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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



Air/electric hybrids would recover heat from the engine. You could store the heat from the compression on board, but you have so much from the engine, it would not matter. No compressors in the garage, it is not like a plug hybrid. It stores braking energy and turns that compressed air into torque for take off and/or runs an alternator to run the motor and charge the batteries.

Henry Gibson

Long range travel can be done ,as it is now, with hydrocarbon fuels by means of a low powered engine generator. Batteries can do the trips to market, school and most of the way to work. Switched reluctance motors need no metals other than iron and copper or aluminum.

Fireless, steam locomotives were very cheap to make and to operate and a very few are still in operation. Fireless steam automobiles could be kept ready to roll with vacuum insulated tanks and natural gas and electric heaters. Even using electricity to make steam might be less costly than gasoline at times, but any fuel can be used including charcoal. There are several ways to make small steam turbines that are more efficient than piston engines. A propane tank could supply emergency heat for extended travel. The steam would be condensed so little water would be needed. Actually a long distance heat powered Stirling engine automobile was designed by Philips for a US car maker. It proposed the use of molten lithium salts for heat storage. ..HG..

Henry Gibson

Range extenders are a requirement for good engineering of battery powered cars. The high power lobby for electric car with TESLA as the best example has people believing that electric cars must have the power of a HUMMER when no TOYOTA car has the power of a HUMMER. A range extender does not need to get you home through city streets at 160 kilometers an hour, and an average of twenty or thirty would be more than enough. The ZEBRA battery was moving cars a hundred kilometers on a single charge before the first lithium battery was used in a laptop computer. ..HG..

jeff ray

What we need is a president that is willing to stand for somthing other than supporting the Military industrial complex. We should stop playing God and start focussing on homeland environmental issues at hand.


Air hybrids are too inefficient.

Recovery and storage of heat from the engine would make them as complex as a recuperated gas turbine and likewise still leave them inefficient.

Without storage you would need to run the ICE to heat the expanding air.

What ever came of using the engine to compress air on decel and injecting the air back in the engine for re-accel? It's an attractive dream.

But like so many others, it has been thought of many times and is apparently a lot of ineffective clap trap that is not marketable.

Steam is romantic but so ineffective that no one has even been able to come up with a conspiricy theory that explains why it is workable but not used.

Vacuum insulated tanks, natural gas, propane, electric heaters, charcoal; there are multiple ways to make steam for small steam turbines - none are worth while, even compared to an ICE.
No analysis will satisfy the dreamers, so I just say - dreaming is good, but when old ideas are not being used, you really have to believe there are reasons. If the reasons go away or change (battery prices drop) you might argue they are now practical, but no matter, you really should not believe no one has thought of it or that no auto maker in the world can see it's marketability.

I think the belief that there is a high power lobby for TESLA type electric cars is paranoia. A lobby for them? Sure there is a lobby for everything – that’s our new way of government. But a high power [effective] lobby? Not a chance, Tesla sales will only cut into Porsche type sales.

A range extender only needs to get you home on the highway or through city streets at 55 MPH; even less if your odds of needing to limp home are low enough.

GM has chosen to provide enough ICE power to have the transition be transparent.

I think they believe this will be what will sell best or gain them the most “greenness” (and Toyota already has the simple-HEV crown).

GM lost the chance to lead in EVs back in 2003 and now Toyota has the market.

– Oh wait, Toyota only has most of the EV market , and after 10 years that market is holding at 3%.

Well, GM could still have invested billions in EVs, gained the admiration of the fringe and gone bankrupt sooner – umm, I am not sure that is a sound business plan.



Since fews days you can (ok in France) order your air car ... !

Happy face :)


'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'

I don't understand why the energy in compressing air has to be 'lost as heat'. For 6 months of the year I am short of heat and currently burn fossil fuel to get it. Why can't the heat producted in compressing the air be transfered(by heat pump if required) into an insulated domestic water tank. Wake up to a car charged with compressed air and a tank full of hot water. Would such an arrangement raise the thermal efficiency of the compressed air powered car?

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