Getting There on Air

By | February 6, 2006
8 Comments

Adopting any replacement fuel source for internal combustion engines requires solving a number of problems. With electric cars the problems are:

  1. Range.
  2. Expense of the batteries
  3. Frequency of replacing batteries
  4. Environmental impact of battery disposal
  5. Lack of infrastructure (gas stations are not presently recharging stations)
  6. Time required to recharge.
  7. The danger of electric discharge after an accident.

It has been very difficult to compete with the price/mile cost of internal combustion. Only recently have hybrid vehicles begun to offer drivers some of the benefits of electric cars while maintaining most of the advantages of gasoline cars. Of course electric hybrids have batteries…and all the problems that come with batteries.

There is another alternative fuel technology that doesn’t require batteries. It hasn’t gotten nearly the attention it deserves – the compressed air car. It’s just what it sounds like. These cars are powered by compressed air instead of electricity. While they are not as quiet as electric cars, they are quieter than internal combustion vehicles – the engine produces power from expansion rather than explosion.

The company that is leading the way on air car research, MDI, has designed several prototypes. All of their prototypes cut weight by using aluminum tubing. To avoid the problem of shrapnel from an exploding tank, the air tanks are made of plastic surrounded by a carbon composite. A failure would split the tank. No pieces – plastic or metal – would go flying.

Power comes from fresh air stored in reinforced carbon-fiber tanks beneath the chassis. Air is compressed to 4,500 pounds per square inch (about 150 times the pressure of the typical car tire). The air is fed into four cylinders where it expands, driving specially designed pistons. About 25 horsepower is generated.

I find this technology interesting because it completely eliminates four of the seven problems associated with electric vehicles: expense of the batteries, frequency of replacing batteries, environmental impact of battery disposal, and the danger of electric discharge after an accident.

The “lack of infrastructure” problem would probably not be as hard to address as with electric. I would guess that turning a gas station into a quick-charge electric station would require significantly greater cost than providing supercompressed air at each pump. While the air compressors that are currently at your local station probably wouldn’t be up to this task, one new compressor and a little plumbing could convert an entire station.

Refueling with compressed air from a station would take about the same amount of time as refueling with gasoline, but you would have to refill more often (every 120 miles).

MDI is considering whether an onboard air compressor would be worth the weight. The onboard compressor could be plugged to an electrical outlet for a six hour refilling process – comparable to recharge time of current all-electric vehicles.

Perhaps a removable compressor would be the best solution. You could leave the compressor (and its weight) in your garage normally. Every night you plug your car’s air tank into the compressor for a refill. But, if you’re going on a trip where you’ll have the opportunity to plug in overnight, then you’ll actually reinstall the compressor in the car.

These vehicles are reportedly equal to the electric vehicles in range.

Though technical problems are being worked out, company officials say the car is capable of 70 mph and a 120-mile range under normal city conditions, performance that is comparable to electric cars.

mdi car 2.JPGWhile 70 mph is a relatively low top speed, I could live with that in a commuter car. MDI is researching the possibility of hybrid versions that could use gas from the local station for longer trips. I would suspect that this would also raise the maximum speed.

Of course the exhaust is just air.

The minicat prototype (pictured) reminds me of the stackable cars that Phil recently wrote about. Why not combine the two ideas?

UPDATE: An MDI prototype was demonstrated on the Beyond Tomorrow program last fall.

UPDATE II: You could heat and cool this vehicle with a simple device called a vortex tube.

A Vortex tube has no moving parts. It separates hot and cold air flows from compressed air – giving you both heating and cooling. The heat is important because you wouldn’t have as much heat produced by this expansion engine as an internal combustion engine.

Cooling with a Vortex tube would mean that you can avoid the weight and complexity of an air conditioner compressor. You also wouldn’t have a refrigerant to maintain and disposal of. This would be a very green approach to the heating and cooling issues.

Also, as long as you’ve got compressed air in the tank, you could have heating or cooling without running an engine.

The blogger Engineer Poet has pointed out that vortex tubes are an inefficient way to heat or cool. That may be the case generally, but here we already have compressed air on hand, weight is at a premium (vortex tubes are light compared to the alternatives). Also, the volume of the cabin space is small – you wouldn’t need a lot of heating or cooling to stay comfortable.

  • Phil Bowermaster

    Well, it’s as cute as those stackable cars. I wonder how much impact the cute factor is going to have on the car market going forward? Everyone seems to be building it into their prototypes. Maybe cute is the new bitchin’. Somebody ask Virginia Postrel.

    Compressed air strikes me as more of a companion technology for moving a car (in the context of a hybrid) than it does a good standalone solution.

    As for having an on-board air compresser…if the car is a hybrid, why not turn the brakes into an air compression system? You can use compressed air to boost acceleration and then you can use the brakes to compress more air. This is the model currently used with gas/hydraulic hybrids.

  • Micah Glasser

    This idea strikes me as incredibly inefficient. I believe that within five years cars will be running on all electric via carbon nanotube supercapacitors or a similar technology. The reason is simply that this technology will be a reality very soon and this technology is far more efficient than any kind of mechanical solution (pneumatic or otherwise). Think about it – any air compressor station would have to be powered by an incredible amount of electricity any ways. That electricity would be put to use far more effectively by storing it in a capacitor and using it to power 95% efficient electric motors than converting the electricity into compressed air and then converting the compresses air into mechanical energy via pistons or turbines. I’m not sure what the numbers look like exactly but the compressed air model is probably about 10% efficient and that sucks (i.e. only about 10% of the electricity used to power the air compressor would end up being converted into usable horse power). Whereas electric vehicles have the potential of being about 80% efficient. Remember this – there is simply no kind of motor out there that can compare to electric motors in efficiency even in theory. Our hope lies in the efficient storage and release of electrical energy and we are well on our way to achieving this.

