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 by Frank Granelli |
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Do you recall the intake process I described in the preceding? Consider the same process but with the engine running at full speed. The piston is at BDC with most of the exhaust gases gone, receiving a fresh charge of fuel/air from the crankcase into the now-vacant volume above the piston, right? Well, not really. The exhaust port opens only slightly before the transfer ports, called the "exhaust lead" or "blowdown." The exhaust gases have not fully exited the cylinder when the transfer ports begin to open. The relationship between these openings is part of the engine's timing. The accompanying illustration summarizes many sport engines' timing in this regard. In practice, this timing means that fresh fuel/air mixture is flowing into the cylinder even as exhaust gases are exiting. Why would an engine designer do this? The hot, still expanding exhaust gases are exiting at a high velocity. This forms a low-pressure area just above the piston, "behind" the exiting exhaust gases. The fresh fuel/air mixture is "pulled" through the transfer ports, into the low-pressure area in the cylinder at the same time the descending piston is compressing the mixture in the crankcase and pushing it into the bypasses. We say the exhaust gases "scavenge" the fuel/air mixture into this section. The scavenging effect increases the velocity, and hence the amount of the fresh fuel/air mixture that is drawn into the engine. Just as the scavenge action is finishing (the exhaust gases' momentum is exhausted) and the pulling of intake from the crankcase through the bypasses is ending, the rotary valve opens. This helps start the flow of fresh fuel/air mixture into the crankcase for the next power stroke. At exceptionally low speeds, such as idle, the scavenging action goes to completion, and you are back to having pressure in the crankcase at the moment the rotary valve closes because of the descending piston. You can sometimes tell that this is happening as the engine spits fuel from the carburetor at slow speeds. Therefore, the scavenge effect is the major force our engines use to put fuel and air into the combustion chamber, but crankcase pressure does play an important part in the initial charge's transferring into the cylinder. Together, these alternating, thermodynamically produced high- and low-pressure conditions, neither a true or even partial vacuum, allow our engines to run. Several exhaust systems are available that will increase the scavenging effect. I will discuss them later, but now you understand how and why they could increase an engine's power by increasing the scavenging effect. During the charge cycle, some fresh fuel/air mixture is drawn out the exhaust along with the escaping gases. This is lost power and poor fuel economy that engine designers strive to recover as much as possible. An additional complication is that the combustion occurs before the piston reaches TDC. It continues even when the engine reaches TDC and ends at or after TDC. The amount of advance is shown in the drawing. It may seem strange to put combustion pressure against the piston's upward movement, but combustion takes time, and our fuel doesn't explode all at once. Therefore, the prolonged explosion used to burn as much of the fuel/air charge as possible is made achievable by the "advanced timing." The relationship between the piston's movements and ignition is a delicate balance. Too much advance, and the piston is damaged; too little means that insufficient combustion occurs. However, running an engine too lean produces extra heat that can change this delicate balance. Hot engines can experience timing that becomes so advanced that detonation occurs, meaning that the fuel/air mixture ignites before it should. This condition may sometimes be identified by a loud "frying egg" sound (crackling) as the engine is run at full speed. When you hear this sound, your engine may be in for problems from overheating and detonation. Land and readjust the high-speed mixture. I am going to stop discussing the process at this point. The preceding is a far more complete and technically correct explanation of two-stroke engines' operation than I wrote in "Engines 101." In deference to that article, this installment has required nearly 2,000 words to cover the same topic as did its roughly 600 words, without adding new operational information that less-experienced RC pilots could use to run their engines better. The long explanation would have left little space for all the other topics I discussed in "Engines 101," but the shortcuts caused confusion that would have been avoided with the longer version. Yet even this explanation covers only the basics of our easy-to-use but complicated machines. If you want to learn more, Dave Gierke has written the excellent engine book Two-Stroke Glow Engines, Volume 1, available directly from him at 1276 Ransom Rd., Lancaster NY 14086. It is $18.95 including shipping. In "Engines 101," I erred in writing that the piston in an aluminum-brass-chrome (ABC) engine is larger in diameter than its respective cylinder. I took the liberty of exaggeration to make the thermal expansion point. Actually, the piston is the same diameter as the cylinder, which still expands more than the piston to allow space for the piston to move efficiently. Sometimes the piston is larger in new engines, but by no more than one to two ten-thousandths of an inch. This quickly wears to the same diameter. I took poetic license to make the point in few words, but it was technically incorrect.
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