A system for retrofitting an internal combustion engine to use a proportion of hydrogen gas and existing fossil fuel is disclosed. The system is comprised of a source of hydrogen gas and means for delivering hydrogen gas to the combustion chamber of the engine. The engine operating parameters are adjusted and the delivery of hydrogen is controlled to provide selective introduction of hydrogen gas throughout the engine's operating cycle. The source of hydrogen gas can comprise an onboard hydrogen reformer, or hydrogen and carbon monoxide can be reformed inside the cylinder from an on board carrier of hydrogen. A single point injector placed at the intake manifold in close proximity to the engine intake valve can be used to mix the hydrogen gas to the air/fossil fuel mixture or sequential injectors can deliver hydrogen in close proximity to each engine intake valve, or directly into each cylinder.