There a several companies that offer bi-fuel options for diesel engines. Gas (usually natural gas) is injected into the intake manifold. The amount can be up to 50%, depending on the quality of the gas, without derating the engine. No special modifications are needed for the diesel fuel system, although, vibration sensors are recomended to detect knock (detonation) which is the main concern of using this tecnology. It is a way of lowering the diesel costs by using a lesser expensive gas. I have seen a couple of these installations running with little problem. Gas Tecnologies Incorporated in Fort Lauderdale Fl. is one of the companies offering these systems.