred from the air stream to an internal fluid such as cryogenic helium. 

Once in the engine, the air goes through a cycle involving compression, combustion, regeneration, and finally, expansion as it passes through the engine’s nozzle — creating a propulsive force. The thermal energy which was transferred to the precooler is used to drive the engine.

Once it reaches a fast enough velocity, a conventional LH2/LOX rocket engine provides the final push into space.

3. The evaporation engine is a very unusual engine indeed

unusual engines evaporation engine
An evaporation engine. Source: ExtremeBio/YouTube

A new, bacteria-powered evaporation engine is unlike any other engine. Developed by Columbia University, the engine does not use conventional fuel, instead, it relies on bacteria.

In nature, many types of bacterial spores remain dormant until they come into contact with humidity. Researchers placed these spores onto plastic film to take advantage of the bacteria's ability to absorb and re-release water very quickly. The spore-coated films bend and straighten in response to changes in relative humidity.

The 'engine' is made by securing hundreds of the bacteria-coated tapes onto the side of a wheel. Using a shutter mechanism to create oscillations, a difference in humidity is created. As the film absorbs humidity, the bacteria to expand, closing the shutters. The lack of humidity then causes the bacteria to shrink, opening the shutters, leading the bacteria to absorb humidity and expand, etc.

The technique is fascinating. However, while researchers were able to power a vehicle the size of a toy car, the engine will not be powering a human-carrying vehicle any time soon. Nevertheless, in theory, the spores could produce more energy per square foot than wind farms at a much lower cost.

unusual engines engine
Source: Peter Miller/Flickr

And that, as they say, is a wrap. 

As times change, so do the technologies which will carry humans from one point to another. Currently, there are hundreds of engines undergoing development.

While some may seem incredibly bizarre, it is through outlandish innovation that humanity will 'propel' itself into the future.

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