My knowledge is regular car engines. On a typical combustion engine, you don’t want oil spraying or leaking into the combustion chamber as you would develop carbon deposits and degradation of performance.
How do these mix engines work both from a function aspect (lubricating via the gas) and maintenance (not building up catastrophic amounts of carbon)?
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Oil designed to mix with gasoline (usually a two-stroke) isn’t the same oil as used in a separate oil system (usually a 4-stoke). If you put the wrong oil in the wrong thing it will eventually screw things up.
Two-stoke oil is designed to mix nicely into the gasoline and burn (relatively) cleanly to avoid exactly the problems you’re talking about. Since it only goes through the engine once, it doesn’t need much longevity or thermal stability and it’s OK (and even desirable) for it to break down rapidly. Exactly what you *don’t* want in a segregated system where the oil will potentially spend a year or more cycling around.
Edit: for the lubricant part, two-stokes run the air/fuel mixture through the crankcase (under the pistons, where the crankshaft is) to expose all those moving bits to the oil.
I think you are talking about two stroke gas engines here. These do not have a lubricating oil system but instead use the fuel air mixture to lubricate the engine. Gasoline is too “dry” to be used as lubricant which is why you add lubricating oil to the fuel. This does mean that lubricating oil does get into the combustion chamber. But the engine is tuned for this. In order to avoid carbon buildup the fuel air mixture have more air. The carbon will therefore burn in the extra oxygen rather then build up. But in general oil does not burn very well and you do end up with a smelly blue exhaust rather then the transparent clean exhaust from normal engines.
You do also see fuel being used as a lubricant in diesel engines. Specifically in the fuel pump and injectors. But diesel is thick enough that it can act as a useful lubricant if the parts are built with the tolerances for it. This is why diesel engines will not run on gas, but from experience they do handle the mix of gas and lubricating oil you use in two stroke gas engines.
Two stroke engines use a different type of bearing usually ball or needle bearings. It doesn’t require as much lubrication as an oil bearing which requires constant oil pressure. So having a small amount flowing through the air is enough to cool and lubricate the engine. The oil is specifically formulated not to create excessive deposits and to burn well. It also lubricates the cylinder walls on top and bottom of the piston so that helps with lubrication.
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