Public Electric Car Trials in UK West Midlands Begins with 25 Mitsubishi i-MiEVs
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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



Looks like a cheap vehicle to buy &'s all about the costs vs. benefits.


Compressed air is a thermodynamic cycle, its efficiency is very limited. Most of the energy in compressing it is lost as heat, most of the energy in expanding it must be provided as heat (by burning fuel). Li-ion have charge-discharge efficiencies has high as 95%. The only way compressed air has a chance is if it could be made cheap enough, and that a hard achievement when all the investment is in batteries.


According to, around half of an EV’s manufacturing cost comes from its lithium ion battery. Mass adoption of EVs depends largely on improving the competitiveness of their batteries. But lithium is also used in batteries for other electrical technologies including laptop computers, digital cameras and cell phones. As demand rises faster than supply, price increases. Unfortunately the supply of lithium is limited by both geological and political factors.

While Lithium is a naturally occurring element, it is a finite natural resource: only so much of it exists in the world. And here’s the crunch point for many environmentalists - half of the world’s known Lithium supply is located in Bolivia, in a nature reserve.

The world will go from peak oil to peak lithium overnight.



Large deposits of lithium have been located in the Ungava peninsula. Mining operations may be 2 or 3 years away. Other Northern parts of Canada and Russia are potential areas for other large deposits.

Future nano-technology batteries will use a lot less lithium per Kwh of stored energy. Other lower cost composites will also be used. The world should not worry too much about shortages, specially at this early deveolpment stage.


Lithium is not consumed like gasoline, its recyclable, and reserves of it are essentially limitless but expensive: we could mine sea water of lithium if needed, the only problem is developing the technology to make that economical. Many batteries like zebra cells, NaS, Zinc-Air cells also do not use lithium and require even cheaper materials. If we were to put in 100Mj of power into an alternative energy power car, we would get 133 km off of Li-ion, only 46 km off of a fuel cells and only 42 km off of compressed air. The 3 times efficiency of batteries over compressed air likely means it net price tag for implementation will be less.



The idea that we can switch from a petrol powered fleet cars to a lithium battery powered fleet cars is not only naive but also dangerous. There is not enough lithium for this period. With dedicating all the lithium to car production we would be able to produce at best 5 millions car a year, when the current market is 80 millions. The forecast is the in 2020 we will produce a paltry 2-4millions car worldwide of aggregated PHEV EV cars. Still 76-78millions of the rest will be petrol powered cars.
Production of lithium will be enough up to 2020 as long as electric are a fraction of the fleet, if we had to fully switch to EV we would have to use more abundant material like Na-S and non permanent magnet motor because they wouldn't be enough Neodymium anyway


I doubt we will run out of neodymium, its one of the least rare of the rare earth minerals, and the few kg for electric motors hardly compares to the hundred kg of lithium needed for a BEV. Lithium is relatively common on earth, the problem is its purity is so low, so the problem is a matter of implementing and scaling up its production from poor concentrated sources like sea water, though this problem is similar to oil its not the same: lithium used is not lithium lost, lithium can be recycled, and lithium is not a form of energy so the net energy considerations in mining it are reduced.



Then check your numbers. A car like the prius needs 10Kg of Neodymium. Even building wind mill will soon be a problem if they don't find a solution to the decision of China to stop exporting neodymium.

As for extracting lithium from sea water : dream on it, it is far beyond any EROI decent number. And yes when you have resource base of 14 Mtons your annual production can only be small fraction of this, simple geological common sense.

Yes it can recycled, but the fact is that it is not at the time.


TH -- The same comments were made in the early 1900s about the switch from horses to ICE vehicles.

About 3 1/2 decades latter, people had to go to a small farms to see a few horses being used.

In about the same time frame, you may have to go out of your way to find an ICE powered light vehicle in daily use.

Dont forget that the switch from horses to automobiles took about 3 1/2 decades during a very severe recession. Today's recession may not be as deep and may not last as long. Secondly, China and India will play a major accellerating role while they did not last.


Air powered cars probably will not catch on, but air/electric hybrids might. Capturing braking energy can be done with compressed air and allow an assist on take off. The air run through an expander cools and can be used for cooling. If needed, the compressed air passed the expander can be used to super charge the engine when extra power is needed.


Relax about Peak Lithium. There is enough lithium within reach to power all the BEVs we could possibly need. The real issue is whether or not the sources of lithium are economical to mine. Some salt-flats in South America are currently the most economical so that's why most of the lithium comes from there. However if there are ever problems and expenses bringing lithium to market, that will only drive prices up. When the price is high enough, entrepreneurs will find new ways to supply it. And if the price of lithium goes too high BEV manufacturers will just use other battery chemistries supplied by other entrepreneurs; advanced lead-acid (like those from Firefly Energy), nickel and/or iron, sodium, etc.

