Energy and environmental problems are more serious at present and people realized that the development of traditional engines is limited, and all countries in the world began studying all kinds of new piston engines. In history, there have been various types of engine to replace the traditional ones. Of all these, the rotary piston engines are the most and most successful, including constant speed types, planetary types, and differential types. But they felled finally. With the development of science and technology, some problems seemingly hard to attack are resolved, new rotary engines, especially the differential type engine, are attracted more attentions gradually. Differential velocity engines are the focus of researchers for its characteristic of high power density, compact, smooth and capability of using various fuels. The technology of sealing, lubrication, and distribution used in traditional engines can also help to promote it.