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Concept: VGT Developing “Plug-in” Gasoline/Compressed Air Hybrid Based on the RoundEngine

Basic design of a two-piston RoundEngine. Click to enlarge.

Canada’s VGT Technologies, the developer of the RoundEngine, has started development of a “plug-in” compressed air hybrid vehicle using the RoundEngine technology. In this application, “plug-in” refers to connecting to an external air compressor to top off the storage tanks. The RoundEngine is a novel variable geometry toroidal (VGT) engine.

The gasoline/air hybrid vehicle is similar to a gasoline/electric hybrid vehicle but uses compressed air instead of electricity for an auxiliary drive. In the first phase of development VGT will focus on the air drive using the RoundEngine technology by modifying an AWD vehicle to incorporate the air drive on the rear axle. The front axle will be powered by the gasoline engine. The purpose is to prove the air drive in an automotive application and to demonstrate fuel and emissions savings. VGT is manufacturing a toroidal engine for this application.

In the first phase of the project, the air drive will be used mostly for acceleration and low speed, while the gasoline engine powers the car at higher speeds and recharges the air tanks. The system will support regenerative braking, and offer the optional plug-in capability to recharge airtanks in off-peak hours. The compressed air can be used to supercharge the combustion engine for short periods without any mechanical supercharger resulting in short power boosts.

In the second phase, VGT plans to apply waste heat from the exhaust and the engine cooling system to the compressed air and converting the expansion/pressure increase into power. Other options are to use fluids with phase changes to produce closed cycle high pressure steam for propulsion.

The third phase of development will focus on a dual-engined vehicle. There will be two RoundEngines, a combustion version and an air motor version. One engine will run on air only while the other RoundEngine will operate with a combustible fuel.

The RoundEngine. VGT was formed to commercialize the variable geometry toroidal engine developed by Rudy Pekau beginning in 1995. In 2002, VGT developed a 625 cc, single-cylinder prototype engine which used compressed air as a power source. In 2004/05, VGT converted the air-powered motor to a combustion prototype.

The external combustion chamber. Click to enlarge.

The RoundEngine is a toroidal rotary engine with two or more pistons fixed around a central disc that connects to the drive shaft, replacing the connecting rods and crankshaft. Combustion occurs in a external combustion chamber, with the gases venting into the toroidal chamber to move the pistons. Inlet, exhaust and combustion chamber valves are set in the base of the toroidal track.

A rotating timing disc, the shaft of which is tangential to the toroid, provides a means for the pistons to compress fresh air and is a backstop which prevents combusted air from moving back through the housing. As a piston passes an inlet port, the air is compressed between the piston and the timing disc until a valve opens upstream of the disc to allow the air charge to flow into the external combustion chamber. The combustion chamber is closed and a direct-injected fuel charge is ignited, then exhausted through a valve on the side of the timing disc.

The disc has a cutout to allow passage of the piston. By the time the combustion chamber exhaust valve opens, the piston has passed through the cutout, and the timing disc once again seals the track.

With two firings per revolution of the output shaft, a two-piston RoundEngine is equivalent to a conventional four cylinder, four-stroke design. VGT suggests that on large engines a second combustion chamber could be used, 180° away from the first.

The RoundEngine can vary two volumes. By using multiple inlet ports, the choice of which can be switching during operations, the engine can vary the volume—and hence displacement—of the compression stroke. Because the combustion chamber is external to the toroid, the actual working internal volume within the chamber can be increased, or decreased. This feature has more of an impact on fuel choice and compression ratios, but nonetheless can also be used to some degree to impact displacement.

VGT does note yet have empirical evidence for power from the engine, but has calculated a range of outputs. For example, a 2-piston, 2.4-liter displacement RoundEngine operating with a pressure ratio of 1.5 to 1 should deliver 88 bhp at 1,500 rpm and 134 bhp at 3,000 rpm, with 310 lb-ft and 236 lb-ft of torque, respectively.