That's the matter? Don't you trust the Free Market System? ;^)


Back to the subject; air powered cars. There are ways to make them better. Instead of using a compressed gas use a liquified gas. I once Googled for "cryocar" and found that University of North Texas developed a car that runs on liquid nitrogen (LN2) and called their cryocar the CoolLN2. Also, the University of Washington in Seattle, converted an old mail truck to run on LN2(but their link is now dead), they called their truck LN2000. The specific energy densities of LN2 are 54 and 87 W-h/kg-LN2 for the adiabatic and isothermal expansion processes, respectively, and the corresponding amounts of cryogen to provide a 300 km driving range would be 450 kg and 280 kg.

There are also descriptions of how CO2 engines work at various places on the internet.



in 1900 you had to build a few millions cars to replace all horses. So the analogy doesn't apply, your statment is ridiculous


there is enough lithium to powered any possible BEV ? sorry but the numbers don't add up here. With base reserves of 14Millions tons that is simple not possible by a huge gap. Extraction lithium from sea water is like saying if we extract all the gold from sea water would make every body rich, it is just a ridiculous statement based on misunderstanding of the law of physics

yes we will need other chemistry, as we will switch back to induction motor rather than permanent magnet


I said "all the BEVs we could possibly need." We don't need to replace every single ICE (although that would be nice) we just need to replace enough to hold GW below 2 degrees.

Base reserves are "natural resources that are economically recoverable." What is "economically recoverable" depends on what people are willing to pay.

Seawater contains lithium at a low concentration of 0.1 to 0.2 ppm. OTOH estimates for crustal content of lithium range from 20 to 70 ppm (by weight), at 20 mg of lithium per kg of Earth's crust, it is the 25th most abundant element. Nickel and lead have the about the same abundance. The only reason seawater is talked about as a source of lithium is that currently the most economical source is brine from natural salt flats [seawater that's been concentrated for us].


If the Bolivian salt plains contain lithium, I wonder about the salt flats of Utah, the Mojave and Owens Valley east of the Sierra Nevada mountain range in California. Maybe there is not as much there, but when lithium prices go up, the exploration and finds may as well.


ai vin

if we need only 5millions EV a year then we agree.

SJC, sure as the demand for lithium picks up we will find new reserves, lithium has been in moderate demand so far. But the ultimate reserves including future discoveries are about 30Mtons, and that's not much.

But I am positive that we should be able to developp Na-S battery with good performances that can be used at room temperature. Ceramatec decreased the temperature from 300C to 95C, so, and I don't see why Na-S battery with an electrolyte can no work just like a Li-S battery.


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 (for bikes among others)


From Treehugger:

"Then check your numbers. A car like the Prius needs 10Kg of Neodymium."

No, it doesn't. Disassemble the motors, remove the magnets, and weigh them.

Not everything that is posted on the Internet is true.


All these cars assume alot of people will have a burning need to be 15 miles or so from where they live ALOT. 20-40 years from now I doubt all that large a percentage of the population will have that need.


The various govt's and scientific orgs take a global approach to energy and transportation. They describe it as "Scattergun."

In I.E. transport, especially in this transitional phase, all comers are welcomed.

Some will not get to the starting post as too dirty or dangerous or horribly inefficient.

The next group will be eliminated early though may find specific application as they are too expensive, not scalable or exist in special environments.

Most of the familiar technologies have to run the race and be pursued to their logical conclusion.

They may find application in aviation that would be impractical or disallowed in ground transport.

Batteries will we hope find many expressions that are yet undescribed.

It is a common trap that commenters and thinkers fall into advocating a one size fits all approach.If the world were that simple.

It is wrong to think that any technology is 'out' or preferable at this early stage.

Correctly, the whole mix will refine and morph but could expect to substantially exist to varying degrees into the future.

The pollies and scientists understand this quite well.
This is reflected in different countries 'loosely agreeing ' to concentrate efforts in specialty areas.


This is where the market system might actually work. The demand and price of lithium will go up and the exploration and development will as well. There are other forms of battery technology, but lithium is the next big thing and it will respond accordingly.


"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).

So,If UC Berkeley had compared the little MDI Airpod to a big Hummer, the Hummer might have won ? ? ?


The main fault with MDI technology is the piston air motor.

Any reciprocating motor has an enormous amount of internal friction ... the MDI air motor is no different.



mobility is considered a great progress and a fundamental right, but the way our modern societies are organized pose problem in that they require so much mobility that generates enormous costs on whole. The question of its sustainability is clearly on the table. I am not quite sure that the BEV fully address this problem and as you underline our mobility need 20 years from now might be very different. Today people take their car no matter what either it is to ride one mile to the grocery store ((when they could use a bike) or to ride 60 miles for a purpose that is rarely justified. I mean I see people riding 100 miles in a single night just to go to a nightclub, when there is plenty around. We don't even know why we need so much mobility, we are just used to the fact that we have to move around whatever, that's what we need to re-think. Our life quality might greatly improve with less mobility in many aspect


All these cars assume alot of people will have a burning need to be 15 miles or so from where they live ALOT.

Winter is right, even today a car[BEV or otherwise] with only a 40 mile range could handle 90% of all the car trips taken in North America, and most other countries have less need. The perceived need for more range is another matter.

As I see it the need for "cars" will also decrease as we return to sensible urban planning.

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