[Galveston1]

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.

[Scleros]

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.

[Galveston1]

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