Beyond Clever - How Two Stroke Exhaust Pipes Really Work

Video Description

To try everything Brilliant has to offer for free for a full 30 days, visit https://brilliant.org/d4a/ . You'll also get 20% off an annual premium subscription. If you have ever observed dirt bikes you have probably noticed that some of them have simple uniform diameter pipes coming out of the engine while some other bikes have very different-looking pipes with large bulges and great variations in diameter. A two stroke exhaust pipe has a weird shape with greatly varying diameters and a large bulge in the form of the expansion chamber. You can only find it on two-stroke motorcycles and never on four-stroke motorcycles. To understand why we need these pipes we must first observe the two stroke engine in a bit more detail. Unlike a four stroke engine a two stroke engine has no camshafts or valves, it’s cylinder head is essentially just a cap. Despite this simple construction the two stroke manages to fire during every single engine revolution which means that we have a combustion event every 360 degrees of rotation whereas in a four stroke we have a combustion event only every 720 degrees of rotation. All of this means that a two-stroke is capable achieving a better power-to-weight ratio than a four stroke. When the piston first uncovers the exhaust ports and the blowdown phase begins the exhaust gas of course rapidly bursts out through the tiny opening. This of course creates a loud powerful sound and as we know sound is a pressure wave so we get a pressure wave coming out of the engine going through the exhaust pipe. Various explanations you might have encountered may have used terminology such as exhaust pulse or combustion pulse or similar and while these are not necessarily incorrect and can help you visuallize things they become useless later in the explanation. What comes out of the engine when the exhaust port opens is in fact a sound wave which is a pressure wave. Here you can see an image shot using a special photography method that shows the end of an exhaust pipe releasing exhaust gas into the atmosphere. Here you can see the pressure wave which is only later followed by the actual exhaust gas. So the pressure wave is traveling through the pipe. It remains pretty constant through the initial uniform part of the pipe. After this, it reaches the diverging or the expanding section of the pipe. So what happens here? Here we have a change in the medium. Or medium is gas and if the pipe diverges our volume increases which means that the molecules of the gas in this space are further apart, in other words, the gas is less dense here. And whenever a wave encounters a different medium or a change in the medium itself the wave or part of the wave gets reflected back. You have probably experienced echo at some point in your life. Echo is simply a sound wave that got reflected back as it encountered a different medium. The sound wave travels through air and reaches a wall. A wall is a different medium and so part of the wave gets reflected back. But here’s the important part: When moving from a medium of higher acoustic impedance to a medium of lower acoustic impedance part of a longitudinal wave will get reflected back AND it will also undergo a phase change. When that longitudinal compressive wave encounters the diverging section of the pipe it transitions from a higher density gas to a lower density gas, or from a medium with a higher acoustic impedance to a medium with a lower acoustic impedance. When this happens part of the compressive wave continues through the pipe but part of it gets reflected back to the cylinder as a negative pressure wave moving in the opposite direction. This negative pressure wave reduces pressure of the gas as it travels back to the cylinder, and of course it reduces pressure inside the cylinder when it reaches the cylinder. So there you have it the shape of the pipe allows us not just to suck in more air and fuel into the cylinder, but also keep that same air and fuel from escaping the cylinder. That's great, but we still have a problem. As you probably know the engine operates at a very wide range of rpm anywhere between 800 to 10.000 for engines like this 300cc single cylinder two stroke. But different engine rpm means different piston speeds and thus different duration of the intake, compression, combustion and exhaust events inside the engine. On the other hand the speed of the sound waves through the exhaust pipe is more or less constant…this means that we can only perfectly match the arrival of the sound waves at the cylinder over a small narrow rpm range A special thank you to my patrons: Daniel Marwan Hassan11 Peter Della Flora Bashuan Allan McKay valkq Zwoa Meda Beda Toma Marini Cole Philips 00:00 The Problem with Two Strokes 04:25 Wave Basics 09:10 How It Works 17:33 Let's Hear it in Practice #d4a #2stroke