A while back this subject came up and tweaked my curiosity so I stick a brass tube and a nail in some fuel and measured the resulting voltage and current. It was 150 mV and 0.3 uA. I don't know what it woudl be with aluminum... but it woudl be something, and the brass is the material that is corroding as electrons are stripped away. Using aluminum tubing would help matters as well.
http://www.rcuniverse.com/forum/m_37.../tm.htm#379179