Compressed air car
A compressed air car is a compressed air vehicle that uses a motor powered by compressed air. The car can be powered solely by air, or combined (as in a hybrid electric vehicle) with gasoline, diesel, ethanol, or an electric plant with regenerative braking.
Compressed air cars are powered by motors driven by compressed air, which is stored in a tank at high pressure such as 31 MPa (4500 psi or 310 bar). Rather than driving engine pistons with an ignited fuel-air mixture, compressed air cars use the expansion of compressed air, in a similar manner to the expansion of steam in a steam engine.
In contrast to hydrogen's issues of damage and danger involved in high-impact crashes, air, on its own, is non-flammable, it was reported on Seven Network's Beyond Tomorrow that on its own carbon-fiber is brittle and can split under sufficient stress, but creates no shrapnel when it does so. Carbon-fiber tanks safely hold air at a pressure somewhere around 31 MPa, making them comparable to steel tanks. The cars are designed to be filled up at a high-pressure pump.
In compressed air vehicles tank designs tend to be isothermal; a heat exchanger of some kind is used to maintain the temperature (and pressure) of the tank as the air is extracted.
Compressed air has relatively low energy density. Air at 30 MPa (4,500 psi) contains about 50 Wh of energy per liter (and normally weighs 372 g per liter). For comparison, a lead–acid battery contains 60-75 Wh/l. A lithium-ion battery contains about 250-620 Wh/l. The EPA estimates the energy density of gasoline at 8,890 Wh/l; however, a typical gasoline engine with 18% efficiency can only recover the equivalent of 1694 Wh/l. The energy density of a compressed air system can be more than doubled if the air is heated prior to expansion.
In order to increase energy density, some systems may use gases that can be liquified or solidified. "CO2 offers far greater compressibility than air when it transitions from gaseous to supercritical form."
Compressed air cars could be emission-free at the exhaust. Since a compressed air car's source of energy is usually electricity, its total environmental impact depends on how clean the source of this electricity is. However, most air cars have petrol engines for different tasks. The emission can be compared to half of the amount of carbon dioxide produced by a Toyota Prius (being around 0.34 pounds per mile). Some engines can be fuelled otherwise considering different regions can have very different sources of power, ranging from high-emission power sources such as coal to zero-emission power sources. A given region can also change its electrical power sources over time, thereby improving or worsening total emissions.
However, a 2009 study showed that even with very optimistic assumptions, air storage of energy is less efficient than chemical (battery) storage.
The principal advantages of an air powered engine is
- It uses no gasoline or other bio-carbon based fuel.
- Refueling may be done at home, but filling the tanks to full pressure would require compressors for 250-300 bars, which are not normally available for home standard utilization, considering the danger inherent at these pressure levels. As with gasoline, service stations would have to install the necessary air facilities if such cars became sufficiently popular to warrant it.
- Compressed air engines reduce the cost of vehicle production, because there is no need to build a cooling system, spark plugs, starter motor, or mufflers.
- The rate of self-discharge is very low opposed to batteries that deplete their charge slowly over time. Therefore, the vehicle may be left unused for longer periods of time than electric cars.
- Expansion of the compressed air lowers its temperature; this may be exploited for use as air conditioning.
- Reduction or elimination of hazardous chemicals such as gasoline or battery acids/metals
- Some mechanical configurations may allow energy recovery during braking by compressing and storing air.
- Sweden's Lund University reports that buses could see an improvement in fuel efficiency of up to 60 percent using an air-hybrid system. But this only refers to hybrid air concepts (due to recuperation of energy during braking), not compressed air-only vehicles.
The principal disadvantages are the steps of energy conversion and transmission, because each inherently has loss. For combustion engine cars, the energy is lost when chemical energy in fossil fuels is converted by the engine to mechanical energy. For electric cars, a power plant's electricity (from whatever source) is transmitted to the car's batteries, which then transmits the electricity to the car's motor, which converts it to mechanical energy. For compressed-air cars, the power plant's electricity is transmitted to a compressor, which mechanically compresses the air into the car's tank. The car's engine then converts the compressed air to mechanical energy.
