SoEngine is the abstract base class for all engines. Engines are objects used for animation and behavior. They are lightweight objects that are connected between nodes, the clock, and other engines to form interesting behaviorial objects (e.g., a spinning windmill).

Engines are used to animate parts of a scene and/or to constrain one part of a scene in relation to some other part of the scene. An engine receives a number of input values, performs some operation on them, and then copies the results into one or more output fields. Both the inputs and the outputs can be connected to other fields or engines in the scene graph. When an engine's output values change, those new values are sent to any fields or engines connected to them.