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Tata Motors Signs Agreement with MDI for Compressed Air Engine

The 4-cylinder compressed air engine. Click to enlarge.

Tata Motors, India’s largest automobile company, has signed an agreement with MDI (Moteur Developpment International) of France for the application in India of MDI’s compressed air engines. MDI has already applied its CAT (Compressed Air Technology) engine in a prototype city car, the MiniCAT.

The agreement between Tata Motors and MDI envisages Tata’s supporting further development and refinement of the technology, and its application and licensing for India.

MDI has for many years been engaged in developing environment-friendly engines. MDI is happy to conclude this agreement with Tata Motors and work together with this important and experienced industrial group to develop a new and cost-saving technology for various applications for the Indian market that meets with severe regulations for environmental protection. We are continuing the development with our own business concept of licensing car manufacturers in other parts of the world where the production is located close to the markets. We have also developed this new technology for other applications where cost competitiveness combined with respect for environmental questions has our priority.

—Guy Negre, founder MDI

The core of MDI’s work is a piston engine powered by the expansion of compressed air. MDI has developed two versions: a single fuel engine that relies solely upon compressed air, and a dual-fuel version that uses compressed air and a combustible fuel. When running under 50 kph (31 mph) in urban areas, the engine runs on air. At speeds greater than 50 kph, the engine switches to fuel mode. In this mode, the on-board compressor also refills the compressed air tanks.

The current production version of the compressed air engine applied in the MiniCat is a four-cylinder, 800cc unit that uses a boxer design. 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.

Tata Motors has more than 4 million Tata vehicles on the road in India, and is the leader in commercial vehicles and the second largest in passenger vehicles. It is also the world’s fifth largest medium and heavy truck manufacturer and the second largest heavy bus manufacturer.



Well that's a neat twist on hybrid. I like it. I also wonder if it'll ever be available in Canada.


Another one is EngineAir from Melbourne-Australia.
So far I have not seen any significant news from that company even though they have a neat rotary engine which I was lucky to see in action a while back.
The problem is that a lot of indipendent "inventors" are actually not savvy business people and treat their inventions like their children rather than a potential money maker...



Compressed air is hugely inefficient!!


As DS notes the take on compressed air is (usually) poor efficiency.

But almost method has some good application. And there is no reason to think these companies are inept.

Roger Pham

To be precise, the efficiency from compressor to wheel of the AIRCAR is calculated to be ~40% according to the following report:

Whereas the efficiency of BEV from battery to wheel is 80%.

Let's say the air compressor is powered by an ICE with efficiency of about 40%, then overall efficiency of the AIRCAR from fuel to wheel is 40% x 40%= 16%. No improvement at all from the ICE, and a lot worse than the Prius at 37% tank to wheel efficiency.


Is regenerative braking possible? Also, in warm climates, wouldn't compressed-air propulsion offer some "free" air conditioning?

Compressor-to-wheel efficiency isn't the whole story. I think it's wrong to judge the compressed-air car by looking at the motor in isolation. That's kind of like ignoring the heat component in a combined heat and power (CHP) setup.


Regenerative braking is part of the design and they also have some interesting patents for stationary generating of compressed air using renewable, non-polluting resources. The range of the air-only car was the biggest hurdle to overcome, but with the hybrid combustion / air model that problem is solved, altho pollution is re-introduced.

In a hydrogen vehicle the same amount of energy is used compressing the gas, but that energy is completely wasted because the pressure is not exploited in the vehicle, the gas just gets burned. I think many people ignore that energy sink in their calculations of hydrogen powered vehicles.


Air conditioning and heating are both natural byproducts of the air engine's operation, so yeah... "free" air conditioning.

Roger Pham

Compressed-air car concept is DOA in term of range and cargo room and load capacity. Here's why:
1) A carbon-fiber Quantum H2 storage tank for 5000psi of 160 liter weighs 170-200 lbs can contain 4kg of H2, costing ~$2000. 300 liters' worth of air tanks of the same pressure will weigh almost 400 lbs and costing >$4000.

2) 300 liters of H2 in those tanks will weigh 8 kg (17.6 lbs). Air weighs 17 times as much as H2, so multiply 17.6 lbs by 17 = 299 lbs. Adding the weight of air and the weight of the tanks will give you almost 700 lbs! Didn't realize "light as air" can weigh that much, did you?

