The Zero-Emissions Air-Powered Car

Politicians have said that in order to regain our energy independence, we need to reduce our foreign oil imports and invest in renewable energy sources, such as wind and solar. While this idea seems to be a valid solution up front, it has a fundamental flaw.  The U.S. imports roughly 66% of it’s crude oil resources and uses approximately 70% of the petroleum that is refined from that oil for transportation. Currently, the energy source for transportation is almost wholly derived from petroleum, and an investment in wind and solar would have little effect on the consumption of foreign oil imports. This is because wind and solar process transfer energy to the U.S.’s electrical grid, which has little to do with petroleum products. Only around 1% of the U.S.’s petroleum usage goes to electricity generation [1].

In order for wind and solar to have a lasting effect on our foreign oil imports, we first must invest in a new transportation mechanisms that derive their energy from the electrical grid. An option that I would like to introduce is a vehicle developed by Motor Development International (MDI), called the CityFlowAir.

MDI CityFlowAir

The primary power source for this vehicle is an engine that runs entirely on compressed air. While compressed air engines are nothing new (the first was developed in the 1820‘s), MDI “believes that their engine will blow the car market away.” The tanks that store the air under 4300 psi of pressure are made of a strong carbon fiber composite, and they costs just $2 to fill [2]. Lightweight composites and aluminum help to keep the weight low—around 1870 lbs, which is about 53% lighter than the average passenger vehicle [3]. With a range of about 125 miles and a top speed of 70 mph, the vehicles are primarily for urban use.

The advantage of a compressed air engine is that external air compressors use electricity from the city electrical grid to compress the air. This opens the door for the use of the electricity produced by wind and solar generators, while eliminating the use of petroleum products as a power source for urban transportation. If more vehicles use electricity as a power source, the demand for electricity would increase. Increased demand would lead to increase in production, which would hopefully be from renewable sources. While it is a possibility that more coal would be consumed to generate the newly needed electricity, governmental incentives for the use of wind and solar could potentially drive a widespread adoption of their use.

A second major advantage of the CityFlowAir vehicle is that there are zero emissions from the use of its compressed air engine. If the compressed air is produced by electricity from renewable resources, CO2 emissions generated from the vehicle’s use would be virtually zero. Even if the electricity used to compress the air is not from a renewable source, CO2 emissions from a fleet of CityFlowAir vehicles would be drastically lower than from a fleet of internal-combustion-engine-driven vehicles.

MDI’s compressed air technology is one of many solutions (battery-powered, hydrogen-powered, and hybrid systems) under development to solve our energy challenges. Perhaps the next step is to begin introducing these solutions on a larger scale, and to improve the energy infrastructure to allow efficient usage of renewable energy sources.


[1] Annual Energy Review (2008). U.S. Energy Information Administration.

[2] Modern Marvels: Environmental Tech. 2 (2007). The History Channel.

[3] Amenda, James M. (2007), EPA Fuel Efficiency Report Reflects ‘Incremental, Hard-Fought’ Gains.

[4] Motor Development International.



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6 responses to “The Zero-Emissions Air-Powered Car

  1. jetengine10

    I’m pretty sure I don’t want to be in a car crash with a tank behind my seat that’s at 4300 psi. I’d want to see significant safety testing before I’d ever consider getting in that car.

  2. Do you know how long it takes to fill a tank? Compressed air seems like it could be an alternative to battery powered cars, and one of the knocks against using electric cars (right now) is that it takes a long time to charge up the batteries. Just curious.

    And ditto to jetengine..

    • Christopher


      According to the Modern Marvels episode, it takes about three minutes to refill the tanks at a station that uses an industrial air compressor to fill a rack of high-pressure air (HPA) tanks connected together. MDI includes a home electric air compressor with each vehicle, which takes three to four hours to fill the tanks.

      To address the safety issue of HPA tanks, the carbon-fiber tanks are strong enough “to withstand small-caliber gunfire.” In the event of a crash, the tanks don’t explode and send pieces flying; They merely crack down the side, and the air rushes out.

  3. patrickpace

    This is an interesting idea. It can’t, of course, solve any CO2 or fossil fuel dependence issue until the electric grid is powered by something other than coal and natural gas (currently the source of ~70% of US electric power). Given the convenience and availability of fossil fuel, I can’t help but compare how big a tank you would need to match a 300 mile range (10gal tank at 30mpg).

    Given 20,000kj/kg (Ashby, Mat’l Selection in ME, pg 125) for gasoline energy density, and a 10 gallon tank, I calculate a typical car would hold at least 558Mj of energy on a full tank. To get the kind of energy storage from compressed air @ 4300psi (rather high, but definitely necessary!), how big would the carbon fiber pressure vessel need to be?

    A VERY casual ideal gas law calc (ignoring losses and entropy) says you would need a 4970 gal tank (one expensive piece of carbon fiber). Or, to match the MDI range, a 2500 gal tank (equivalent to 5 gal tank @ 30mpg – 150mi range). Call me crazy, but I’m not buying a 125mi range, given that the entire interior of the car is less than 2500gal.

    Running a motor on compressed are is not any different, really than IC engines, all fuel does is burn and expand (ie – create compressed air) that pushes on a piston, or rotor. I’d have to see one go 125mi to believe it, I’m thinking these cars are good for more like 10-15mi between refills (obviously dependent on tank size).

    Here’s my calculation in case I made a big mistake or am not using applicable analysis:

    nRT is the energy in the air, which needs to be ~280Mj shown below:

    E/p gas 20,000 kj/kg
    p gas 737 kg/m^3
    1 m3 / 1 gal 0.003785
    5 gal = 0.018925 m3 =(5*.0037)
    5 gal mass = 13.947725 kg =(.019*737)
    5 gal energy= 278954.5 kj =(14*20000)

    To get this @ 4300psi (~30MPa) we find we need:
    V = nRT / P
    =279Mj / 30 Mpa = 9.41 m^3 of gas (~2500gal)
    (Assuming STP you need about 3300kg of air as well).

    Unless I am just really missing something, I just can’t believe MDI’s claims, I think MDI is lucky if they get 10% of their claim- not to mention the real benefit isn’t until we generate electricity w/o fossil fuel.

    • Christopher


      What a thoughtful response! I think your energy density calculations are quite logical. It is true that it would require a very large amount of pressurized air to reach the amount of energy present in the average automobile-sized tank of gas. However, a key think to note is the difference in the use of the energy to produce useful work.

      In an internal combustion engine (with an efficiency of about 18-20%) much of the energy released when the fuel is combusted escapes as waste heat. On the other hand, MDI states that its compressed air engine employs a pressure reducing valve that operates “quasi isothermally” (to avoid losses in energy due to temperature changes) and a constant-pressure expansion chamber, which “produces significant work on the crankshaft” and “doubles” the efficiency of a conventional expansion.

      Now, these are certainly things written by MDI for positive public relations, and I believe it would require a skilled thermodynamicist to validate their claims.

      • Patrick Pace

        This does change the numbers a bit. So If a typical car has 20% energy efficiency (large oversight on my part, mainly to keep it simple), and assuming 100% efficiency with the MDI (just to get a grasp of the comparison), then you would need a 500 gallon tank. This is the about the capacity of a semi truck (the two long cylinders alongside driver and passenger doors). And of coarse, if the real eff. is only 0.5, than you would need ~1000gal to be the equivalent to 5gal of gas (and get the 100mi they claim). However, it is a valid proposal, just needing clean energy to initially compress the air, and it would be fine for short trips, like 5mi to and from work; just unlikely to actually take you 100mi. Glad you read read my length comments.

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