- When air expands in the engine it cools dramatically and must be heated to ambient temperature using a heat exchanger. The heating is necessary in order to obtain a significant fraction of the theoretical energy output. The heat exchanger can be problematic: while it performs a similar task to an intercooler for an internal combustion engine, the temperature difference between the incoming air and the working gas is smaller. In heating the stored air, the device gets very cold and may ice up in cool, moist climates.
- This also leads to the necessity of completely dehydrating the compressed air. If any humidity subsists in the compressed air, the engine will stop due to inner icing. Removing the humidity completely requires additional energy that cannot be reused and is lost. (At 10g of water per m3 air -typical value in the summer- you have to take out 900 g of water in 90 m3; with a vaporization enthalpy of 2.26MJ/kg you will need theoretically minimally 0.6 kWh; technically, with cold drying this figure must be multiplied by 3 - 4. Moreover, dehydrating can only be done with professional compressors, so that a home charging will completely be impossible, or at least not at any reasonable cost.)
- Conversely, when air is compressed to fill the tank, its temperature increases. If the stored air is not cooled while the tank is being filled, then when the air cools off later, its pressure decreases and the available energy decreases.
To mitigate this, the tank may be equipped with an internal heat-exchanger in order to cool the air quickly and efficiently while charging.
Alternatively, a spring may be used to store work from the air as it is inserted in the tank, thus maintaining a low pressure difference between the tank and recharger, which results in a lower temperature raise for the transferred air.
- Refueling the compressed air container using a home or low-end conventional air compressor may take as long as 4 hours, though specialized equipment at service stations may fill the tanks in only 3 minutes. To store 2.5 kWh @300 bar in 300 liter reservoirs (90 m3 of air @ 1 bar), requires about 30 kWh of compressor energy (with a single-stage adiabatic compressor), or approx. 21 kWh with an industrial standard multistage unit. That means a compressor power of 360 kW is needed to fill the reservoirs in 5 minutes from a single stage unit, or 250 kW for a multistage one. However, intercooling and isothermal compression is far more efficient and more practical than adiabatic compression, if sufficiently large heat exchangers are fitted. Efficiencies of up to 65% might perhaps be achieved, (whereas current efficiency for large industrial compressors is max. 50% ) however this is lower than the Coulomb's efficiency with lead acid batteries.
- The overall efficiency of a vehicle using compressed air energy storage, using the above refueling figures, is around 5-7%. For comparison, well to wheel efficiency of a conventional internal-combustion drivetrain is about 14%,
- Early tests have demonstrated the limited storage capacity of the tanks; the only published test of a vehicle running on compressed air alone was limited to a range of 7.22 km.
- A 2005 study demonstrated that cars running on lithium-ion batteries out-perform both compressed air and fuel cell vehicles more than threefold at the same speeds. MDI claimed in 2007 that an air car will be able to travel 140 km in urban driving, and have a range of 80 km with a top speed of 110 km/h (68 mph) on highways, when operating on compressed air alone but as of August 2017 have yet to produce a vehicle that matches this performance.
- A 2009 University of Berkeley Research Letter found that "Even under highly optimistic assumptions the compressed-air car is significantly less efficient than a battery electric vehicle and produces more greenhouse gas emissions than a conventional gas-powered car with a coal intensive power mix." However, they also suggested, "a pneumatic–combustion hybrid is technologically feasible, inexpensive and could eventually compete with hybrid electric vehicles."
- It is often accompanied by a small petrol powered engine that helps it with various tasks such as starting and maintaining working speeds. This engine emits carbon dioxide.
Safety claims for light weight vehicle air tanks in severe collisions have not been verified. North American crash testing has not yet been conducted, and skeptics question the ability of an ultralight vehicle assembled with adhesives to produce acceptable crash safety results. Shiva Vencat, vice president of MDI and CEO of Zero Pollution Motors, claims the vehicle would pass crash testing and meet U.S. safety standards. He insists that the millions of dollars invested in the AirCar would not be in vain. To date, there has never been a lightweight, 100-plus mpg car which passed North American crash testing. Technological advances may soon make this possible, but the AirCar has yet to prove itself, and collision safety questions remain.
