So this quote "...All of the energy concentrated in one gallon of gasoline is enough to charge an iPhone once a day for almost 20 years...." got me thinking; why in the heck don't we have a micro-sized bio-diesel engine in our cell phones?  

It might sound crazy, but the energy density in bio-diesel surpasses that of gasoline, bio-diesel is not toxic, it is not flammable (it is combustible, but there's a difference - look it up if you don't know), produces low emissions, and it's easily produced.

Conservatively, an ounce of fuel would provide enough energy for more than a  weeks worth of smartphone use.  One oz is about 300 watt/hrs of energy, and the average smart phone uses about 1 watt at full throttle and only a few milliwatts the majority of the time; I'm assuming 136k btu/gal energy density.  

In a time where we are building nano-machines, a micro engine like this seems like it could be feasible.  Who's smart enough to kickstarter this?  
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