3) Range: Look into this pdf report, page 9, ( will give you the fact that the compressed energy of H2 is but 5% of the total heating value of H2. According to ideal gas law, air and H2 when compressed to the same pressure will have nearly the same mechanical energy. The Ford Airstream hybrid FCV can travel 280 mi using 4.5 kg of H2 at an estimated 50% efficiency. If this car uses only the compression energy of the H2 at 80% efficiency, the calculation will be 280 mi /50% x 5% x 8%= 22.4 mi. That is for a tank of 180 liter. For 300 liter compress air storage as in the original AIRCAR, the total travel distance will be 22.4/180 x 300= 37 miles for the Ford Airstream. This is assuming slow speed travel wherein the expanded cold air from each stage of airmotor has the chance to warm up to nearly ambient temperature (near isothermal expansion) At normal highway cruising speed, expect a range of about 1/2 as much, or ~20 miles. Look in for further details, but beware that the number you see there for the AIRCAR's range is for a much smaller and lighter car of plastic construction, weighing but ~1600lbs, and the number is still very much hype!

Now, can anyone justify having an energy storage weighing ~700 lbs costing ~$4000 and can give you a range of only ~20 miles in a typical compact 5-seat family car, with a fuel-to-wheel efficiency of 16%?


Roger Pham,

Saying that a compressed-air vehicle has a fuel-to-wheel efficiency of 16% isn't fair. Nobody is suggesting we should build ICE vehicles with compressed-air "transmissions" (ICE -> air tank -> air motor -> wheel).

Some of us are, however, curious if there might be situations where compressed-air propulsion would make economic and environmental sense.

Compressed-air vehicles are extremely flexible. The energy to compress the air could come from any number of sources. As we move into/through peak oil, supply disruptions will become more and more of a problem, especially for the poorest of the world's citizens.

The entire world does not travel by 5-seat family cars.


Roger -- Thanks for all those numbers and analysis.
I wish that the numbers were not so discourageing because I love the air-car concept. The only hope that I now see is for some breakthrough to greatly lower the cost of the tank. A twenty mile range on a $400. tank would look great to me.


the air engine is just plain silly b/c of air storage problems.

Tata probably invested b/c they are interested in the fueled engine. Such an engine could possibly operate without fuel injectors (b/c it is an external combustion engine instead of internal) and other expensive electronic parts, allowing them to produce a cheaper engine for a market that is extremely cost sensitive. Such an engine would require little to no air storage and may have easily controlled pollution characteristics.

Paul Dietz

There's certainly a serious efficiency and range concern, but compressed air cars do have some potential areas of advantage over BEVs. The compressed air tanks are simpler and cheaper than batteries and likely have much longer cycle lifetimes and lower self discharge rates. The tanks can also be refilled very quickly compared to most batteries. These could be advantages in very cost conscious markets, like India.


What I like about this aircar is all the work they put into designing an engine that gets the most out of the compressed air. Years ago I heard about another type of air powered car. It had a better idea for storing energy but the prototype they built still had limited range because they used a cheap but inefficient WWII surplus winch motor. See the link- -If we could combine the best of both maybe we'd have something.

Rafael Seidl

Pure compressed-air vehicles suffer from five problems:

1 - low well-to-wheels efficiency
2 - very limited range
3 - large, expensive pneumatic accumulators
4 - high risk of uncontrolled energy release in a crash
5 - high noise levels

For these reasons, they are probably a dead end as prime mover technology.

The one area where pneumatics could prove useful in a passenger car is in reducing the lag characteristic of turbocharged gasoline engines at low RPM. In that scenario, a compressor (e.g. two-stage centrifugal) attached to the transmission via a wet clutch and planetary gears would recuperate kinetic energy during braking and hill descents. The engine clutch should be disengaged while the tank is filled up. The already present intercooler would be "hijacked" using a set of bypass valves, to increase the air mass stored and, to reduce process temperature levels. When the vehicle needs to accelerate again, it initially receives its fresh charges from this already pressurized tank until it is empty.

The required size of this tank would still be significant, e.g. 40 liters, at 10 bar absolute and 200 degC, since only the first compression stage would be cooled. At this intermediate pressure level, there would be no need to resort to expensive composite constructions. Also, since we're just talking about air not combustible fuel, it might be acceptable to regulators to integrate the tank directly into the chassis to save weight.

