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Are Compressed Air engines as clean and green as claimed?

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Tags: Compressed Air

Several companies and inventors around the world are working on compressed air engines to power cars or motorcycles. Every so often their stories make a splash of interest. Compressed air engines have a long history behind them, dating back to the 1840's, but have to overcome inefficiencies before being practical in vehicles. Because the only "exhaust" is air, it's easy to claim these vehicles are clean, but the truth is not as simple as looking at what comes out the tailpipe.

What are compressed air engines?

Let's start first what understanding what a compressed air engine is. They are "a type of motor which does mechanical work by expanding compressed air." That is, compressed air stores potential energy. Releasing the air through a compressed air engine converts that energy into mechanical motion. To be useful in a vehicle the engine has to create rotary motion on a shaft to drive wheels to propel the vehicle.

Compressed air engines in vehicles are either a rotary design (wankel engine) or use pistons. Ultimately both power a driveshaft.

Energy analysis

Just how clean is an engine whose exhaust is air? Can you judge an engine purely by the exhaust it produces?

It takes energy to compress air. We have air compressors in widespread use so to understand this go to your hardware store, buy an air compressor, and bring it home. Or maybe not. In any case what you get is an electric motor driven pump and an air tank, with a hose leading from the air tank to compressed air tools. It takes electricity to compress the air.

The same would be true with compressed air engines on a car, the air storage tank in the vehicle gets its compressed air from an air compressor which in turn is powered by electricity.

The Air Car design by Guy Negre (links below) has an optional mode to include burning petroleum in the compressed air engine to increase engine efficiency by heating the air.

In other words, while the exhaust is air, the electricity to compress that air comes from somewhere. The environmental impact of this depends on the source for your electricity. Is it from a coal fired power plant? Or what?

The notes below indicate that compressed air engines are relatively low efficiency. For example the deal between MDI and Tata fell through because of low efficiency making the car impractical.

History

One widespread use of compressed air engines (a.k.a. pneumatic motors) is in air powered tools. They operate by releasing compressed air through a tool, causing the tool to turn and do stuff like tighten lug nuts enough to make it hard to change your tires when it's 3AM on a snow bound deserted road.

Historically compressed air engines have been used in torpedos, and in railway trains. One especially interesting use of compressed air engines is in coal mine trains where a combustion engine could actually be dangerous. However electric motors have largely replaced compressed air engines in these uses.

Companies

The Energine Corporation is a South Korean company that delivers fully-assembled cars running on a hybrid compressed air and electric engine. The compressed-air engine is used to activate an alternator, which extends the autonomous operating capacity of the car.

EngineAir, an Australian company, is making a rotary engine powered by compressed air, called The Di Pietro motor. The Di Pietro motor concept is based on a rotary piston. Different from existing rotary engines, the Di Pietro motor uses a simple cylindrical rotary piston (shaft driver) which rolls, with little friction, inside the cylindrical stator.

K'Airmobiles vehicles were intended to be commercialized from a project developed in France in 2006-2007 by a small group of researchers. However, the project has not been able to gather the necessary funds. People should note that, meantime, the team has recognized the physical impossibility to use on-board stored compressed air due to its poor energy capacity and the thermal losses resulting from the expansion of the gas.

Motor Development International (MDI) was founded by inventor Guy Negre. In his original engine design, one piston compresses air from the atmosphere to mix with the stored compressed air (which will cool drastically as it expands), and that mixture drives the second piston, providing the actual engine power. In 2001-4 they switched to a design from Armando Regusci's in which the air engine couples the transmission system directly to the wheel, and has variable torque from zero to the maximum, enhancing efficiency. Regusci's patents date from 1990. Recently there was a joint venture between MDI and India's Tata to develop a compressed air car, but this fell through with Tata claiming it was proving difficult to develop due to its low range and problems with low engine temperatures.

References

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