This is an interesting area. I looked at the Chicago Taxi articles.
I also did some looking several months ago on the prospects of LNG (Liquefied NG) as a replacement for Propane for home heating among other uses.
A road to riches !!!!:
Come up with a practical, affordable home type storage tank/system similar to that used for Propane (a Crude derivitave) and expensive.
Combustion of Propane and NG are essentially identical. Just a small change in the size of the hole in the burner orifice to account for the difference in heat content per cubic foot. No other change to equipment such as furnaces, fryers etc. necessary. Not sure on an internal combustion engine, but I'm sure is minor.
What's the Problem/Difference ???
First: LNG is produced and shipped in a "commodity like fashion" similar to crude. Need a specialized plant to compress the NG and store/ship on specialized "carrier/transport" ships. These ships are "unique" with high pressure/low temperature storage vessels/tanks.
LNG is a lot like Liquid Oxygen, Nitrogen or Hydrogen. Very low temperatures and high pressures required to maintain the "liquid state". If not in a "liquid state" the volume is so large that it is impractical to transport. That is why all you generally see for NG transport is gaseous state pipelines. Then, on the "receiving end" you need a specialized plant/facility to store/expand the liquid to a gaseous state and pump it into the distribution pipeline system.
Solve this problem for home heating storage, you will be another Bill Gates !!!
CNG: Compressed NG is a very close cousin to Propane except that it has storage problems as well. It can be compressed into "high pressure" tanks similar to those for welding, or Oxygen/Nitrogen gas distribution but the volume of gas contained is not adequate for say home heating storage.
This is the primary issue with CNG fueled vehicles. It is not a MPG issue but travel range achievable on the amount of fuel capable of being carried. [about half the range of a conventional gasoline vehicle] That is what is great about the move to city taxi fleets, buses, etc. etc. where local refueling stations will serve the needs of the vehicle fleet.
If NG prices stay down with all of the apparent availability this whole thing should mushroom !!!
Thanks Trades for re-triggering these thoughts. Now we have to figure out how to "make money" on it !!!!!