Wow, those Arrow engines are SWEET! Nice website literature, also!
I could really see myself turning into a dairy farmer, and setting up a big methane digester system to feed one of these engines, and power the whole operation (heat, electricity, etc.) with cow poop. There is quite a bit of that type of methane production in Europe, but it's only just starting to catch on here.
I did a science fair experiment using poop from my pet rabbit to make methane back in middle school. Didn't work half bad!
I think this would be an even better renewable resource than biodiesel. Now, if we could only harness the methane directly out of the cow, we could assist in reducing greenhouse gasses somewhat (although we'd be making CO2 by burning the methane...)
Chris