A Suggestion
"Alternative Fueks", in my view, fall into 3 broad categories:
A) Liquid fuels
1) gasohol
2) biodiesel
3) alcohols
B) Gaseous fuels
1) Propane
2) LNG
3) Hydrogen
4) Other
C) Electric - for purposes of discussion, electricity is a de facto "fuel"
1) Mains sourced (charging battery packs)
2) Fuel cell generated (lots of options becoming available)
3) Solar array produced
4) Onboard generation (which might include fuel cells)
5) Other sources - wind, microhydro, methane digester/engine/generator
For transportation purposes, electricity is in many ways ideal. Compact, cheap to produce, efficient power exchange from electricity to mechanical motion. It has serious drawbacks, as well. Principally, power storage densities are relatively low as compared to various fuels. Power storage systems are relatively expensive, and for a reasonable life-span to be achieved, very expensive. Range is seriously limited for a pure electric vehicle.
Gaseous fuels have advantages and they have drawbacks as well. LNG is relatively cheap - but not readily available in many areas, and as primarily methane, it suffers from a pair of physical drawbacks; low specific energy, and incompressability. Getting a reasonable range takes a relatively large and heavy tank. Propane has the advantages that LNG does, and rather less of the drawbacks - higher specific energy, more easily compressed to liquid state, relatively low tankage weight. Hydrogen suffers from a very serious set of disadvantages, and benefits from some very real compensating advantages. First, it is difficult to work with - it will leak through ANYTHING, even steel. Second, it is extremely inflammable. Third, it has to be highly compressed for effective storage, yet the more compressed it is, the faster it leaks. Fourth, it reacts very readily with almost anything - it's difficult to work with in engines because it quickly degrades lubricants. In compensation, it burns cleanly, it has an excellent specific energy, and as a gaseous addition to a liquid fuel/air charge in an engine, it works very well indeed.
The various liquid fuel alternatives would require volumes to discuss adequately. In brief, pure alcohols (ethanol or methanol) have low specific energies, and from most more "traditional" sources actually cost more energy to produce than they provide. There are exceptions to that. They poise special concerns for handling (seals, etc) but those have been dealt with, for the most part. Biodiesel is not as straightforward a subject as it might appear. Biologically sourced oils (effectively, low molecular weight fats) might come from any of hundreds of plant sources, of wildly varying quality and production rates, or might well include animal sourced oils (whale oil lamps, anyone?) So, what's the source? What's the molecular weight mix (which determines viscosity, flammability, etc)? There are a multitude of questions to be asked (and answered) before one can reasonably discuss details.
