oh yeah, it's making a comeback. As a rule of thumb, if you open up the cylinder bore surface for ports, it's a game over. HC, CO and soot will just go up.
The 2 stroke needs to have poppet valves as in the 4 stroke engines.
gas 2-stroke with intake ports (blown air) and exhaust valves in the head like 2-stroke diesel engine can reach acceptable cleanliness level if direct injection is used, but the engine becomes as heavy as a 4-stroke, so no gain there if weight is a concern
conventional gas 2-stroke engines can be pîston port, reed valve or rotary valve designs
That's not my understanding. Piston port engines have the intake port in the cylinder barrel, reed and rotary valve engines have the port on the crankcase, well below the cylinder...
You still have transfer ports in the cylinder to move the air (and fuel)from the crankcase into the cylinder and an exhaust port in the cylinder to remove burned gases. Intake port, reed valves, or rotary valves are just the different ways to control the movement of the air (and fuel) from the intake manifold to the crankcase.
reed valves generally float around the 10500 RPM mark, and make more torque. Piston port will give more rpm and performance but will sacrifice duration of port open timing. rotary valve will give most duration and RPM but will sacrifice low end torque I guess it depends on what the application the engine will be used in. Scavenging of a engine is the transfer of bottom end pressures to top! that is a diffrent kettle of fish. Loop charging a cylender will help wiht filling and HC in exhaust, and stratisified chargening will help eliminate most HC from exhaust. They do this by adding a non fuel charged air circuit in the carburator. There for allowing a "fresh" charge of air into the bottom end that will "purge" the cylender of exhaust and allowing the fresh air fuel charge from the bottom end to not end up as a wasted HC with about a 2%loss. As for EPA and enviroment, Stihl can build a 4 stroke engine that uses mixed fuel and no oil sump. A great idea, a 4 stroke that can be used in any angle of operation. As for BRP they can built a out board that is 2 stroke and can burn cleaner then most 4 strokes due to direct injection and less combustion chamber pressure/heat so less NOX, then most 4 strokes can provide. In saying that 2 stokes are the way to go but 4 strokes sill make more torque due to longer durataion of MEP on the piston head. I think they can be improved on again, this is just a small step in the evolution of the 2 cycle engine and internal combustion engines in general!!!
