Diesel engines remain the dominant means of ship propulsion. In recent years, they have shown continuing progress in areas of refined manufacturing, higher power output per unit of engine volume (power density) and per unit weight, reduced fuel and lubricating oil consumption, improved tolerance for fuels of low quality, simplified maintenance requirements, suppression of vibration, application of electronic monitoring and control, and reduction of exhaust emissions. There have been promising developments in the use of natural gas as a marine fuel. There has been a dramatic consolidation in the diesel-engine manufacturing industry, resulting from the increasing sophistication (and cost) of the research, design processes, prototype testing, manufacturing methods, and after-service sales obligations that have become part of the manufacturing process, leaving fewer engine designs of increasing similarity to be built worldwide by closely cooperating manufacturers.