A rotary engine without pistons is an internal combustion engine that does not use pistons as an alternative engine does, but uses one or more rotors, sometimes called rotary pistons. An example of a rotary engine without pistons is the Wankel engine.

As an alternative name for these engines to distinguish them from principles (usually until the early 1920s) aircraft engines and motorcycle engines also known as rotary engines. The basic concept of a rotary motor (without piston) avoids the reciprocating movement of the piston with its inherent vibration and its mechanical tension related to the speed of rotation. As of 2006 the Wankel engine is the only rotary engine without successful pistons, but many similar concepts have been proposed and are under different stages of development. The green engine is one of the most interesting discoveries of the new millennium. It has some unique features that were first used in engine manufacturing. This engine is a piston minus one with characteristics such as sequential variable compression ratio, direct air intake, direct fuel injection, the use of various fuels, etc.

The efficiency of this engine is high compared to contemporary engines and also exhaust emissions are almost zero. The importance of the engine lies in efficiency when considering the current global conditions of limited energy resources. Engine prototypes have been developed. The generators have been produced with the green engine. The United Nations Organization for Energy names all of them "elementary energies" as well as "conventional energies". Electricity is simply a "second energy" derived from these sources. At present, the energy consumed around the world depends almost entirely on the supply of the five main energy sources. Oil consumption constitutes about 60 percent of the energy used from all sources, making it the main energy consumer.