With the parameters given, the full tank would contain about 0.3 kg of air. Note that the engine must be fed air at e.g. 2 bar absolute, so a regulated throttle is needed, which cools the air back down. To prevent excessive condensation or ice formation between the throttle and compressor wheel of the turbocharger, the storage tank would have to be insulated and the air stored inside used in short order or else blown off. In other words, the tank will be empty immediately after a cold engine start.

After the first braking maneuver from ~60 kph, the stored air mass would be enough to operate a 2L inline 4 at 1800 RPM and wide open throttle at a boost pressure of 1 bar for ~4.5 seconds. This would permit a lot of low-end torque even as the turboshaft spins up, so the vehicle dynamics would approximate those of a supercharged engine as the driver accelerates quickly up to urban cruise speed. Turbo lag is the primary reason for low customer acceptance of fuel-saving downsizing concepts for gasoline engines.

You could reasonably argue that the same effect, with much higher efficiency, could be obtained with a mild hybrid setup comprising a motor-generator, power converter and traction battery or ultracapacitor bank. However, the pneumatic one outlined above could be a lot cheaper.

tom deplume

If compressed air is so inefficient then why is it so widely used in industry? As for the MDI engine I see an air motor that is more complicated than neccesary.

John Donaldson

Actually this looks like an application of the scuderi Patent which is an internal combustion compressed air engine with phenomenal implications for efficiency see:
for more details on the actual engine. It has the design possibilities to double engine efficiency be it gasoline or diesel and reduce air pollution by 80%.

Roger Pham

Very promising idea! The problem with centrifugal comprssor is the non-linear response of pressure with rpm, but, by coupling the compressors with the transmission, the car can downshift as it slows down, hence maintaining near-constant rpm at the compressor. A simple clutch that simultaneously declutches the engine while engages the compressors is all you would need to have. A car with a CVT would be even better. Centrifugal compressor should be relatively cheaper than electric motor/generator. Each of centrifugal compressor should generate up to 5:1 pressure ratio, which, when taking into account diabatic compression, then the compression ratio from each stage should be ~3:1, and with intercooling between the stage, should easily allow for over 10 bars of compressed air.
Engine shut-off feature should be offered with a starter-generator hefty enough to restart the engine and match its rpm to the transmission when engine re-engagement is needed (ie. micro hybrid)
A complete quantitative cost/benefit analysis should be the next step to compare this with full electric hybrid.

I'm not suggesting ICE coupled with pneumatic transmissin. The ICE is only used at the air-station to compress the air into large storage tanks to recharge the AIRCAR.

The airmotor is only good with pre-compressed air, as its inherent design is for air pressure of up to 300 bars, via 3-4 stages of expansion with inter-reheating. Hey, the airmotor can work with steam from a boiler as well, with reheating stages.
If you wanna use combustion engine, might as well go with the Scuderi concept which separates the compression cylinder to the combustion cylinder to allow for pneumatic hybrid arrangement.


Compressed air locomotive engines were used in the past, most recently in coal mines, where they were safer then electric due to absence of electric sparks. Nice site for the history (including compressed air bicycle):


So, has anyone heard any news from/about these guys?



Hi, does any one know any additional information on the Massive Yet Tiney engine from Angel Labs? Would it fall into the Green Car or engine catagory? It is an engine that is supposed to be able to replace truck and automobile engines but it is many times smaller. The size of the engine that replaces a car engine will be the size of a coffee can. The engine is supposed to run best on soybean oil but it can run on comressed air as well. They have a video showing the engine running on compressed air. The engine supposedly will get some where between 100 to 150 mpg on the soybean oil. Here is a link to one of the sites it can be found on:

Is this an engine that could hold lots of possibilities or would it be bought out by big business and then shelved in some big warehouse?



Everyone is forgetting to quantify the big picture of the air engine vs the liquid fuel engine. The Air Engine with all of it efficiency addressess the pollution problem, especially in densly populated areas. The medical cost benefits from this fact alone is a huge improvement. Additional, the cost of the efficiency is still lower than any combustion engine. Don't forget we have many modes of transportation that are not very efficient, but we are using them anyway at a high cost to the environment and people. So, I would say the trade offs offer more for our future that combustion engine.

girish vilekar


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