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ETH Zürich Developing Pneumatic Hybrid Engine; Approaches Hybrid-Electric Performance at Lower Cost

Pneumatic hybrid engine on the test bench. Photo: P. Rüegg / ETH Zurich. Click to enlarge.

Researchers at ETH Zürich are developing a pneumatic hybrid engine—an internal combustion engine connected to a compressed air tank instead of a battery system. A member of the ETH research team, which is led by Lino Guzzella, Professor of Thermotronics, will present a paper on their work at the upcoming SAE World Congress 2009 in Detroit in April—one of a number to be presented on the topic there.

The pneumatic hybrid engine, which follows the downsizing and supercharging paradigm, offers a fuel-saving potential that is almost equal to that of hybrid electric powertrains while inducing much lower additional mass and cost penalties, according to the ETH Zürich researchers.

When required, e.g. when starting from rest or after changing gear, compressed air flows into the engine through an electronically controlled valve. If fuel is also injected, the engine responds quickly.

The compressed air supply also allows extreme downsizing. Conventional car engines can have peak powers of 150 hp or more, but usually need no more than 30 hp for everyday driving. Downsizing the engine in this instance halves the number of cylinders from four to two. This also halves frictional losses and increases the engine’s average efficiency. To keep the maximum power and thus satisfy the consumer’s drivability demands, the engine is highly supercharged by a turbocharger—which exploits the exhaust gas enthalpy as an energy source, and which boosts the to the desired levels.

Initial results from a prototype on the test stand in the ETH Zürich Machinery Laboratory show an increase in the engine’s average efficiency in the European Test Cycle from 18 to 24%. This corresponds to a fuel saving of one-third. Energy savings of up to 50% are achievable in purely urban traffic, because the engine can pump air into the compressed air tank during braking, thus recovering the kinetic energy.

The fuel saving achieved by the air hybrid is about 80% of that of a full electric hybrid, but the price-performance ratio is “distinctly better”, according to the research team.

Guzzella estimates the cost increment for the air hybrid over a conventional gasoline engine is approximately 20%, while the incremental costs for an electric hybrid are at least 200%, according to his calculations. With the cost advantage, Guzzella suggests that the air hybrid also would be suitable for use in poorer countries.

ETH Zürich says that the engine concept has interested several major motor companies and automotive suppliers.

Other papers on aspects of pneumatic hybrids and their control systems that will be presented at the SAE World Congress 2009 include contributions from Lund University, Sweden; Université D’Orléans, France; National Taipei University of Technology, Taiwan; the University of Waterloo, Canada; and Brunel University, UK.


  • Dönitz et al. (2009) Realizing a concept for high efficiency and excellent driveability: The downsized and supercharged hybrid pneumatic engine. (SAE 2009-01-1326, not yet published)

  • C Dönitz, I Vasile, C H Onder, L Guzzella (2009) Modelling and optimizing two- and four-stroke hybrid pneumatic engines. Journal Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering pp. 255-280 doi: 10.1243/09544070JAUTO97

  • Andrej Ivanco et al. (2009) Energy Management Strategies for Hybrid-Pneumatic Engine Studied on a Markov Chain Type Generated Cycles (SAE 2009-01-0145, not yet published)

  • Kuohsiu David Huang et al. (2009) Validation of Dynamic Model of Hybrid Pneumatic Power System. (SAE 2009-01-1304, not yet published)

  • Sasa Trajkovic et al. (2009) Simulation of a Pneumatic Hybrid Powertrain with VVT in GT-Power and Comparison with Experimental Data (SAE 2009-01-1323, not yet published)

  • Amir Fazeli et al. (2009) A New Air Hybrid Engine using Throttle Control (SAE 2009-01-1319, not yet published)

  • Cho-Yu Lee and Hua Zhao (2009) Analysis of a cost effective air hybrid concept (SAE 2009-01-1111, not yet published)



Whenever I now see SAE and performance in the same post, I am immediately skeptical. Adherents to the Otto Motive paradigm are interested in satisfying consumer’s drivability demands, but don't make planet livability demands on them.


Would it be possible to build a Pneumatic Electric Hybrid? I understand these people are using the "supercharging paradigm" but the same reasoning of needing only 30 hp to maintain speeds should allow for fewer or less expensive batteries, I would think.

I am asking, because I just do not know.


Why not do a PHEV hydraulic hybrid, where you plug it in at night and charge both the battery and the compressed air tank.
By sizing the compressed air tank you could modify the ICE-off performance.
By charging the battery, you would use the alternator less. By modeling the daily power consumption, you might be able to do all the battery charging from the mains.

You would have the extra cost and weight of an electric compressor, but it might be worth it.


They probably could get the same recovery efficiency as an electric system by re-heating the compressed air using the heat of the exhaust but that would make the system significantly more complicated.


This is good.
Scientists and engineers should explore all possibilities.
Conservation/green enthusiasts should note that the words "developing", "producing" and "selling" all have different meanings.


