Many people characterize two-stroke engines as not having valves, but that’s not true. Every two-stroke engine actually employs three different valve functions, but the valve mechanisms are easy to overlook because they do double duty, such as also being part of the piston. And the various methods of configuring these valves is a fascinating lesson in engineering problem-solving.
The 2-Stroke Combustion ChamberIn a traditional two-stroke engine, gasses enter and exit the combustion chamber through ports cut into the chamber wall. As the piston goes up and down, it covers and uncovers these cut-outs, opening and closing the ports. So the crown of the piston is, itself, the intake and exhaust valve. This means that the engine can only perform the sucking part of the suck-squeeze-bang-blow process during the lower portion of the piston’s downward travel (unlike a four-stroke, which draws a deep breath throughout the downward stroke of the pistion). A two-stroke needs to fill the cylinder with unburned oxygen and fuel as rapidly as possible once the piston descends far enough to uncover the transfer ports (the combustion chamber’s intake ports are actually called “transfer ports,” for reasons that will be clear to you a little later). Combustion is still occurring in the cylinder at this point, so the gasses in there are still expanding and creating positive pressure within the chamber — right where the fresh charge needs to go — exactly the opposite of the intake suction we need to do the taking-in. That means we need some method of pressurizing the incoming air-fuel mixture. That pressure needs to be pretty significant, because The incoming gasses must have enough pressure to not only overcome the positive pressure in the combustion chamber, but actually help push those still-expanding hot gases out the exhaust port in a timely manner.
The 2-Stroke’s Secret “Supercharger”What pressurizes the incoming air-fuel mixture? Two strokes not only cleverly use the sides of the piston as valves, they use the descending piston as a supercharger, pressurizing the crankcase below it as it descends. First, during the upstroke of the piston, vacuum created by the piston draws air through the carburetor, into the crankcase. Then, when the piston starts descending, bingo, positive pressure right when you need it. The trapped gasses are compressed until the piston drops below the tops of the transfer ports, at which time they rush upstairs to be compressed again and ignited. (Using the crankcase as a part of the intake tract is one reason why traditional 2-strokes are an emissions nightmare; the crank bearings need lubricating oil, which ends up mixed with the air and fuel as they all head up into the combustion chamber to be burned.)
“But wait!” I hear you say, “You said the intake mixture gets trapped — how? If the crankcase is open to the carb throat, what stops the gasses from just getting pushed back out into the atmosphere as the piston drops?”
Ah, that is the third valve function in a two-stroke engine, and the specific topic of this post.
Piston Port InductionIf you look at the animation above, you can see the earliest and simplest solution: the bottom of the piston skirt covers and uncovers yet another cut-out in the cylinder wall. This one is low enough that the top of the piston is still above it at the bottom of its stroke. But, as it rises, the bottom edge of the piston skirt slides away from this port, opening the crankcase to the atmosphere just as the pressure in our crankcase-cum-supercharger goes negative. Sucking commences. When the piston changes direction and heads back down, the piston skirt slides back over the port, trapping the the air/fuel mass in there to be pressurized. Voilà! We now have one chunk of cylindrical metal doing five different jobs simultaneously as it moves up and down: exhaust valve, cylinder intake valve, crankcase intake valve, supercharger compressor and, of course, transferring combustion force to the crankshaft through the connecting rod. Brilliant!
But there are two big limitations with this piston-ported crankcase: the first is that the piston must rise partially before it can draw in the fresh charge, and it must drop past the port again before generating any meaningful crankcase pressure. Secondly, intake timing is inherently symmetrical. That is, the piston must close the intake port the same number of degrees after top-dead-center as it open it before TDC.
Reed Valve InductionSo engineers decided that adding some other discrete crankcase valve was needed. One option was passive — the reed valve. Like any other one-way check valve, the reed valve is easy to understand: when there’s negative pressure on the proper side, it opens. Positive pressure from that direction forces it shut. It’s simple and has the flexibility of being locatable anywhere on the crankcase. It also allows our crankcase supercharger to “breath in” at least a little bit throughout the entire upward stroke, and at least begin to help pressurize the charge right from the moment the piston starts descending. However, since it is passive, it still is pretty tied to piston travel, and thus symmetrical.
Rotary Valve InductionThat brings us to the third type of intake valve. When some Hoon says “rotary,” you probably think of the Wankel and perhaps Mazda’s “Ours goes Hmmmmmm…” ads. But that’s something totally unrelated. If you have an interest in things two-stroke, rotary means rotary intake valves, aka disc valves, a very effective way to get more power out of the two stroke cycle. A rotary valve is a sort of rotating shutter that is positioned against the crankcase intake port. On motorcycles, this is typically a thin metal or composite disc, mounted to one side of the crankshaft. On smaller engines, a gear driven cylindrical mechanism or even one of the crankshaft flywheels is used as the valve shutter. The valve opens and shuts as a window in this shutter passes in front of the port.
Obviously, you can configure this cut-out in such a way as to open and shut it at any point in the crankshaft’s rotation, allowing even asymmetrical timing for the most accurate fine tuning of the intake. Compared to reed valves and even more-so compared to piston-ports, rotary-valve engines can produce more total power. More importantly, rotary 2-strokes can be tuned to generate that power over a wider rev range, or (in the case of racing motorcycles and constant-speed stationary engines), positively stratospheric maximum output in exchange for a peaky, knife-edged powerband.
For motorcycle use, not everybody thought the added combustion efficiency of a separate crankcase valving mechanism was worth compromising the elegantly simple manufacturing and maintenance efficiency of the piston-port design, and some piston-port engines are still made today. But reed valves and rotary valves have made it clear that you can ask a piston to handle too many functions.