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KLM Testing MDI AirPod Compressed Air Cars at Schiphol; UC Berkeley Study Finds Compressed Air Cars Significantly Less Efficient than Battery Electric Vehicles

The AirPod. Click to enlarge.

France-based MDI (Moteur Development International), the developer of a compressed air powertrain and several derivative vehicles, officially handed over the keys to two AirPods to KLM earlier in December. The AirPods are under testing for a minimum period of three months at KLM E & M at Schiphol Airport. One of the AirPods is a cargo version adapted to transport parts and maintenance equipment at Schiphol-Center and the other is for personnel transport at Schiphol-Oost.

The AirPod is one of five derivative vehicles designed by MDI based on its Compressed Air Engine (CAE) invented by Guy Negre, CEO and founder of MDI. In 2007, MDI signed an agreement with Tata Motors for the application of CAE technology in India. (Earlier post.)

The core of MDI’s work is a piston engine powered by the expansion of electronically injected compressed air. MDI has developed two versions: a single fuel engine that relies solely upon compressed air, designed for urban areas only (e.g., AirPod); and a dual-fuel version that uses compressed air and a combustible fuel (petroleum-based or biofuel). The compressor is onboard in the MDI vehicles, with the exception of the single-fuel Airpod where it will be outboard but supplied with the car.

The MDI Engines consist of an active chamber and are made up of modules of two opposing cylinders. A proprietary connection rod allows the retention of the piston at top dead center during 70° of crankshaft rotation—providing enough time to establish the required pressure in the cylinder. These modules can be coupled to make groups of 4 or 6 cylinders for a range of uses from 4 to 75 hp.

The AirPod, equipped with a 4.5 kW/15N·m motor, stores compressed air at 350 bar in a 175 liter tank. Range is 220 km (137 miles) on the EEC urban cycle, with a maximum speed of 45 km/h (28 mph). The energy requirement of the MDI AirPod on the EEC urban cycle is 0.56 kWh.

The standard AirPod is designed for the transport of people. It has four seats (3 adults and one child) and has space for luggage. The AirPod Cargo version with a single seat has a load volume greater than one meter cube that makes deliveries easy in town.

The purpose of the use of AirPod at Schiphol is to reduce CO2 emissions on a portion of the distribution chain for which KLM is currently using traditional cars and trucks that run on diesel.

Video of the AirPod at Schiphol.

UC Berkeley Study Concludes Compressed Air Cars Not as Efficient as BEVs. A recent study by researchers from UC Berkeley and colleagues from ICF International and Stanford University analyzed the thermodynamic efficiency of a compressed-air car powered by a pneumatic engine and considered the merits of compressed air versus chemical storage of potential energy.

The study, published in the journal Environmental Research Letters, concluded 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, the team concluded, a pneumatic–combustion hybrid is technologically feasible, inexpensive and could eventually compete with hybrid electric vehicles.

In their analysis of thermodynamic efficiency, the authors concentrated on air compression and air expansion, two stages that are specific to the compressed-air car. Tank leakage loss is negligible compared to the loss of air compression and air expansion.

The compressed-air car should be regarded as a car similar to the common BEV, powered by electricity from the grid but different in storage technology. In principle, compressed-air cars [CAC] could compete with BEVs in substituting for gasoline cars. The life-cycle analysis of the compressed-air car, however, showed that the CAC fared worse than the BEV in primary energy required, GHG emissions, and life-cycle costs, even under our very optimistic assumptions about performance.

Compressed-air energy storage is a relatively inefficient technology at the scale of individual cars and would add additional greenhouse gas emissions with the current electricity mix. In fact, the BEV outperforms the compressed-air car in every category. Uncertainty in technology specifications is considerably higher for CACs than for BEVs, adding a risk premium.

...Overall, the CAC does not appear to offer any advantage over purely electrical means of storing energy on board a vehicle. Batteries are common and improving almost daily, while the compressed-air cycle has no present role in any popular automobile platform. Since there are great pressures on battery performance from other applications such as cell phones, it is hard to imagine that CAC will gain an advantage over BEV in the foreseeable future.

Automobiles must become lighter and more efficient if even the best batteries are to provide longer autonomous ranges. At the same time, combustion technology itself is evolving rapidly in the face of concerns about oil and climate change. As long as there are no substantial innovations in compressed-air technology and its deployment, the real progress in this sector may be the emphasis on light materials and small car design, for which the competition between batteries and fuel will just intensify.

—Creutzig et al.