VGT suggests that initially, the RoundEngine can achieve savings of 15-35% in fuel consumption over reciprocating combustion engines and smaller gas turbines.




The website says (in effect) that one of the advantages of their system is storage of energy in compressed air rather than in an expensive battery system.

It's a good point, however I have been proposing for several years now that these expensive boutique batteries have never been fully justified in the first place in the FULL hybrid. I see myself ahead of the curve here because of the many years I've been looking into this solution.

I think it is fair statement that the hybrid systems installed in the Prius and the VOLT gain minimal advantage with their on board storage in relation to their exhorbitant cost. The Phev battery is something of a scam in my opinion, forcing people to part with huge sums of money for storing the same anmount of energy as the ten dollar plastic container I have to store gas for my lawn mower.

Of course the high voltage bus has to be energised in order to start the vehicle or even drive the vehicle itself at very low speeds in the absence of a chemical battery. A 'virtual battery' is required which will take its energy from the existing 12 volt Pb-acid battery. I'll spare you the description of the electrical circuitry involved in that, but suffice it to say even Mr Lutz himself has mentioned the possibility of a Battery-free Volt.
However if this comes to pass it will dilute the usefulness of VGT's system.


It is true that compressed air has not be seriouly investigated for a plug-in vehicle except by MIDI including the possibiliy of efficient waste heat recovery. Keep in mind that in order to be efficient you need at least 3 stage of expansion with re-heating in between and still you returned energy will be around 50%.


It's very true that every time you convert energy into another form there is a lose of energy in doing so; burning gasoline in an ICE, no matter what form, is quite ineffecient; then storing the energy as compressed air and then diving a drive line with it...well! how efficient can it be?

Seems to me that if you plug in and charge batteries then discharge them in an electric drive line...that's about as efficient as a practical car gets. And, if one charges the batteries using solar power, it is even clean, practical and efficient.


@Lad " clean, practical and efficient "

You forgot to add "and very expensive."

How much do you think the owner of the $20,000 solar PV array, needed to recharge your electric vehicle while you are at work, is going to charge ? As much as a $20,000 rental car perhaps ?

Then how many recycles will your battery pack stand ?
Say 500 cycles at 100miles/chg = 50k miles.
Let's say pack costs $10,000 and that's generous.

But gas at $5 that is equiv to 2000 gallons
A Prius or Yaris @ 50mpg will give you 100kmiles
for that.

And I needn't remind you that you haven't paid the parking attendant the fee for your daily use of his solar array yet.



I wonder when the readers of green car congress are going to start demanding more complete specifications accompanying announcements like this. If there is real substance to reports like these, the readers deserve to know about them. The web site listed has no specific information that I could find, only generalized goals. Almost every day, someone makes an announcement about a product and fails to give adequate information. Does anyone agree?


Gentlemen. Air engines are well-known by their low efficiency, motors and compressors, and here you have to use the two. Example , in rotary tools like grinders you can compare pneumatic vs electric in the same work measuring the electric power in the compressor inlet and in the electric tool inlet, this late consume 1/3 of the pneumatic.
Compressed air cars may be good for certains specific environments but not for every day use.
Batteries and fuel cells are very expensive also, I think there´s a second chance for ICE's but we have to redefine the entire car ,to create new paradigms like VW 1 litre, BMW Clever... vehicles with very low consume and emmisions.
By the way, anybody knows where´s Rafael Seidl?


a capacitor makes a battery free electric.. I still want my car with a 120 mile range and a 15 minute recharge, and not too expensive.



Not so fast, current air engines are very inefficient because their are mainly adiabatic, isotherm engine are much more efficient but requie multi-stage expension with re-heating in-between. That's how the MIDI air cars works.


