There should be enough engineers out there to answer this question.
What percent hydrogen would be required in the fuel-air mixture in an internal combustion engine for the engine to run? I understand that hydrogen burns in concentrations from 4% to 75%, but haven't found anything to answer the above question. This is not an idle question, so any educated guess is welcome.

Depends on engine, load, fuel delivery method, fuel form, etc. etc. See Hydrogen fuelled V-8 engine for city bus application and Hydrogen Use in Internal Combustion Engines. Running and being a usable engine are two different goals.

Thanks for the links. Now I have to find out why my program won't let me read pdf.