It sounds to me like this should not be called a hybrid drive system. This is a hybrid supercharger, that uses recovered braking energy to push in the air, rather than the motor to spin the supercharger. How much efficiency does that deliver over a standard supercharger? Is it any more cost effective? Will it run out of compressed air on a long uphill?
Is it preferrable to a 2-stroke engine with similar properties? Does it have emission problems? Does it wear out faster and cost more to maintain?

John Thompson

@ jcwinnie:

The entire world runs on the 'otto motive' paradigm right now. That indisputable fact can not be changed overnight.
This engine can be run on algae-based ethanol, for one thing. If so, will you stop whining?

Peace Hugger

"Guzzella estimates the cost increment for the air hybrid over a conventional gasoline engine is approximately 20%, while the incremental costs for an electric hybrid are at least 200%, according to his calculations."

That is a great selling point. Watch out, lithium.


Finally a practical alternative to high cost electric hybrids.

Most upsetting to folks that are biased who have been pushing electric hybrids as a steeping stone to full electrics.


There are a number of turbo equivalents IE twin turbo or electric turbo besides electonc valve timing that are available on the market.

If we assume that we are stuck with some ice for the present.

I don't see this as any sort of hybrid, but why let the truth get in the way of a good story?

Electric motors , large and larger alternatorss(and all
sort of accesories)
freqency controlled motor capability, solar or wind recharging and demonstrated 40mile range and only at series 1.
I would question mechanics wanting to get involved in the xtra hassle.

Electric motoring will be second nature to the kids growing up today along with understanding of digital, media and the world we live in.

This will make a pleasant change from the unimaginative world being left behind.

Don't see this as part of it other than specialist application.


They improve the ICEs energy efficiency from 18 to 24% and call that a victory?

Petty vague details:
1) The compressed air acts like a supercharger, on a motor that is already super/turbocharged?
2) Is this like the hydraulic transmission hybrids Eaton/UPS are using? (Doesn't sound like it)
3) There is just NO WAY to avoid 50-75% energy loss with a heat engine unless you reuse the HEAT, they are not doing that, and they’re just trying to downsize to reduce friction around the edges. The thing still has 600 moving parts compared to 1 with an electric motor



I don't think this is hydralic hybrid. I think the turbo charger is assisted by compressed air at low RPM, so that it eliminates turbo lag. This may make a small engine perform like a big engine, which offers some (perhaps 18-24%) efficiency gains, but in all likelihood, the compressed air only provides a portion of that, and most of it comes from the small displacement and turbo charger.

It doesn't sound that revolutionary.


I guess it is a turbo system that stores excess compressed air in a tank, instead of shutting off the turbo with a wastegate when too much pressure is available. All it really needs is a storage tank so it is cheap.. but I would not call it a hybrid, that is just marketing bull****. There is no brake energy recovery, an important aspect of hybrid cars, either pneumatic, electric or flywheel.

Alex Kovnat

A disadvantage of this concept, which is also true for electric hybrid concepts, is that you have regenerative braking only on wheels driven by the engine/drivetrain. So I would suggest placing a pneumatic pump/motor on the front axle of rear wheel drive vehicles, or the rear axle of front wheel drive vehicles. That way, you have all wheel drive when needed, and regenerative braking on all wheels at all times.


Herm, "but I would not call it a hybrid, that is just marketing bull****. There is no brake energy recovery, an important aspect of hybrid cars"

If you read the article, you will find that "Energy savings of up to 50% are achievable in purely urban traffic, because the engine can pump air into the compressed air tank during braking, thus recovering the kinetic energy."


As the SAE report is unpublished it is difficult to be accurate however that there is regenerative air pumping (an no small amount of fanfare) I could see how air could be used to drive pistons in hybrid mode.
Again there is a complexity issue there that is somewhat offbeat.
Tyre pumping, air driven alternater, air conditioner compression may be tasks compatible with this but as they are not current market technologies, one woul question the sense of this focus.
It reminds me somewhat of Citroens excellent hydraulic suspension pumps that lost favor by overloading the technical demands of the average technician.

If air pumping technology eventually found a market an development with the accompanying improvisation and reliability together with the simplicity required to mass market. Then -- why not?


Sounds almost an exhaust compound engine as described by many other but using the primary piston as the secondary piston.


@ Bernard,

So is this an air car, ala that French guy who is trying to build them in India?

Perhaps if you only use the compressed air for a few seconds at a time, then an air tank could be small enough and cheap enough to avoid the least-efficient RPM ranges of an ICE.


Quoth Mannstein:

Most upsetting to folks that are biased who have been pushing electric hybrids as a steeping stone to full electrics.
IMAO, an improvement is an improvement.  I'd love to have this air-storage system on my Passat TDI.  Being able to stuff energy into a tank instead of just throwing it away while braking would be an enormous improvement, and having air-assisted boost well below turbocharger speed... well, you just can't imagine how great that would be until you've driven a turbodiesel for a while.


I agree "an improvement is an improvement" but only as a way to buy time while we move to a new paradigm in transportation.


The Jetta TDI isn't shabby either.


Wasn't this originally Scuderi's idea?

Felipe Esteve

Other hybrid pneumatic drive system. That system is OK, but it only can recover energy frombraking. I am developing a system that would use the energy from braking and the exhaust gasses too. It can be found at .

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