By Kenichi Yamamoto from his book "Rotary Engine" 1981 1.3 REQUIREMENTS FOR A PRACTICAL ROTARY ENGINE Although various types of rotary engines have been developed to date, there are few that do not qualify as internal combustion engines or do not qualify as a practical engine even if they satisfy the qualifications of an internal combustion engine. The requirements for a rotary engine to be qualified as a practical internal combustion engine can be summed up in the following five items. The practicality of various ideas on rotary engines can be judged by evaluating them by these criteria: (1) Every moving part, including the timing mechanism, should make a rotating motion. A mechanism having a reciprocating inertia will increase mechanical noise and vibration, and will work against high speed and high revolution. [In addition mechanisms that reciprocate generate reversing loads. Reversing loads lead to reversing stresses. Reversing stresses lead to metal fatigue and limited life. The Wankel rotary engine has no reversing stresses therefor the life of the major parts such as the eccentric shaft and rotor are unlimited unlike, crankshafts, connecting rods, pistons and poppet valves. Paul Lamar] Accordingly, structures that require intake and exhaust valves and mechanisms using the rotors oscillatory motion are not desirable. (2) Gas seals or the working chamber should be three dimension-ally reliable. The gas seal mechanism of a rotary engine should be constituted by connecting the individual seals three dimension-ally. Among the many ideas we come across are those that don't show such three dimensional thoughts. (3) Appropriate gas exchange of intake and exhaust should take place. Together with having a mechanism that can correctly open and close the ports, sufficient time for intake and exhaust should be provided especially for high speed and high revolution. Among the ideas on rotary engines, there are some that ignore this point. (4) Every component part should have the strength to endure high speed and high pressure. As the component parts are exposed to high pressure, high sliding velocity and high heat load, every part should have sufficient allowance in size and in shape. (5) Sufficient cooling and lubrication should be provided. In order to qualify as an internal combustion engine, although this is related to above (4), durability against high heat load, high sliding velocity, etc. are required. Therefore, the structure should be simple, and the ideas on rotor cooling, lubrication of seal parts, and oil seal structure should be those that are given adequate thought. Also, it is desirable for a practical rotary engine to have a simple and compact structure. When we evaluate the various type's of rotary engines devised to date in view of the above mentioned requirements, the NSU Wankel type rotary engine is the engine that best satisfies the above mentioned requirements.