As the air is pulled through the carburetor, it speeds up to go through the narrow carburetor intake passage. The added velocity means that the intake air gains kinetic energy and, in order to maintain balance, the potential energy (temperature and pressure) drops. When the engine is running or hand-cranked, this lowered pressure is seen at the fuel jet, and the difference between this low-pressure area and the outside air pressure (seen at the fuel tank vent) "sucks" fuel into the carburetor as if your thumb were still there!

When the piston reaches as far upward as it can—TDC—the fully open rotary valve begins to close but draws fresh air and fuel into the crankcase for another 70°-90° of crankshaft rotation. The valve closes completely before the exhaust port begins to open. The crankcase and combustion chamber are again sealed. But the piston still has a ways to go before reaching BDC. It continues downward, compressing the fuel/air mixture inside the engine's crankcase.

That results in the crankcase's now being a high-pressure region. I left the following part out of "Engines 101" because it is not the major reason why the fuel/air mixture flows into the combustion chamber. But still, this high-pressure condition does exist, and for now, when there has been no combustion, it is the only transfer mechanism in operation. The piston continues compressing the crankcase mixture and increasing the pressure.

But before reaching BDC, the piston uncovers the transfer and boost transfer ports (bypass ports). The high crankcase pressure now has an exit. The fuel/air mixture under pressure rushes up through the transfer ports and into the volume just above the piston. Since the exhaust port is also fully open at BDC, some of this precious mixture is lost out the exhaust port. But some remains above the piston.

(One advantage of the Schnuerle boost transfer port system is that less incoming fuel/air mix flowing from these side-mounted ports is lost out the exhaust. The Schnuerle ports are not aimed straight out the exhaust port, as is the main transfer port.)

As BDC is passed, the piston travels upward, pushing more of the fuel/air mixture upward and into the already filled combustion chamber. Yet some still goes out the exhaust port—another inefficiency. Once the exhaust port closes, the piston begins to compress the fuel/air mix as it continues upward. If the glow plug is lit, and the fuel/air mixture is in the proper proportions, a prolonged, controlled explosion called "combustion" occurs.

The model two-stroke is part of the class of engines known as "combustion ignition," which includes diesels. But there is a subclass known as "catalytic enhanced combustion ignition" engines. Our engines fit into that category, as do many automobile diesels with "glow plugs" that are constantly receiving electric current (still not a true chemical catalyst effect) and are therefore always "lit."

It seemed easier to just call our engines "diesels" in the original article to differentiate them from model gas ignition engines rather than go through the true technical explanation, as I just did.