MDI response. MDI took great umbrage at the paper, calling it “an act of bashing.” In a document posted on its website, MDI says that the researchers erred by comparing the AirPod to a smart (gasoline and electric), because the weights between the two are so different. The Smart gasoline version weighs 837 kg, the Smart electric weighs 924 kg; the MDI Airpod weighs 330 kg (with driver).

A more appropriate comparison to the smart would be MDI’s larger format variants, will be equipped with dual fuel technology, MDI said Taking into account differences in mass, MDI said, the AirPod is as efficient as the smart electric drive.

MDI also said that while its compressed air tank has a life of 12,000 discharge cycles—approximately 30 years—the batteries have a life ne twelfth as long.


  • Felix Creutzig, Andrew Papson, Lee Schipper, and Daniel M Kammen (2009) Economic and environmental evaluation of compressed-air cars. I 4 (2009) 044011 (9pp) doi: 10.1088/1748-9326/4/4/044011



Here's a thought; couldn't this air engine, with its light weight, be used as a range extender? We could have a 40 mile BEV with a clean engine powered from a tank that can be quickly refilled.


There's plenty of lithium to last the next few years and now that Chevron no longer controls the NiMH battery, development of that will continue and there is no shortage of Nickel.


"the absolute abundance of an element doesn't necessarily help for his avaibility. Ti is extremely abundant still is an expensive material that we use sparingly despite its outstanding properties "

Dude, we use titanium in toothpaste.

Roger Pham

I have repeatedly voiced the inefficiency involved with compressed air as method of propulsion, and glad that UC Berkley has concurred in this assessment. There are other methods of utilizing renewable energy that are much more efficient and convenient, such as BEV, PHEV, FCV, and HEV utilizing clean fuels such as biomethane, cellulosic ethanol, or H2.



I looked at air as a range extender and concluded that 40 miles was about all that was practical, so you have 40 miles on batteries and 40 miles on air, or a combined 80 miles. It just was not good enough to justify.

However, air/electric hybrids could do some interesting stuff. Most HEVs have too small a battery pack to recover much braking energy. Air compression and expansion could recover a lot of the braking energy to be used in acceleration from a stop. The costs of a small system are better than adding a lot of expensive batteries. Cooling, supercharging and heat recovery are side benefits.

jeff ray

I think that this design could prove to be very green if applied correctly. If one were able to have solar panels set to directly supply the needed energy to operate a dedicated compressor, eventually after the manuacture impacts were offset, this design would have zero polution or carbon impacts; except for future replacement parts.


Try compressed air, if it isn't economical somewhere - it's abandoned, but don't license, promise vehicle release dates, and place nothing on the market, like Tata.

Lithium/it's batteries have been around for decades, so why whine about it's reserves so many years before any vast EV/lithium demand has/might occur.

In five years, battery chemistries, bio-fuels, alternate energies, etc. will have different profile and perhaps a minimal Lithium demand.



The cryocar I mentioned earlier would be even cleaner. When you liquify air the different gases come out in order; first atmospheric air is passed through a dust precipitator and pre-cooled using conventional refrigeration techniques to remove all traces of dirt and water, this also removes pollution. Next CO2 is liquified at 216.6 K so it can be sequestered. Then comes O2 at 90.20 K, O2 can be sold for a profit. Dido for argon at 87.30 K. Finally you get N2 at 77 K.

A car powered by liquid N2 is not a low CO2 car or a zero CO2 car - it's a minus CO2 car.

Chris Jensen

Treehugger - how is mobility a fundamental right?


It takes more energy to liquefy air than to compress it, and the CO2 emitted to generate the energy for either compression or liquefaction is far larger than the amount of CO2 in the air used as working fluid.  These things may have niche applications (liquid-air vehicles would do really well in S. Africa's deep hot mines) but the inefficiency will keep them out of broad use.


One of the guys that invented a really good air expander motor put them in carts used in produce warehouses. They do not like exhaust fumes in there and he had a ready customer based that really like how clean and quiet they are.


"It takes more energy to liquefy air than to compress it,"

Yes it does but if there is a relative gain in the amount of energy you can store on board the car [The specific energy densities of LN2 are 54 and 87 W-h/kg-LN2 for the adiabatic and isothermal expansion processes, respectively.] it's a win.

"and the CO2 emitted to generate the energy for either compression or liquefaction is far larger than the amount of CO2 in the air used as working fluid."

Wouldn't that depend on the source of energy used for compression or liquefaction?


If there is a relative gain in the amount of energy you can store on board the car then an ICE with a big gas tank - or even a Hummer with a 80 gal tank - is a winner.

