You're asking really good questions :)

In my layman's understanding, those are related. Playing with the piston dimensions limits (but doesn't entirely prevent) fuel/exhaust mixing. Unburned fuel in the exhaust makes them dirty.

The site says:

> ... This expels the exhaust gasses out the exhaust port, usually located on the opposite side of the cylinder. Unfortunately, some of the fresh fuel mixture is usually expelled as well.

Another factor - since the fuel enters through the crankcase, it needs to be mixed with oil for lubricating the moving parts. That oil burns when it gets to the combustion chamber. Pretty sure that also increases unwanted emissions.

It's entirely possible modern CFD and chemistry could improve on those issues... I'd be curious if anyone else knows about recent R&D on two-strokes. They're hard to beat on power-to-weight.

Direct injection two-strokes like ETEC's avoid the burnt fuel in the exhaust by not injecting any fuel until the exhaust port is closed. They also typically have specific oil injection points reduce the amount of oil needed to be burned. They are generally as clean as modern 4-stroke engine.

Thanks for the reference! I'll have to read about those more.

Sounds like that also eliminates any fuss with oil/fuel ratios, since they're introduced separately.