IMHO it has nothing to do with size. A model engine can be a working engines when used to do work in the form of powering something. it may power a model boat a model airplane, a dynamo etc. A tiny gnat power co2 engine on a tiny airplane still does work.
an engine for the sake of an engine can do work as well.
A model engine may just be built to represent the real thing and run for the sake of running and not power anything.
I do not think folks here really worry about the definitions or semantics. we build engines. some just for fun, some to power things, some for the sake of education like my see though engines. some for the sake of saving and interpretation of history in the case of the guy that built the only known copy of the uss monitor engine and some build engines to learn machining and because they can. no right or wrong reason.
I do and have asked folks if an engine is a working engine . this is because there is more stress on an engines and its components that an engine that just runs at idle. These stresses are more significant at horsepower level than a small mouse power engines. so I like to know use if recommending material.
hope this helps.
What you need to know that is more important than a defintion is the law of scaling.
linear dimensions are directly proportional area are proportional to the square of the change and volumes and power proportionality to the cube.
If you take the plans for a larger, model engine, and scale them up...will the resulting engine be considered a small, working engine? For example, a one cylinder, model steam/gas/compressed air engine:
Bore: 1.00"
Stroke: 2.50"
Scale up the engine to:
Bore: 3.00"
Stroke: 5.00"
Has the "model engine" now become a small, working engine? What is different, generally speaking, about a working engine?
Just asking.
In your example you doubled the size of a model. it is twice as long and twice as high and twice as wide.. the area of the piston is 4 times as big the power increased 8 times and the weight of the engine also increased 8 times. your material cost likely increased 8 times as well.
unless you need a big engine I recommend beginning work with an engine of about 1/2 inch bore (3/8 to 3/4) the engine is small enough for most lathes. the parts are not too tiny . it is not difficult to get running and it will not suck the tank of a compressor empty in minutes.
Tin