Efforts to develop rotary internal combustion engines have been undertaken in the past, and are continuing. One main advantage to be gained with a rotary engine is reduction of inertial loads and better dynamic balance. The Wankel rotary engine has been the most successful example to date, but sealing problems contributed to its decline. The Hanes rotary engine uses an eccentric circular rotor in a circular chamber with sliding radial vanes. This engine has never been fully tested and commercialized, and has a sealing problem similar to that of the Wankel. A more recent development, the Rand Cam engine , uses axial vanes that slide against cam surfaces to vary chamber volume. Currently under development, it remains to be seen whether the Rand Cam can overcome the sealing problems that are again similar to those of the Wankel. In the compressor and pump arena, reduction of reciprocating mass in positive displacement machines has always been an objective, and has been achieved most effectively by lobe, gear, sliding vane, liquid ring, and screw compressors and pumps but at the cost of hardware complexity or higher losses. Lobe, gear, and screw machines have relatively complex rotating element shapes and friction losses. Sliding vane machines have sealing and friction issues. Liquid ring compressors have fluid turbulence losses. The new design concept of the Ball Piston Engine uses a different approach that has many advantages, including low part count and simplicity of design, very low friction, low heat loss, high power to weight ratio, perfect dynamic balance, and cycle thermodynamic tailoring capability. These aspects will be discussed in more detail below.