The Hydrogen Program at Sandia National Laboratories is developing internal combustion engine generators for application in series hybrid vehicles and stationary power units. The program consists of two approaches : investigating the utilization of hydrogen in a conventional crankshaft driven engine and in an advanced free piston configuration. The conventional engine program has taken the direction of utilizing the unique ability to spark ignite homogeneous fuel/air mixtures of hydrogen at low equivalence ratios (f approximate to 0.4) to achieve low NOx emissions and high thermal efficiency. The goal is to translate the indicated thermal efficiency of single-cylinder engines into multicylinder configurations achieving at least 40% brake thermal efficiency. When coupled to an electrical generator, the fuel to electricity conversion efficiency would be approximately 37%. A modified Perkins 3.152 Diesel engine (8.50 cm(3) per cylinder) is currently being tested and has achieved an indicated thermal efficiency of 45% in preliminary operations. The advanced free piston design utilizes a new approach to IC engine combustion. A double-ended free piston is used to compression ignite homogeneous fuel/air mixtures as it oscillates inside a closed cylinder. Through this oscillation, electrical energy is generated in a linear alternator which acts also to control the piston motion by actively adjusting the electromagnetic forces felt by the piston. Electricity is the output of this engine, and electronic control of the compression ratio is achieved with extreme mechanical simplicity. Fresh charge is introduced to the engine in a modern two-stroke cycle fashion. Initial testing in a single-cycle experiment has validated the potential of this alternative combustion system, as it has produced an indicated thermal efficiency of 56% with essentially zero NOx emissions, Development of these advanced generators continues.