This paper describes the experimental evaluation of a prototype free piston engine - linear alternator (FPLA) system developed at Sandia National Laboratories. The opposed piston design was developed to investigate its potential for use in hybrid electric vehicles (HEVs). The system is mechanically simple with two-stroke uniflow scavenging for gas exchange and timed port fuel injection for fuel delivery, i.e. no complex valving. Electrical power is extracted from piston motion through linear alternators which also provide a means for passive piston synchronization through electromagnetic coupling. In an HEV application, this electrical power would be used to charge the batteries. The engine-alternator system was designed, assembled and operated over a 2-year period at Sandia National Laboratories in Livermore, CA. This report primarily contains a description of the asbuilt system, modifications to the system to enable better performance, and experimental results from start-up, motoring, and hydrogen combustion tests. Passive synchronization of the pistons was successfully demonstrated through continuous motoring tests. However, operation was limited to less than a minute by the somewhat weak restoring force of the passive synchronization. Low equivalence ratio HCCI combustion with hydrogen was successfully demonstrated. Hydrogen/air mixtures with phi = 0.04 to 0.25 were compression ignited with compression ratios ranging from 20:1 up to 70:1. Net indicated thermal efficiencies between 50% and 55% were common with some results as high as 60%.