ᕕ( ᐛ )ᕗ Herman's blog

Who killed the air-powered vehicle?

Compressed air is a sustainable alternative to batteries and fossil fuels. Despite this, it has been disappearing from the public eye.

I first became interested in compressed air as a fuel source during a visit to The Hartebeesthoek Radio Astronomy Observatory (HartRAO) during a cub-scout camp many years ago. We made rockets out of Coca-Cola bottles pumped up with a bicycle pump; makeshift fins held on with cello-tape. After a brief pumping the pent-up energy overcame the nozzle’s friction, the bottle-rocket would detach from the pump, and would blast off into space.

Alternative forms of energy storage have always been fascinating. From pumped hydroelectric energy storage to flywheel energy storage . I recently stumbled upon this article on Low-Tech magazine about compressed air energy storage (CAES) . It rekindled my interest in using compressed air as a way to conserve power, overcoming the duck’s head problem of some sustainable energy production techniques like solar and wind power.

The concept of CAES is fairly simple: compress air while there is energy production (the sun is up) for later use. The flow of energy can then be turned up or down depending on the demand by increasing or decreasing the flow of air.

Larger-scale CAES plants are fairly inefficient since compressing air generates heat (the second law of thermodynamics) and decompressing air cools it down significantly. Due to this, we will not be talking about CAES for large scale power storage, but for powering vehicles. Specifically, the humble bike.

A compressed-air vehicle is fuelled by tanks of pressurised atmospheric gas and propelled by the release and expansion of the gas within a pneumatic motor. These vehicles are comparable to battery-powered vehicles, with energy generation being displaced from the vehicle itself to a power plant where lower source emissions are possible and the net production of pollutants can be reduced.

The energy is stored in a pressurised tank on the vehicle which can be refilled or swapped out (hypothetically) at service stations, or refilled by plugging an onboard compressor into the electricity grid. Compressed air in 300 bar containers has energy density comparable to lead-acid batteries.

There have been few examples of compressed air vehicles in history such as the Gottardbahn pneumatic locomotive and Fireless locomotives .

 Gotthardbahn

Torpedos have also been known to use compressed air as a propulsion mechanism.

What makes pneumatic vehicles interesting?

The sounds of a city are the sounds of traffic. During the first week of lockdown was the first time I had ever heard a truly quiet city. The sounds of vehicles dominate the soundscape of cities. A city without internal combustion engines would be a much more peaceful place to live.

The cost of building pneumatic bikes are potentially much smaller than both internal combustion and electric motorbikes due to the simplicity of their design and the lack of special materials required in their construction. Lest we forget, internal combustion engines work by carefully exploding fuel and directing the explosions in a useful way.

Compressed-air vehicles are comparable in many ways to electric vehicles in their advantages but without the need for environmentally hazardous chemicals such as gasoline or battery acids/metals. Cylinders also do not need to be changed out every few years like electrochemical batteries and have a much longer operational life due to the simplicity of their structure.

The rate of self-discharge is very low when compared to batteries that deplete their charge slowly over time. Therefore, the vehicle may be left unused for longer periods than electric alternatives.

These cylinders can be swapped out at filling stations easily or charged while parked using solar power or done at home by hooking it up to the electrical grid.

Due to social distancing measures taken in big cities such as London and Paris, cycling for transport has spiked again, with new cycle-only paths and roads becoming the norm. Having personal vehicles as opposed to taking public transport will be standard for the foreseeable future while we deal with this pandemic.

There are some downsides

Any conversion of energy between forms results in a loss. For compressed-air cars, energy is lost when electrical energy is converted to compressed air, and when that stored energy is converted into kinetic energy to turn the motor.

When air expands it cools dramatically. This energy loss could be used for something, but for now let’s just assume that it is lost to the atmosphere on an air-powered motorbike. Similarly, tanks get very hot when filled rapidly. SCUBA tanks can be immersed in water to cool them while being filled, but this also results in energy loss.

Refuelling a compressed air container at home may take as long as 4 hours, while refilling on specialised equipment may fill the tank in 3 minutes while dealing with the excess heat in some (hopefully productive) way.

The final issue with CAES for propulsion is that while batteries can somewhat maintain their voltage throughout their discharge, the pressure of compressed air tanks falls as the air is released, effectively providing less propulsion over time.

Experimental cars and bikes

For this article, we will be focussing specifically on bikes, as there are a few problems with air-powered cars as the power necessary to propel a car is vastly greater than the amount necessary to propel a bike. While the majority of the energy required for driving a car is used in moving the car itself, the inverse is true for a bike where an air propelled bike would likely weigh half of the weight of the rider.

There have been a whole host of functional prototypes for bikes running on nothing but compressed air. The Green Speed Air Powered Motorcycle using the Angelo Di Pietro compressed air engine made some noise in 2009.

O2 Pursuit

In 2011 the O2 Pursuit Prototype by Dean Benstead boasted a range of 100km per tank and top speeds of 140km/h, using a modified Yamaha WR250R and also using EngineAir’s Angelo Di Pietro motor. It even won a James Dyson award

There have been multiple successful engineering projects. Some built by engineering students, some by hippies or handymen at home. Cory Little

Even Peugeot, Suzuki, Toyota, and Tata had some form of air-powered vehicles in the works.

Then things go silent

My research on pneumatic vehicles took me on nostalgia-inducing trips through the “old internet” of personal blogs, unmaintained news sites, and enthusiast journals. There were a LOT of broken links as web pages were removed over the years, domains expired, and server fans stopped spinning.

Around 2012 talk about pneumatic and air-powered vehicles goes quiet. The plans of releasing some promising pneumatic motorbikes to the public seems to completely disappear. Toyota, Tata, Peugeot and Suzuki (as far as I can tell) abandon their plans to release air powered vehicles. 1 2

The technology is sound, and if prototypes built by engineering students can work well enough, imagine what could happen if a well-funded company developed an air-powered motorbike. The big issue is still energy loss during compression and decompression, but with green energy, that becomes moot.

Or maybe I just want an air powered motorbike.

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