Bob Wallace

Compressed air cars might fill a niche between bikes/electric bikes and EVs. They avoid the expense of batteries. Priced very low and given reasonable road speed they might well be the short commute/shopping vehicles that some would find useful. Think of the two car family or someone with the pickup/SUV to pull their boat on the occasional weekend and limited driving needs otherwise.

There's also a rotary compressed air motor which is quite a bit more efficient than the piston version. You'd be looking at a propulsion system about the size of a lawnmower motor and some carbon fiber tanks.

With a little ingenuity the heat generated during compression could be utilized. Heat household water? Use it to generate electricity? (Progress is being made in that field.)


Lithium - looking like we might be able to get a decent supply from the waste water at the Salton Sea geothermal plant. And we did have a lithium extraction operation in North Carolina prior to the Chinese undercutting the market and causing it to shut down.


It’s time for these people to get serious or open the technology up for those that could make use of it. Although they want to highlight the innovative technology with an innovative design, no one I know would ever drive the Air pod as built even if it ran on zero point energy. The work they have done has great signs of potential if properly applied. If mass produced this tech would be extremely inexpensive to make, durable low maintenance, and capable of providing a quick recharge from an off peak maintained local storage bank. A few rough calculations indicate it has about 5 hours of run time at 6 HP, sounds a lot like the specifications of a riding lawn mower to me. Play the video it looks about the size of a riding lawn mower. Play the video with sound it evens sounds like a lawn mower. Walks, looks, and sounds like a duck? In North America just about every suburban house has an inefficient polluting riding lawn mower waiting to be used about 4 hrs and then ignored for the remainder of the following week. I will admit this is not as impressive a market as the auto industry but the efficiency that this technology offers could find a welcome home in the personal and professional lawn care industry.


The Air Pod is not the only design they have;
Think of the Air Pod as a motorized rickshaw.


The basic problem with cars like this is if you told me I had to drive it I would kill you.


Relax about Lithium. Even at $8/kg the raw materials cost for Lithium in the entire Volt battery pack is about $200. Current laptop style lithium cells tended to attribute about 60% of the raw materials cost to COBALT! It's the cobalt that contributes the most to lithium batteries and that's why cells used in cars use a lot less, and hence why Nissan is able to make their cells 'at cost' for substantially less then $300/KWh. Lithium Ion batteries specifically for mass produced electric vehicles are in the first generation, you can expect a lot of improvements in the future, and boy do we need it.


It is niches that will see applications. Small powered flat bed cart trucks for two passengers in fruit and vegetable storage facilities, where no pollution is tolerated and quick refill is desired. You can fill an air tank up in 1 minute versus hours with electric. There ARE applications.


"The basic problem with cars like this is if you told me I had to drive it I would kill you."

Ok.. your deposit is being refunded ;(


"The basic problem with cars like this is if you told me I had to drive it I would kill you."

Ok.. your deposit is being refunded ;(


hehehehe:) Thanks I needed that laugh.


ai_vin:  the amount of energy storage isn't at issue here; what matters is the efficiency, which drives the cost of energy and the emissions from any fossil-fired energy source employed.  If compressed-air cars powered by coal-fired plants will emit more CO2 than hybrids, liquid-air cars will be much worse.

You can say that carbon-free energy will eliminate that consideration, but you are faced with greater investment in physical plant to generate more power, so that comes back to bite you.  Inefficiencies affect everything.


E-P, when I first read about the Washington State cryocar one of the ideas was you could liquefy the air from the smokestacks. Because the car would be powered by LN2 you could use flue gases because even though it's been through the powerplant it's still mostly N2. Liquefying it would just be a part of the CCS system we'd have to be installing on any fossil-fired energy sources anyways.

If I understood it right[It's been a while so I'm prepared to be wrong.] the LN2 powered car should use less N2 than the compressed air(CA) car because its actually a heat engine car. The energy to power a CA car is held in the tank but the energy to power a cryocar is taken out of the environment surrounding the car. The engine works by heating the liquid nitrogen in a heat exchanger, extracting heat from the ambient air and using the resulting pressurized gas to operate the engine. It's a steam engine, kind of.


You don't want to try liquefying powerplant stack gas because all the contaminants will corrode your equipment, not to mention the stuff with odd freezing points giving you fits as it clogs the gear and requires defrosting cycles to clear out.

The cryocar is even less efficient than the compressed-air car because there are even more entropy increases in the high ΔT between liquid air and ambient than compressed air and ambient.  You also can't use cryocars when the ambient temperatures are too low and the boiler will frost up.  If you think stopping at regular intervals to charge batteries is a problem, think about stopping whenever a shot of humid air covers the boiler with ice and kills the engine.

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