Semantic difference between "engine" and "motor"

I like the definitions provided by WordNet and their definition is the way I've understood the difference:

• A motor is a machine that converts other forms of energy into mechanical energy and so imparts motion.

• An engine is a motor that converts thermal energy to mechanical work.

wordnetweb.princeton.edu

So an engine is a specific type of motor. That's why it's not incorrect to speak of a motorboat, or a motorcar, or a motor speedway, even if the boat or car is clearly powered by combustion.

Note that if there's no combustion, there's no engine. Purely electric cars don't have engines.

As nouns, motor can also refer to a nonspecific agent that causes motion: "happiness is the aim of all men and the motor of all action", and engine can refer to something used to achieve a purpose: "an engine of change", a railway locomotive, or a machine used in warfare: "medieval engines of war".

I picked the relevant parts from my dictionary and would say that these terms have been (over)used figurative.

Googles services are called a search-engine but never would be called a search-motor. In the field of software development we talk about graphic-engines, physical-engines etc. but never about motors.

motor |ˈmōtər| noun a machine, esp. one powered by electricity or internal combustion, that supplies motive power for a vehicle or for some other device with moving parts. • a source of power, energy, or motive force : hormones are the motor of the sexual functions.

engine |ˈenjən| noun a machine with moving parts that converts power into motion. • a thing that is the agent or instrument of a particular process : exports used to be the engine of growth.

In a strictly physics sense an engine converts energy into mechanical work.
A motor is a subset of engines that produce motion as the mechanical work.
So all motors are engines but not all engines are motors.

Sometimes they are used interchangeable eg. rocket engine and rocket motor. Motor is generally used for electrical devices but it's also used as an abbreviation for motorized eg. motorboat or motorcycle.

The real difference, is the fact that "motors" run on electricity, while "engines" run on combustion. However, many people interchange the usage of these words:

On the rare occasions we encounter one, we refer to a steam locomotive as an engine, the same word that we give to the motive power of an aircraft. But all electrical devices are driven by motors. In Britain at least, one’s personal transport is a motor car (with compounds such as motor trade, motor vehicle and motor sport), even though it’s always powered by an engine. Small boats may have outboard motors and then are often called motor boats.

From a mechanic:

So, what is the difference between an engine and a motor?
Motors are electric. Electricity powers motors.
Engines are powered by come sort of pressure or combustion, ie: steam engines, automobile engines, diesel engines.
Pretty simple, really.