The Hot-Air Car

Those of us of a certain age remember the conspiracy theories that involved geniuses in garages inventing fabulous carburetors that would enable a car to get 100 or 200 miles per gallon -- but then the serpent crept into paradise: Big Oil bought the patents and kept the wonder off the market.  A Google on "mythical 100 mpg carburetor" produces over 600,000 hits, including this, a succinct explanation of what kept those wonders off the market: it was not Big Oil, but the First Law of Thermodynamics.

Such rumors swirled for decades.  I met a man 30 years ago who swore that his dad and uncle had invented one of those miracles after World War II and sold it to Standard Oil for $50,000, never to hear of it again.

Carburetors are passé now, so those who wish to strike a blow for the green movement, love conspiracies, and love to hate Exxon and BP have moved on to cooler technologies, like hybrids and electrics and hydrogen cars and compressed air vehicles.  Tata Motors of India announced a compressed air car several years ago, but its manufacture always seems to lie just over the horizon -- a conspiracist's dream.  Big Oil must be keeping it off the market, as an email that I received last week claimed.

Compressed air is a fantasist's dream.  Zero emissions!  Big Oil can't patent the air, can they?  A Google on "tata compressed air car" returns well over 200,000 hits, like this one from Gizmag: "The Air Car - zero pollution and very low running costs."  Money for nothing and chicks for free -- the dreams never die.

If "journalists" wrote "coal-fired" instead of "zero emissions," they would hardly generate the excitement that keeps people dreaming.  Ask a greenie if she has hugged a coal-fired electric plant lately.  Here are just some of the problems with the compressed-air car:

  • Not Big Oil, but Big Government, aka National Highway Traffic Safety Administration, will keep it out of the USA: with a frame made of PVC and glue, this thing would crumple like a tinfoil hat in a crash.
  • When full, the tank will be pressurized to 4,500 psi.  When a tank ruptures, as can happen in an accident, the energy in the compressed air is released instantaneously, unlike a gasoline fire.  The energy required to drive a car 200 miles is roughly 40 kw-hr, or the energy contained in 34 pounds of high explosive.
  • There are some significant hurdles here, such as the First Law of Thermodynamics.  What do we need all these laws for, man?  Can't we just do our own thing?

But I'm no engineer, so I asked a man who has spent his entire career designing power-generation systems.  He emailed back the numbers:

Compressing air (four stages of compression, 4/1, 88% stage efficiency): 60% efficiency

Expanding compressed air and converting to shaft power in car: 60%

Making the electricity to run air compressor: 40%

Electrical transmission efficiency: 93.5%

Net efficiency = 0.6 * 0.6 * 0.4 * 0.935 = 0.1346 = 14% primary energy source to shaft power

By comparison, a piston engine car that gets 30 mpg runs at 16 to 20% efficiency, while an electric car comes in at about 21% (70% charging efficiency, 80% battery to shaft, 40% power plant, 93.5% transmission).

If electricity is generated in a coal-fired station, each 200 miles the car drives will require 89 pounds of coal or the generation of 300 pounds of carbon dioxide.  The equivalent consumption of gasoline at 30 mpg for 200 miles is 6.7 gallons (44 pounds).  This combustion process uses 37.6 pounds of carbon, generating 138 pounds of carbon dioxide.  Of course, if the power is generated by nuclear or solar, the carbon generation is zero.  But then, environmentalists do not like nuclear, and solar is neither available nor cost-effective.

It would be so easy for "journalists" to ask an engineer to verify the claims in their stories, but then they would not have a story.

Here's an idea.  Maybe we could get the EPA to issue permits for that backlog of applications for mining pixie dust they've been sitting on.  Pixie dust, or PD, is said to be the ideal propellant: organic, renewable, infinite, noncarboniferous, earth-friendly, and producing zero emissions.  Oh, and harvestable with no harm to the pixies.  Yay, a win-win-win-win-win-win and win!  I'll be circulating a petition next week to get the EPA to do the right thing.  Free the Pixie Dust Now.  Just imagine -- a world of infinite energy living in peace.  Perhaps one day you'll join us?

Henry Percy is the nom de guerre for a technical writer living in Arizona.  He may be reached at saler.50d[at]