Air engine

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The air engine is an emission-free piston engine using compressed air. The engines are similar to steam engines as they use the expansion of externally supplied pressurized gas to perform work against a piston.

The most recent development uses pressurized air as fuel in an engine invented by Guy Nègre, a French engineer. A similar concept is currently being developed by the Uruguayan engineer Armando Regusci, an Australian Angelo Di Pietro and a South Korea Chul-Seung Cho. Despite interest in the technology, no company has yet put a vehicle using this technology into mass production. A successful vehicle would offer many of the advantages of a battery electric vehicle with the additional ability to quickly restore the stored energy - in a few minutes rather than the hours required to recharge batteries.

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[edit] History

The air engine, and its concept to use air as an energy carrier is not new. Historically it has been used since the 19th century to power mine locomotives, and has been the basis for the propulsion of naval torpedos since 1866. After this, it was used (and is still being used) in car racing to give the first power to start the car's main power plant, the internal combustion engine (ICE).

In 1991 the inventor Guy Nègre started up MDI and invented a dual-energy engine running on both compressed air as on regular fuel. From this moment on he managed to create a compressed air only-engine, and improved his design to make it more powerful. In the 15 years he's been working on this engine, considerable progress has been made: the engine is now claimed to be competitive with modern ICEs. It is probably still not as powerful as an ICE (although depending on which model of air engine vs model ICE). Proponents claim that this is of little importance since the car can simply be made lighter, or the tanks be put on a higher pressure, pushing the engine to above a comparable ICE-engine.

Other people that have been working on the idea, among them Armando Regusci, Angelo Di Pietro and Chul-Seung Cho. They too have companies, Rugusci's RegusciAir, Di Pietro's EngineAir and Chul-Seung Cho's Energine, selling their engines.

[edit] Engine design

It uses the expansion of compressed air to drive the pistons in a modified piston engine. Efficiency of operation is gained through the use of environmental heat at normal temperature to warm the otherwise cold expanded air from the storage tank. This non-adiabatic expansion has the potential to greatly increase the efficiency of the machine. The only exhaust gas is cold air (−15 °C), which may also be used for air conditioning in a car. The source for air is a pressurized carbon-fiber tank holding air at around 20 MPa (3,000 psi, 200 bar). Air is delivered to the engine via a rather conventional injection system. Unique crank design within the engine increases the time during which the air charge is warmed from ambient sources and a two stage process allows improved heat transfer rates.

Armando Regusci's version of the air engine has several advantages over the original Nègre design. In the original Nègre air engine, one piston compresses air from the atmosphere, holding it on a small container that feeds the high pressure air tanks with a small amount of air. Then that portion of the air is sent to the second piston where it works. During compression for heating it up, there is a loss of energy due to the fact that it cannot receive energy from the atmosphere as the atmosphere is less warm than it. Also, it has to expand as it has the crank. Nègre's engine works with constant torque, and the only way to change the torque to the wheels is to use a pulley transmission of constant variation, losing some efficiency. In Regusci's version, the transmission system is direct to the wheel, and has variable torque from zero to the maximum, enhancing efficiency. When vehicle is stopped, Guy Nègre's engine has to be on and working, losing energy, while the Regusci's version need not.

In July 2004, Guy Nègre abandoned his original design, and showed later a new design that he stated to have invented in year 2001, but his new design is identical to the Armando Regusci's air engine which was patented back in 1989 (Uruguay) with the patent number 22976, and back in 1990 (Argentina). In those same patents, it is mentioned the use of electrical motors to compress air in the tanks.

Besides the compressed air engine designs by Regusci, Nègre, and EngineAir, the Quasiturbine is also capable of running on compressed air, and is thus also a compressed air engine.

[edit] Advantages

The principle advantages for an air powered vehicle are:

  • Fast recharge time
  • Very low self-discharge (most batteries will deplete their charge without external load at a rate determined by the chemistry, design, and size, while compressed gas storage will have an extremely low leakage rate)
  • Long storage lifetime device (electric vehicle batteries have a limited useful number of cycles, and sometimes a limited calendar lifetime, irrespective of use).
  • Potentially lower initial cost than battery electric vehicles when mass produced.
  • Expansion of the compressed air reduces its temperature and heat from the passenger compartment may be cooled using a heat exchanger, providing both hot weather air conditioning and increased efficiency.

[edit] Disadvantages

Having solved most of the high pressure storage and handling problems, the main remaining disadvantages are related to the thermodynamics of air compression and expansion, the consequent temperature changes, and the resultant heat transfers.

  • At the supply station, compressing the air heats it, and if then directly transferred in a heated state to the vehicle storage tanks will then cool and reduce the pressure. If cooled before transfer, the energy in this heat will be lost unless sophisticated low grade heat utilization is employed (see cogeneration).
  • Within the vehicle, expansion and consequent pressure reduction in the throttle or engine chills the air, reducing its effective pressure. Addition of ambient heat will increase this pressure and this addition leads to a more complex propulsion system. While an attempt was made in the Nigger system to warm the air in a long portion of the stroke at top dead center, it appears that this scheme has been abandoned due to inherent imbalances causing unacceptable levels of vibration.
  • Passenger compartment heating is more difficult since the propulsion system does not provide a source of waste heat. Some form of heat pump device would probably be required.