  • legion

    Micah is right about the efficiency of electric motors compared to other prime movers of vehicles.

    In situations where compressed air is already a plentiful resource, this type of engine might be practical. Likewise in environments where electric motors would be hazardous, or where any type of internal combustion engine would be hazardous, a pneumatic motor might well be appropriate.

  • http://pfdietz.blogspot.com/ Paul F. Dietz

    Some comments on this, and on the previous comments:

    ‘Efficiency’: these comments seem to be under the impression that efficiency is important here. It’s not very, by itself. The problem with plug-in vehicles is not that they use too much electrical power, it’s that the vehicles themselves are too expensive, have inadequate range, or take too long to charge. Compressed air vehicles help with all these problems vs. battery-electric cars.

    Having said that, compressed air car designs are inherently more efficient than your typical compressed air tools, since they do care about efficiency to the extent that it affects range. The designs I’ve seen use heat exchangers to expand the air quasi-isothermally, vs. the single stage, unheated expansion you see in simple air motors.

    One could get even more range with another heat source, so the expansion occurs at a temperature above ambient. Combustion could do this, or you could use a ‘thermal battery’ — an insulated thermal mass that is heated electrically when the vehicle is recharged. The thermal energy/mass in optimized materials can be many times higher than the electrical energy/mass in chemical batteries. This isn’t terribly efficient, but, to repeat, efficiency is not the point.

    My problem with these compressed air cars is that they aren’t good enough. The one shown in the original post is flimsy looking and too expensive. The market has shown that a vehicle has to be a lot like a conventional car or people won’t buy it. The Honda Insight hybrid, for example, is too small and compromised: it sold fewer than 700 units in the US last year. Electric cars like GM’s EV-1 did even worse when they were being trialed.

    If compressed air cars are to succeed at all, they will be as hybrid vehicles in which a conventional IC engine is modified with electronically controlled valves so it can also act as a compressor and air motor. Ford has been looking into this; adapting the engine to be able to store energy in an attached compressed air tank would add about 30 pounds to the weight of the vehicle. This should be significantly cheaper than an electric hybrid, since no expensive batteries or electric motor/generator are required.

    See here for an entry on compressed air/IC hybrids from my blog.

  • http://pfdietz.blogspot.com/ Paul F. Dietz

    Correction: added 30 kilograms, not 30 pounds. Still, that’s not very much.

  • Micah Glasser

    Efficiency is precisely the point. We already have a relatively efficient and convenient method of transportation with the ICE – why would anyone want to downgrade to a laughably inefficient and expensive ‘air car’? The reason why the all electric vehicle is better is because it is potentially more efficient which makes it potentially more effective and affordable. Granted that the technology is not quite in place yet but it is very close. With the newest supercapacitors an electric vehicle would have an enormous amount of horsepower and range; it would be highly efficient, and it could be recharged almost instantly. Obviously such a device would have a high market demand and hence it would be produced on a mass scale leading to drastically lower costs than at present. This technology is here now and will be going intp production in the next few years. The air car is a toy. It is a fun idea but to even consider it as some kind of challenger of the status quo is, in my opinion, a misunderstanding of our present circumstances.

  • http://ergosphere.blogspot.com Engineer-Poet

    The Air Car has its own list of problems, old and new:

    1. Range.
    2. Lack of infrastructure (gas stations are not presently compressor stations, and the capital investment would be greater)
    3. Energy losses in compression.
    4. The danger of explosion in an accident (even if the tank does not generate shrapnel, a wall of 3000-psi air can kill).
    5. Bulk of air tankage.
    6. Thermal difficulties such as freezing of moisture inside engines and valves.

    It would be relatively easy and cheap for places where people park to offer charging too.  Restaurants, motels, rest areas, parking lots, even the meter on the street could have a plug (and take debit cards).  You can’t increase the energy in a tank of compressed air, but there are ultracapacitors, at least two different Li-ion technologies and an up-and-coming lead-acid battery (Firefly Energy) which offer radically improved lifespan and charge/discharge performance.  AC Propulsion has shown range close to 300 miles at highway speeds using old-tech 18650 Li-ion cells; stuff on the shelves today already whips it several times over.

    You can beat the crash problem with fusible links and impact disconnects.  The charging time problem is gone.  And the batteries don’t make any noise.

    The air-car is going to have really good A/C, as long as it’s moving (it makes cooling like there’s no tomorrow).  However, I don’t see it being practical except as a taxicab in warm climates in the near term; the plug-in hybrid offers the best combination of performance and infrastructure compatibility for general-purpose cars in the near term, and batteries outperform air tanks for the long term.

  • Alice M. Ocskay

    Is there a current list of stations that offer Compressed Gas to run the car/buses that are now equipped to utilize this efficient energy???

    Are there opportunities for companies such as ours to get Grants/Low Interst Government Loans to develop these stations through out the United States today.

    Thank you for your attention to this matter!