The key to achieving an acceptable range with an air car is reducing the power required to drive the car, so far as is practical. This pushes the design towards minimizing weight.
According to a report by the U.S. Government's National Highway Traffic Safety Administration, among 10 different classes of passenger vehicles, "very small cars" have the highest fatality rate per mile driven. For instance, a person driving 12,000 miles per year for 55 years would have a 1% chance of being involved in a fatal accident. This is twice the fatality rate of the safest vehicle class, a "large car". According to the data in this report, the number of fatal crashes per mile is only weakly correlated with the vehicle weight, having a correlation coefficient of just (-0.45). A stronger correlation is seen with the vehicle size within its class; for example, "large" cars, pickups and SUVs, have lower fatality rates than "small" cars, pickups and SUVs. This is the case in 7 of the 10 classes, with the exception of mid-size vehicles, where minivans and mid-size cars are among the safest classes, while mid-size SUVs are the second most fatal after very small cars. Even though heavier vehicles sometimes are statistically safer, it is not necessarily the extra weight that causes them to be safer. The NHTSA report states: "Heavier vehicles have historically done a better job cushioning their occupants in crashes. Their longer hoods and extra space in the occupant compartment provide an opportunity for a more gradual deceleration of the vehicle, and of the occupant within the vehicle... While it is conceivable that light vehicles could be built with similarly long hoods and mild deceleration pulses, it would probably require major changes in materials and design and/or taking weight out of their engines, accessories, etc."
Air cars may use low rolling resistance tires, which typically offer less grip than normal tires. In addition, the weight (and price) of safety systems such as airbags, ABS and ESC may discourage manufacturers from including them.
Developers and manufacturers
Various companies are investing in the research, development and deployment of Compressed air cars. Overoptimistic reports of impending production date back to at least May 1999. For instance, the MDI Air Car made its public debut in South Africa in 2002, and was predicted to be in production "within six months" in January 2004. As of January 2009, the air car never went into production in South Africa. Most of the cars under development also rely on using similar technology to low-energy vehicles in order to increase the range and performance of their cars.[clarification needed]
MDI has proposed a range of vehicles made up of AIRPod, OneFlowAir, CityFlowAir, MiniFlowAir and MultiFlowAir. One of the main innovations of this company is its implementation of its "active chamber", which is a compartment which heats the air (through the use of a fuel) in order to double the energy output. This 'innovation' was first used in torpedoes in 1904.
As of January 2009[update] Tata Motors of India had planned to launch a car with an MDI compressed air engine in 2011. In December 2009 Tata's vice president of engineering systems confirmed that the limited range and low engine temperatures were causing problems.
Tata Motors announced in May 2012 that they have assessed the design passing phase 1, the "proof of the technical concept" towards full production for the Indian market. Tata has moved onto phase 2, "completing detailed development of the compressed air engine into specific vehicle and stationary applications".
In February 2017 Dr. Tim Leverton, president and head at Advanced and Product Engineering at Tata revealed was at a point of "starting industrialisation" with the first vehicles to be available by 2020. Other reports indicate Tata is also looking at reviving plans for a compressed air version of the Tata Nano, which had previously been under consideration as part of their collaboration with MDI.
Engineair is an Australian company which has produced prototypes of a variety of prototype small vehicles using an innovative rotary air engine designed by Angelo Di Pietro. The company is seeking commercial partners to utilise its engine.
Peugeot and Citroën announced that they intended to build a car that uses compressed air as an energy source. However, the car they are designing uses a hybrid system which also uses a gasoline engine (which is used for propelling the car over 70 km/h, or when the compressed air tank has been depleted). In January 2015, there was "Disappointing news from France: PSA Peugeot Citroen has put an indefinite hold on the development of its promising-sounding Hybrid Air powertrain, apparently because the company has been unable to find a development partner willing to split the huge costs of engineering the system." Development costs are estimated to 500 million Euro for the system, which would apparently have need to be fitted to around 500,000 cars a year to make sense.  The head of the project left Peugeot in 2014.
- Air engine
- Charging station
- Compressed air energy storage
- Compressed air battery
- Compressed air vehicles
- Engine swap
- Plug-in hybrid
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