[edit] Uses of air engine

The Nègre engine is used to power an urban car with room for five passengers and a projected range of about 160 to 320 km (100 to 200 miles)[citation needed], depending on traffic conditions. The main advantages are: no roadside emissions, low cost technology, engine uses food oil for lubrication (just about 1 liter, changes only every 50,000 km (30,000 miles) ) and integrated air conditioning. Range could be quickly tripled[citation needed], since there are already carbon fiber tanks which have passed safety standards holding gas at 70 MPa (10,000 lbf/in²) .

The tanks may be refilled in about three minutes at a service station[citation needed], or in a few hours at home plugging the car into the electric grid via an on-board compressor. The cost of driving such car is projected around 0.75€ per 100 km, with a complete refill at the "tank-station" at about US$3.

Small single cylinder engines are also incorporated into small toy flying airplane models.

[edit] Compressed air as an energy carrier

[edit] Energy density and efficiency

Ideal air compression and expansion is described by the isothermal process. Compressing air however heats it up and expanding it cools it down. Therefore practical air engines require heat exchangers in order to avoid excessively high or low temperatures and even so don't reach ideal constant temperature conditions. Nevertheless it is useful to describe the maximum energy storable using the isothermal case, which works out to about 110 ln{\frac{P_A}{P_B}} kJ/Nm3 at 24° Celsius. A Nm3 is a cubic meter of gas volume at normal, i.e. atmospheric pressure conditions. Thus if 1.0 m3 of ambient air is very slowly compressed into a 5-liter bottle at 200 bar, the potential energy stored is 583 kJ (or 0.16 kWh). A highly efficient air motor could transfer this into kinetic energy if it runs very slowly and manages to expand the air from its initial 200 bar pressure completely down to 1 bar (bottle completely "empty" at ambient pressure). This is practically impossible and if the bottle is emptied down to 10 bar, the energy extractable is about 330 kJ. The efficiency of isothermal compressed gas storage is theoretically 100% but in practice the process is not isothermal and the two engines (compressor and motor) have various losses.

A standard 200 bar 5 liter steel bottle has a mass of 7.5 kg, a superior one 5 kg. Bottles reinforced with or built from high-tensile fibers can be below 2 kg in this size, always regarding legal safety codes. Thus we get energy densities from roughly 75 up to 300 kJ/kg. Ordinary steel bottles thus have about the same energy density as lead-acid batteries and advanced fiber-reinforced bottles that of superior electrochemical storage batteries. However, modern batteries provide almost their full energy at a nearly constant voltage, whereas the pressure of compressed air storage varies greatly. It is technologically difficult for air engines to maintain high efficiency and sufficient power values over such pressure swings.

The advantage of compressed air over electric storage is the longer lifetime of pressure vessels compared to batteries and the lower toxicity of the materials used. However for this to count, air engines must become as light, efficient and cheap as available electric motors. Compressed air tanks can also be charged more safely than those with inflammable fuels. For example, grocery store parking spaces could be fitted with pressure hoses, thus not requiring large central stations.

As with electric technology, it must be stressed that compressed air is only an energy vector therefore can only be as clean as its source.

[edit] Safety

As with most technologies, compressed air has safety concerns, mainly the catastrophic rupture of the tank. Rigid safety codes make this a rare occurrence at the cost of weight: codes may require the working pressure to less than 40% of the rupture pressure for steel bottles and less than 20% for fiber-wound bottles. High pressure bottles are fairly strong so that they stay unruptured in crashes, but if a failure does occur, a violent explosion could result. Therefore it is important how the bottles fracture and how well the fragments can be contained.

[edit] Technical boundaries

For practical application to transportation, several technical problems must be first addressed:

  • As the pressurised air expands, it is cooled, which limits the efficiency (see combined gas law). This cooling reduces the amount of energy that can be recovered by expansion, so practical engines apply ambient heat to increase the expansion available.
  • Conversely, the compression of the air by pumps (to pressurize the tanks) will heat the air. If this heat is not recovered it represents a further loss of energy and so reduces efficiency.
  • Storage of air at high pressure requires strong containers, which if not made of exotic materials will be heavy, reducing vehicle efficiency, while exotic materials (such as carbon fiber composites) tend to be expensive.
  • Energy recovery in a vehicle during braking by compressing air also generates heat, which must be conserved for efficiency.
  • It should be noted that the air engine is not necessarily emission-free, since the power to compress the air initially can involves emission at the point of generation.

[edit] See also

[edit] External links

Energy Development and Use   Edit
2000 Watt society | Air engine | Energy development | Environmental concerns with electricity generation | Flywheel energy storage | Future energy development | Hydrogen storage | Hydrogen station | Hydrogen economy | Hubbert peak theory | Hypermodernity | Inertial fusion power plant | Liquid nitrogen economy | Low-carbon economy | Renewable energy | Technological singularity | World energy resources and consumption
Sustainability and Development of Energy   Edit
Conversion | Development and Use | Sustainable Energy | Conservation | Transportation
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