something I've always wondered ..

[CalM]

Steam engines, Gas engines... Stirling engine? or is it a Stirling motor? I see a pattern here... A conversion of energy states, although an electric motor does convert electricity to heat...
An engine could be described as something that converts energy states and creates "work" all in the unit. A hydraulic motor, electric motor, etc, are being fed the energy that drives them. An engine takes a fuel, converts it, develops work out of it.
I am sure my description can be packaged much better by some one.

We may as well forget it, It's all just common usage.

There is an engine in my motor vehicle. I can live with that. Like we live with a gasoline engine powered air compressor, and an electric motor powered air compressor

But,..... there is NO SUCH THING as an electric engine, or a hydraulic engine, or an engine cycle. (Although all Internal Combustion Engines have cycles ;-)

SOMEONE on the internet is WRONG! ;-)

[PStechPaul]

The word "engine" derives from the same roots as "ingenuity", and "engineer", which apply to various mechanisms, devices, structures, and contrivances such as software search engines and database engines.

The word "motor" derives from roots involving "motion" and "mobility", which would apply to most devices which we identify as engines, but not so much to software.

What I've always wondered was why we get ON a ship, bus, train or airplane, but we get IN a canoe, boat, car or truck. And we get ON a bicycle or motorcycle or horse - or a bandwagon. We might get ON or IN a hay wagon. You could go ON a bed, or IN bed, or TO bed.

The opposites are also interesting. You get OFF the bus, train, or airplane, and OUT of the car. There may be others...

[Dave C]

We put cargo in a ship called a freighter but put freight on trucks. Ain't that bassackwards? Then we have freight and cargo on airplanes. My brain is starting to hurt.

[Noitoen]

In Portuguese language we use "motor" which is spelled the same for every device that produces motion being it electric, fuel or fluid activated. The word "engenho" is used for "device", usually complicated that "could" only be made a "professional" engineer or a "tinkerer" we usually call "engenhocas" so, I think it all comes down to what people start to call things and the name sticks.

We have a (relatively) recent addition to our vocabulary which is "telemovel" that everybody uses for cellphone and it is a registered trademark word from our first mobile phone company, so, language is very dynamic and even in a small country like ours, we have a lot of used names for the same things.

By the way, I'm not an Engineer, I consider myself as a Engenhocas