Anyone who has worked with two-stroke racing engines automatically thinks about cylinder scavenging as cream pours into coffee or milk into tea. The pleasant white stuff becomes the analog of fresh charge entering the cylinder, which is represented by the cup. I always pour at the edge because then the flow will descend to the bottom of the cup, turn to flow across its bottom to the other side, then up that side to appear as a white upwelling in the dark drink.

This is an upside-down analog of what happens as a two-stroke’s piston begins to uncover the transfer ports in the cylinder wall. Fresh charge, propelled by pressure in the crankcase, emerges through the transfers to form a coherent stream that crosses the piston crown, hits the non-exhaust cylinder wall, and flows up it. That flow then hits the cylinder head—the bottom of the coffee cup—flows across it to the exhaust cylinder wall, and down it to the exhaust port.

The goal, after all, was to fill the cylinder as fully as possible with pure fresh fuel-air mixture while driving out inert exhaust gas.

While performing the cream or milk ritual, I have the pleasure of anticipation of that first sip and the cheerful energy it releases. In similar fashion, so many of us have paused at the parts washer while cleaning a two-stroke cylinder to direct the stream of solvent into each transfer port in turn to see the resulting flow direction. Ideas come to us humans in moments like these.

In the two-stroke case as modeled by pouring milk or cream, much of the flowing stream of white stuff survives the pour to reappear on the surface at the far edge of the cup. This lack of mixing was essential to two-stroke power, and tremendous work went into exploring minute variations of transfer-port direction, number, and shape. The goal, after all, was to fill the cylinder as fully as possible with pure fresh fuel-air mixture while driving out inert exhaust gas. In fact, there was mixing, and that was the eventual downfall of carbureted two-stroke engines. Inevitably, some fresh charge did mix with the spent exhaust gas in the cylinder and get carried out the exhaust port and pipe to atmosphere. There, the EPA detected it as unburned hydrocarbons and said, “No.”

Modern two-strokes either wait to add fuel until after the rising piston has closed the exhaust port (direct fuel injection) or inject the fuel into the transfer ports too late in the cycle for it to reach the exhaust (Transfer Port Injection). The result is a simple low-emissions engine that was briefly studied by the auto industry in the late 1980s and early 1990s. A few motorcycles are built with such systems today.

Idle thoughts as you pour cream or milk have the virtue of being free and spontaneous.