I know why they're quoting pressure ratio instead of compression ratio. To cover the fact that their engine will never be efficient with such a low compression ratio. I can't understand the claim that the engine will consume 15% - 35% less fuel than the equivalent power conventional internal combustion engine. The cycle efficiency for the round engine is lower, that means more energy in the form of fuel energy will have to be put in, to get the required output.

Hint... why are manufacturers developing downsized engines with high compression ratios and forced induction? This round engine seems to be flying in the face of all that.

A case of a design that is not good at being an air motor, or an internal combustion motor.


jmilner - No question. Articles about compressed air power and new weird ICE designs are usually intended to gain publicity and money. They probably lack basic technical details because the supporters do not, or will not understand the inherent shortcomings of the designs. It’s not easy to separate the blind true believers from the scam artists, but this lack of details is a common trait.

michael Bryant

compress might not be the most effecent engery system but very cost effective. Compress air is cheeper to produce than hydrongen. Air power system is much cheaper than batter power system. Compress air tanks can out last batteries. Also much cheaper than batteries. Plug in hybrids only means that plug in to electric out to charge up. The storage medium can be hydralic ,pneumatic or battery. A pneumatic plug in hybrid with 40 mile range will cost around 20,000$ easy. A battery plug in hybrid with 40 mile range will cost 30,000$+.


It is very obvious that busses and garbage trucks, with their constant, repetitive stopping, stopping, stopping (with friction brakes wasting ALL the energy) are absolutely IDEAL for cheap compressed air hybridization/regeneration, of almost any efficiency. Almost any.
Compressed air system cost – Pennies
Technology – Mindlessly simple, even reheat is not complex.
Where are they all?
Few exist because the efficiency is SO bad - even with semi-costly reheat.
Not understanding why it is impractical, does not make it practical.
The round engine is a fine example of Reuben Goldberg engineering.


I know I'm in good company when a resounding chorous of CRAP , burp, thats it.

michael Bryant

It seems to me that only thing keeping compress air tech and new Ice tech is personal opinion. Engery meduiem types all of sudden become very personal. Believe only like what they know. Any do not know or understand full they attack.


The schematics show the large surface area exposed to hot gases which coupled with the large thin wall (hence) unstable working chamber. While it is not obvious that air tight sealing would be required for good output, hot exhaust blowby would not be tolerated by whatever unlikely radially matched seal may be designed.

The engine would surely grow with temperature and could not be expected to remain dimensionally stable as would be required to minimise hot gas (blowby) damage.

If cooling was employed to help dimensional stability, this would reduce the heat available to do work.

At the same time free passage of the piston past the rotary front of the piston will need to occur instantaneously.
Combustion intake air seems to need forced induction.

How will the seal pass this point (and the exhaust port openings.
Manufacturing such a design to any tolerance would appear to be quite a feat.

As described in the report, this is effectively a two stroke engine and would appear to use both piston and rotary ports.
Surely low friction turbine operating at high speed is the ultimate expression of this idea.


" in order to be efficient you need at least 3 stage of expansion"

I do not understand why people do not use the ideal that were discover log time ego to solve current technical problems.

Question: Why we do not used 3 stage expansion (that was used in steam engine) in current ICE. The principle are very similar. It definitely will improve efficiency of today engines.

Roger Pham

@michael Bryant | Aug 9, 2008 10:26:33 AM,

Please look in to see how far this compressed air idea has gone. An idea so simple, yet always in development...What happened to Tata motor's development of the aircar? What happened to Ford's development of pneumatic hybrid and hydraulic hybrid at least over 5 years or more?

The physics of it is very simple: compressed air at best can store only 1/20th the energy density of hydrogen at the same pressure and volume, and 1/60th the
energy density of NG at the same pressure and volume. This means that for the Honda FCX Clarity with 270-mi range on H2 can only travel 14 miles if compressed air is used! How practical is that?



500 cycles on the battery is on the low side. I believe the A123 batteries do thousands of cycles. That changes your calculation drastically.

