The joy of learning

Have you ever experienced real excitement at learning something new? I seem to do so quite often, either because I’m not really that smart or because I find it easy to get excited about things I am interested in. Although some moments of discovery have been associated with my field of professional interest, many have not. Before I embark upon a discussion of a learning discovery made over the weekend, let tell you of another by way of demonstrating to you what I am talking about. As a professor of Biology I have discussed the topics of Cell Structure and Function to a number of students over the years. When investigating such things, especially in the laboratory, I have felt that it was too bad that most students learn about real cells by using microscopes to view slides of tissues or of cultured preparations. This has the unfortunate consequence of giving students the impression that cells are static things of little interest. Although I have tried, in lectures, to give my students a feel for the excitement which is a living cell, this point has mostly been lost to my audience. So you may well imagine my joy when I first viewed the video below. It provides a fresh vision of processes and of an environment which I thought I knew well. It was, and still is, a stunner. Showing it to students has the same effect it did on me, that of opening a window of insight which was previously unavailable. Thanks to the folks at BioVisions for opening my eyes,  wide.

Ok, interesting anecdote but you may be wondering where this is going. If you follow this blog you will know that I had a great time at
a powerboat Regatta over the recent Labor Day weekend. It may sound silly but as a result of attending the event, watching, and talking to a few of the participants I learned something really interesting about … get ready for this … tuned exhaust! My adventure began when I saw, and photographed, what I thought was a very strange sort of muffler on several of the boats. Take a look.

In order for me to properly express the joy of my discovery you must first endure a description of the mechanics of a two stroke engine, the sort which many of these small racing boats were running. There are a number of places around the internet where you can learn about this type of engine and what you will find when you look is that the two stroke is a deceptively simple and very powerful machine. Here is how it works. At the moment the piston rises to the top of the combustion chamber a spark ignites the fuel mix which has entered and been compressed there. At the very same time more fuel is being delivered to the crank case below. The explosion which has just occurred drives the piston down, compressing the newly delivered volume of fuel. Notice that as the piston falls a port is exposed allowing exhaust to exit the right side the engine. As the piston drops even further it uncovers another port, on the left side this time, which allows the now compressed fuel mix to flow up and into the combustion chamber. The explosion has pushed the piston down with enough force that it can both compress the next bit of fuel and continue rotating up and into the combustion chamber. The plug ignites the fuel mix and drives the piston down once more. Remember that the piston is tied to a connecting rod, which is tied to a crankshaft, which is tied to a drive shaft, which goes round, and round and drives your mower, weed trimmer, power boat, or airplane. [The wonderful schematics shown here were found at where else, Wikipedia, but their original source is here under the tabs Know How … and then Zweitaktprinzip (Funktionsprinzip eines Zweitaktmotors).

So then, what did I learn this weekend? What was so exciting? I learned about what is called tuned exhaust. Two stroke engines with tuned exhaust sport a quite unusual looking muffler, as I showed you above. You may have noticed that during the exhaust phase of the cycle above a bit of the fresh fuel mix (in green) escapes the combustion chamber before the piton rises into it. Now, hold that thought … we’ll come back to it in a bit. As a result of each explosion at the top of the cylinder two things happen, exhaust gas is created and a high-pressure sound wave is generated. When the exhaust port opens these go zooming down the manifold and into the muffler. Now a tuned muffler, as shown below, is built in a very particular way. Notice that a thin pipe overlaps the manifold for some distance. This pipe then expands into a diverging cone which merges into a converging cone which merges with a little bit of tail pipe at the end. As the exhaust port opens the gasses and high-pressure sound wave travel the thin pipe from the manifold. Notice that the gasses expand, slow down, and cool – and these actions tend to ‘muffle’ the noise. Also notice that the shock wave which leaves the manifold travels the diverging cone and when it hits the converging cone, it reverses direction and heads back in the direction of the exhaust port. At this very same time fresh fuel mix (which we mentioned above) is escaping the combustion chamber. This happens because of the low pressure which follows the high pressure wave and the fuel mix essentially gets sucked out of the combustion chamber. The advancing (now reversed) sound wave collides with the onrush of fuel and forces the escaping mix back into the chamber. And this increases engine efficiency by super-charging the combustion chamber with fuel and thereby ramps up the engine RPMs. The timing of all of this is critical. Note that the length of the pipe determines the timing of the return pressure wave and adjustments may be made to the system by sliding the muffler in and out of the sleeve which surrounds the manifold. I was told by more than one powerboat skipper that this sort of adjustment is done before each and every race and is dependent upon a number of variables, but is mostly done by feel and how the system sounds. When an engine is running at over 10,000 RPMs even a 10% increase in rotational speed (and torque) can make the difference between winning and losing. Wow. How cool is that? Perhaps this entire discussion has left you feeling flat. If that is the case you’ll have to forgive me. When I learned of tuned exhaust I was struck by the simplicity and elegance of its design and the sheer logic of its function. Check out this link for more discussion of tuned exhaust.

Schematic of a tuned two stroke engine. Blue is air intake. Brown is fuel. Green is fuel mix. Red is combustion. Grey is exhaust. Notice that when the high-energy sound wave hits the walls of the converging cone it reverses direction and collides with fuel mix which is escaping the combustion chamber. The effect of the collision is to force the mix back in to the engine thereby increasing its efficiency.

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