Clear as mud, eh? Actually, it is once you know how to read their inscrutable logic.
Part I ~~~~~~ The first set of letters seems to primarily identify the manufacturer. 'JL' being Jinlun, 'HW' being Huawei, etc. These can be omitted entirely, so if they aren't there, don't panic.
Part II ~~~~~~~ Okay, now it can get a bit tricky but bear with me.
The first number is always the number of cylinders - 1, 2, 4, 9, etc.
Then there *might* be a code letter for the layout of those cylinder(s), as follows: < > = Upright, or slightly inclined. (NB:- No letter or even a space) <V> = V-type, like Mr. Davidson's famous motor-bicycles <P> = Horizortal, or "Prone" if you prefer
This *might* be followed by a stroke identifier, like: < > = Four-stroke engine (NB:- No letter or even a space) <E> = Two-Stroke engine. Or one with nasty "Emissions", if it helps one remember
The last two numbers are the nominal diameter of the cylinder(s) in millimeters - 57, 49, 40,
Part Three ~~~~~~~~~~ So, now we have a seeming random series of two or three letters.
The first denotes how the engine is cooled: < > = Water-Cooled (NB:- this will result in only two letters in this section) <F> = Naturally Air-Cooled... and I'm all out of clever memonics <Q> = Forced Air-Cooled... ditto <Y> = Oil-Cooled... (probably means something in Chinese)
The next is a bit of a gift. It will always be an: <M> = Motorcycle - unless you are looking at jetskis, or something.
The finally letter is code for the displacement of the engine, in cc's. A <50 N 300 B 50 P 350 C 60 Q 400 D 70 R 500 E 80 S 600 F 90 T 650 G 100 U 700 H 110 V 750 I 125 W 800 J 150 X 900 K 175 Y 1000 L 200 YG 1100 M 250 YL = 1200
(Bad table, sorry)
Part IV ~~~~~~~ Still with me? Good, because the last section is simple. Some letters or numbers *might* follow a "-". If these are numbers, then there are the successive designs of the engine ('1' usually being omitted). If therer is a capital letter, then this will indicate a modification of the engine design that follows the letter - I've not seen any of these so far... so let's gloss over that.
Putting it in to practice ~~~~~~~~~~~~~~~~~~~~~~~~~
157FMI - would be an unspecified, single-cylinder, upright, 4-stroke engine, with a 57mm cylinder; naturally air-cooled, for motorcycles, and 125cc.
JL2V49FMM - would be a Jinlun, twin-cylinder, V-engine, 4-stroke engine, with 49mm cylinders; naturally air-cooled, for motorcycles, and 250cc.
JL244FMI - would be another Jinlun, twin-cylinder, upright, 4-stroke engine, with 44mm cylinders; naturally air-cooled, for motorcycles, and 125cc.
HW147FMF - would be a Hauwei, single-cylinder, upright, 4-stroke engine, with a 47mm cylinder; naturally air-cooled, for motorcycles, and 90cc.
1PE40QMB - would be an unspecified, single-cylinder, horizortal, 2-stroke engine, with a 40mm cylinder; forced air-cooled, for motorcycles, and 50cc.
152QMI - would be an unspecifiec, single-cylinder, upright, 4-stroke engine, with a 52mm cylinder; forced air-cooled, for motorcycles, and 125cc.
139MB - would be an unspecified, single-cylinder, upright, 4-stroke engine, with a 39mm piston; water-cooled (NB:- the missing letter), for motorcycles, and a 50cc.