In the history of internal combustion engine development, hydrogen has been considered at several phases as a substitute to hydrocarbon-based fuels. Starting from the 70's, there have been several attempts to convert engines for hydrogen operation. With the development in gas injector technology, it has become possible to control the injection of hydrogen precisely and accurately for safe operation. Since the fuel cell needs certain improvements before it is widely used in vehicles, the conventional internal combustion engine has to play an important role during this transition. This book examines the performance characteristics, combustion analysis and emissions of a conventional compression ignition engine with hydrogen enrichment. Slight modifications were made for hydrogen enrichment which does not change the basic characteristics of the original engine. Comparison is made between neat diesel and hydrogen operation. Certain remedies to overcome the backfire phenomena were attempted like flame arrestor, NRV, etc.