{{Main|Compressed air energy}}

An '''air [[engine]]''' or '''air [[motor]]''' is a device for converting [[potential energy]] from [[Pneumatics|compressed air]] into [[kinetic energy]] to drive other [[machine]]s. As in a [[steam engine]], expansion of externally supplied pressurized gas performs work against one or more [[piston]]s or [[rotor]]s to move [[wheel]]s or other tools.

A compressed air vehicle can offer many of the advantages of a [[battery electric vehicle]] without the need for heavy and potentially toxic batteries, which take hours to recharge instead of the few minutes required to refill the tanks for an air engine. Like an electric vehicle, a compressed air vehicle will usually be pollution free during operation. However the energy required for compression must be sourced, and will usually be derived from electricity, or an internal combustion engine. Depending on the method used to generate the electricity, the energy may contribute significant quantities of [[greenhouse gas]]es and other pollutants, especially if [[fossil fuel]]s are used. Although some air engines may be highly efficient, the system efficiency, including compression of the air, heat rejection, [[Electricity transmission#losses|electricity losses]] and [[Thermal efficiency#Carnot efficiency|electricity generation]], may be less than 25%.

==History==
The air engine and its idea of using air as an energy carrier is not new. Air has been used since the 19th century to power [[mining|mine]] [[locomotives]], and has been the basis of naval [[torpedo]] propulsion since 1866.

Compressed air is still currently used in [[racecar]]s to provide the initial energy needed to start the car's main power plant, the [[internal combustion engine]] (ICE).

The most recent development uses pressurized air as fuel in an engine invented by [[Guy Nègre]], a [[Formula One]] [[France|French]] engineer. In 1991, Guy Nègre started [[Moteur Developpement International]], and invented a dual-energy engine, capable of running on both compressed air and 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 of [[compressed air vehicles]], among them [[Uruguay]]an engineer [[Armando Regusci]], [[Australia]]n [[Angelo Di Pietro (inventor)|Angelo Di Pietro]], Tony Salvino, [[South Korean|South Korea]] Chul-Seung Cho, and more recently, Kernelys' ''K'Airmobiles'' [[compressed air vehicles]]. They too have companies, Regusci's [[RegusciAir]], Di Pietro's [[EngineAir]] [http://www.engineair.com.au/airmotor.htm] and Chul-Seung Cho's [[Energine]], selling their engines. Tony Salvino, however is a high school student who is pursuing a more efficient engine.

Also since 2007, [http://kernelys.free.fr/?lang=en K'Airmobiles] looks at commercialing some urban and [[leisure]] VPPs (Vehicles with Pneumatic Propulsion) and tries to gain partnerships and sponsors. Their goals appears to be individual transport and taxi-bikes as the projects proposed on the site are mainly made of a motor-bike and trikes (with 1 to 3 seats).

In 2008 [[Tata Motors]] plan on producing the low-cost [[€]]3,500 MDI OneCAT that is powered by an air engine.<ref>http://www.popularmechanics.com/automotive/new_cars/4217016.html; http://www.ecogeek.org/content/view/659/ </ref>

== Laws of physics ==

By [[Boyle's law]] it is known that

<blockquote>For a fixed mass of ideal gas at fixed temperature, the product of pressure and volume is a constant.
The formula is P₁V₁=P₂V₂ making pressure and volume indirectly related</blockquote>

Therefore under identical temperature:
* the pressure multiplied by the volume of a gas contained in a tank corresponds to a constant;
* the variation of pressure of the gas is inversely proportional to its volume.

If either the pressure or the volume are altered, the factor T can be modified accordingly. It is what brings to the concepts of [[thermodynamic]], of [[adiabatic]] expansion of [[compressed air]].

More the change of the pressure/volume ratio is fast and brutal, the less time the gas gets to satisfy this change and reflects part of the conversion to its temperature factor.

This is why the methods of use of compressed air in a system explain why, with compressed-air engines, two main trends exist, conceptually rather different:

=== Thermodynamic exploitation ===

In the event of fast expansion of a great quantity of compressed air, corresponding to an important lowering of pressure, the gas cannot physically find its entire original volume; a variation in temperature thus follows, namely meaning an important cooling, while the expansion of useful volume may be limited to approximately 40% of the theoretical volume only. Contrarily, with compression, the voluminal reduction generally involves a rise in temperature resulting once again in a total volume of compressed air lower than its theoretical value.

The technologies exploited by MDI, Energine and Quasiturbine (see hereunder) requiring relatively important flows when exploited to animate powerful engines, must thus obligatorily take account of these thermodynamic constraints.

=== Dynamic exploitation ===

To produce a mechanical push while circumventing this obstacle, or at least while reducing its effects, it is thus necessary to comply with certain rules:
* to allow expansion as slowly as possible, i.e. while working with low flows (but this, of course, results in a reduction in engine power).
* to regulate the abrupt variations of pressure (by the use of pressure reducers and other intermediate decompression rooms),
* to maintain as much as possible a constant gas temperature, which reduces energy losses during compression/expansion by a cooling/heating of the air.

