It runs on piss!!

http://www.automobilemag.com/auto_shows/naias_2006/0601_mercedes_benz_e320_gl320_bluetec

Mercedes-Benz's new Bluetec diesel technology uses urea--derived from
urine--to neutralize particulate emissions and make its GL320 and E320
Bluetec models the cleanest diesels in the world and fifty-state legal
in the States. The urea solution must be replenished during normal
maintenance stops, but there's no word yet on a rumored do-it-yourself
kit based around a compact urinal.