I think Lad never suggested you must have a parking lot with pv cells to be able to recharge your car with solar power. I think he had the smarter option in mind of a solar power plant in the desert feeding into the grid.

And solar power is not necessarily pv, solar thermal now seems to be the cheapest option.


The main advantage of an air powered system is the low tech fast recharge combined with off-peak recharge from the electric power system. The limitations of this tech can be seen in the car’s its promoters develop, they remind me of golf carts or lawn mowers. Not something you would most of us commuting to work or cruising the interstate with but a huge market may exist that is being overlooked, Lawn care. Compressed air tech has a huge market in the professional and private lawn care markets with fast recharge stations located on the transportation trailers in the pro market or the garage for the private market. Anyone who has ever seen a disinterested teen refill a lawnmower can testify to the gas savings alone in spilled gas. This could also have huge environmental benefits.


It has become de rigeur that whenever a new technology is discussed, the naysayers pile on. And they do so with You would think that they really know what they're talking about.

michael Bryant

The air is not always in development. MID had sound concepts for about five years now. The problem has always have been interest and marketability. I been following this teck for five years. How can anyone devellop anything if they always have there funding pulled from the project. Look at popular machincanlics article and you see alot negavtive comments from people who never drove one those air power cars. It's not a quesation about development but money always been problem.

Henry Gibson

Turbines can expand compressed air as efficiently as pistons or more so. They are being used in compressed air UPS systems with preheated heat exchangers. They can even develop high torque. Lets go back to the Pescara dual free piston engines, but add computer control. A steam turbine locomotive ran for many years, including WWII with better efficiency and torque than similar piston locomotives. Modern material can make superiour turbines and gears. Let's use them. ..HG..

Roger Pham

@Henry Gibson,
Low-temp turbines has low Carnot efficiency, and high-temp turbines are very expensive. Large power turbines has hollow blades for air cooling. Plus, the high inertia of turbines takes a long time for acceleration which is not good for automobile application.

Michael Bryant,
The low energy density of compressed air storage is (CAS) the problem, and no amount of money nor time can fix that. If you already invest money to buy a high-pressure air tank, might as well fill it up with NG or H2 and enjoy much farther driving distances (60x and 20x more range respectively, to be exact).

Roger Pham

<<"It has become de rigeur that whenever a new technology is discussed, the naysayers pile on. And they do so with You would think that they really know what they're talking about."

Jack, have faith that there's a lot of knowledgeable people in this website. You're right that they are not cheerleaders, nor ego massagers, nor motivational coaches. But you must realize that with any new and unproven technology, there will be a lot of potential flaws, pitfalls and unanswered questions. If I am to present my project in a technical forum, I would much prefer reading 10 negative critiques over just one complement, because only through critiques and failures that an idea can be prefected, or dropped before wasting too much effort on an unfeasible approach.


@Lad " clean, practical and efficient "

You forgot to add "and very expensive."

How much do you think the owner of the $20,000 solar PV array, needed to recharge your electric vehicle while you are at work, is going to charge ? As much as a $20,000 rental car perhaps ?

Then how many recycles will your battery pack stand ?
Say 500 cycles at 100miles/chg = 50k miles.
Let's say pack costs $10,000 and that's generous.

But gas at $5 that is equiv to 2000 gallons
A Prius or Yaris @ 50mpg will give you 100kmiles
for that.

And I needn't remind you that you haven't paid the parking attendant the fee for your daily use of his solar array yet.


Posted by: T2 | Aug 8, 2008 7:09:20 PM

I think you forget all the money you will save when you change: Oil , Oil Filter, Fuel Filter, Spark Plugs, Air Filter, Gear Oil, Transmision Oil, Coolant, Power steering Oil, taking a Tesla Motor Engine, they be coming out with a consumer model in 2010, it only has about 5 moving parts, all the money you will save in service and repair, no: Injectors, no fuel pumps etc etc.... I think you get it by now.

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