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 by Frank Granelli |
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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. How far upward the piston travels determines the engine’s compression ratio. This is the ratio of the entire cylinder volume above the piston when at BDC to the remaining cylinder volume with the piston fully raised at TDC. For example, if the volume above the piston is 10 times larger when the piston is at BDC than the cylinder’s combustion area at TDC, the compression ratio is 10:1. The higher the compression ratio, the more power is produced by the final fuel/air combustion. But compression ratios can be too high, causing preignition, hot running, burnt glow plugs, and piston damage. Most sport engines do not have high compression. The engine is now 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. Diagram 4 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 air/fuel mixture is “pulled” through the transfer ports into the low pressure in the cylinder at the same time as 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, hence the amount, of the fresh fuel/air mixture that is drawn into the engine. As the scavenge action is completing (the momentum of the exhaust gases is exhausted) and the pulling of intake from the crankcase through the transfer ports is ending, the induction valve opens. This helps start the flow of fresh fuel/air mixture into the crankcase for the next power stroke. At extremely low speeds, such as like idle, the scavenging action goes to completion and you are back to having pressure in the crankcase at the moment of the induction valve’s closing because of the descending piston. You can sometimes tell this is happening as the engine spits fuel from the carburetor at slow speeds. back to top Therefore, the scavenge effect is the major force our engines use to put fuel and air into the combustion chamber. Yet crankcase pressure does play an important part. Together, these alternating, thermodynamically produced high- and low-pressure conditions allow our engines to run. Several exhaust systems are available that will increase the scavenging effect. They are for a later discussion, 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 in Diagram 4.
Click on photo to view large image with caption It may seem strange to put combustion pressure against the piston’s upward movement, but combustion takes time and our fuel doesn’t burn all at once. Therefore, the prolonged explosion used to burn as much of the fuel/air charge as possible is made possible by the “advanced timing.” The relationship between the piston’s movements and ignition is a delicate balance. Too much advance and the piston may be damaged. Too little means insufficient combustion occurs. However, running an engine too “lean”—not enough fuel and too much air in the mix—produces extra heat that can change this delicate balance. Hot engines can experience timing that becomes so advanced that detonation occurs. This means 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. Readjust the high-speed mixture. The piston is rapidly moving up and down. But the propeller needs to rotate. Therefore, the piston is connected to the crankshaft by a device called, for some strange reason, a “connecting rod.” The piston’s up-and down-movement is converted to a rotating crankshaft that turns the propeller. At least one side (sometimes both sides) of the connecting rod uses a bronze bushing that must be well lubricated. The connecting rod is the part that breaks if a too-large or too-small propeller is used. When the rod breaks, the rest of the engine is usually destroyed. As I mentioned, the combustion process creates a lot of heat, which is used to keep the glow plug glowing even when the battery is disconnected. In effect, the glow plug is working to catalytically create each combustion cycle just as a diesel’s combustion, created by a high compression ratio, creates its combustion. Neither requires outside ignition energy to continue the process. Simple and elegant, with an explosion occurring every time the piston moves up and down, two-stroke engines are the definition of power and reliability. |
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