What is Engine?

Legal Definition
An engine or motor is a machine designed to convert one form of energy into mechanical energy. Heat engines burn a fuel to create heat, which then creates a force. Electric motors convert electrical energy into mechanical motion; pneumatic motors use compressed air and clockwork motors in wind-up toys use elastic energy. In biological systems, molecular motors, like myosins in muscles, use chemical energy to create forces and eventually motion.
-- Wikipedia
Legal Definition
This is said to be a word of very general signification; and, when used to an act, its meaning must be sought out from the act itself, and the language which surrounds It, and also from other acts in pari materia, in which it occurs. Abbott, J., 6 Maule & S. 192. In a large sense, it applies to all utenslls and tools which afford the means of carrying on a trade. But in a more limited sense it means a thing of considerable dimensions, of a fixed or permanent nature, analogous to an erection or building. Id. 182. And see Lefler v. Forsberg, 1 App. D. C. 41; Brown v. Benson, 101 Ga. 753, 29 S. E. 215.
-- Black's Law Dictionary