Both engine and motor refer to a device used to convert some form of energy into mechanical motion. They are sometimes used interchangeably in casual conversation, but, technically speaking, they do not mean the same:
motor = a device that converts electric (or hydraulic) energy into motion
Some authors may disagree, but the fact is, the two words are almost never used the other way round. The following collocations with “engine” are commonly found in literature:
while the corresponding expressions with “engine” replaced by “motor” are virtually non-existent (“petrol engine”, for instance, is about 50 times more common in literature than “petrol motor”). On the other hand, only the following adjectives are typical for the word “motor”:
and they both refer to electric motors.
‘Motor’ as an adjective
Somewhat paradoxically, when used as an adjective, the meaning of “motor” is almost the opposite to that of the corresponding noun. It is mostly used in the phrase “motor vehicle” (and in compounds like “motorboat” and “motorbike”), which means:
In other words, motor vehicles are cars, buses, and other vehicles we usually associate with engines, but we never use “engine vehicles” in this sense. In the same vein, the British call their highways “motorways”, not “engineways”.
Note that “motor” is also used in biology in the sense of “connected with movement of the body that is produced by muscles” so that we can speak of motor skills or the motor cortex in the brain. This, of course, refers to the root “mot” in “motor” (which is the same as in “motion”), not to the presence of motors or engines.