==Engine design==
It uses the expansion of compressed air to drive the pistons of the engine, to propulse the vehicle or generate electricity. 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 [[graphite-reinforced plastic|carbon-fiber]] tank holding air at around 20 [[MPa]] (3,000 psi, 200 [[Bar (unit)|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.

== Disadvantages ==
Having solved most of the high pressure storage and handling problems, the main remaining disadvantages are related to the [[thermodynamics]].

*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 Nègre 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]], or more likely, an electric heater would be required.
*Limited range due to available tank technology. The air engine suffers from similar problems to [[hydrogen vehicles]] in this regard.
*Using energy to compressed air is less efficient than charging a battery with that same energy.
*Less efficient than electric motors.
*While the air engine reduces greenhouse gas emissions from the vehicle, the energy used to compress the air may not come from clean sources.
*Long refill times when compared to conventional automobiles, circa 4 Hours using a home or low-end system; a few minutes at a larger, commercial refilling station.<ref>http://www.msnbc.msn.com/id/6138972/</ref>

== Advantages ==
The principle advantages for an air powered vehicle are:
*Fast recharge time, 3 or 4 minutes for volume transfer at a commercial refill station; hours at home or low-end station.
*Very low [[self-discharge]] rate (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). This means that batteries in operation are much more expensive than [[compressed air storage]], and are more pollutant because a lot more pollutant material needs to be used (typical car batteries are made from sulphuric acids and lead).
*Lower initial cost than battery electric vehicles when [[mass production|mass produced]] (€3,000).
*Expansion of the compressed air reduces its temperature and heat from the passenger compartment may be cooled using a [[heat exchanger]], providing both relief from hot weather [[air conditioning]] and increased efficiency.
*Zero pollutant emissions from the vehicle itself.

==Uses of air engine==
{{See|compressed air vehicle|compressed air storage}}
=== Road vehicles ===

The Nègre CityCAT 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) {{Fact|date=February 2007}}, depending on traffic conditions. The main advantages are: no roadside emissions, low cost technology, engine uses food oil for lubrication (just about 1 [[litre|liter]], changes only every 50,000 km (30,000 miles) ) and integrated air conditioning. Range could be quickly tripled{{Fact|date=February 2007}}, 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{{Fact|date=February 2007}} (using volume transfer), or in a few hours at home or in [[parking lot]]s 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.

=== Cars ===
Air engines can be used in [[Car]]s.

=== Ships ===
They can be used as [[outboard motor]]s.

=== Powered model vehicles ===
{{Main|Powered model car}}

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

==Compressed air as an energy carrier==
===Energy density and efficiency===
{{Main|compressed air storage}}

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 <math>ln{\frac{P_A}{P_B}}</math> 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 or hypermarket store [[parking lot|parking spaces]] could be fitted with pressure [[hose]]s or electric grid, thus not requiring large central stations.

As with electric technology, it must be stressed that compressed air is only an [[energy vector|energy ''vector'']] therefore can only be as clean as its source. However, even as an energy carrier it will still provide adequate and cleaner power even by compare to petroleum based fuels, like gasoline or diesel.

===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 and follows the [[ISO 11439]] standard.
===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 may produce emissions at the point of generation. However such emissions from the power to compress the air initially would be far less than the emissions from gasoline powered cars and trucks already on the streets based on [[petroleum]].

==References==
<references/>


==See also==
{{Portalpar|Sustainable development|Sustainable development.svg}}

* [[Alternative fuel]]
* [[Alternative propulsion]]
* [[Battery electric vehicle]]
* [[Torpedo#Compressed air|Compressed air torpedo]]
* [[Compressed air storage]]
* [[Compressed air vehicle]]
* [[Glider]]
* [[Liquid nitrogen economy]]
* [[Pneumatics]]
* [[Quasiturbine]]
* [[Zero-emissions vehicle]]

== External links ==
{{HSW|air-car|Air-Powered Cars}}
*[http://jerryrig.com/convert/step6.html How to convert hook up your air engine to the chassis (originally for electric engines)]
*[http://science.discovery.com/fansites/discoveriesthisweek/videogallery/videogallery.html?myClip=dtw_aircar Movie of Discovery Channel's Science Channel on the air car]
*[http://timesofindia.indiatimes.com/Air-powered_car_no_more_a_distant_dream/articleshow/1565072.cms Times Of India Article- "Air-fulled car catches Tata's gust of wind"]
*[http://www.airenergycars.com/enhtm/armando.htm RegusciAir Club Company site]
*[http://www.aircaraccess.com/index.htm Pneumatic Options (general resource with history, photos, comprehensive external links)]
*[http://quasiturbine.promci.qc.ca/QTPneuLocoValen030908.html Air engines used to power mine locomotives, and might today be used again to power locomotives - compressed air locomotives-]
* [http://www.theaircar.com/further_applications.html Air engine applications]

[[Category:Compressed air]]
[[Category:Engines]]

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