I have an idea for a tuned exhaust system. I'm hoping one of you is an exhaust tuning expert who might know if it could work:
Small, single cylinder engines can get huge power gains (over a narrow RPM range) with an expansion chamber type, tuned exhaust system. But our in-line twin cylinder engines commonly use a relatively untuned exhaust. If each cylinder had its own separate expansion chamber, we could reap similar gains. But our larger displacement and lower RPM engines would need two rather huge expansion chambers, where drag, exhaust cracking, and weight, would probably make them not worth their power gains.
Tuned exhausts are available for our twin cylinder engines. They are not too common on aircraft because they have similar problems. But I'm wondering if there might be a better way. My understanding is that the goal of a tuned exhaust is to pack some extra charge (fuel air mixture) into the combustion chamber from the exhaust pipe.
At the end of the power stroke, the exhaust port opens, letting a burst of high pressure exhaust go down the pipe. A fraction of a second later, the transfer ports open, allowing fresh charge into the cylinder. Some of that charge follows the exhaust pulse out the exhaust. A pressure wave, in a tuned exhaust, reflects back towards the engine, just as the exhaust port is starting to close, and packs that charge back into the cylinder.
My idea is to not use a 2 into 1 manifold, and not have separate tuned pipes either. What if there was a U shaped pipe that connected the two cylinders' exhausts. This U shaped pipe's length could be such that the exhaust pulse from one cylinder would go down the pipe, around the U, and reach the other cylinder right at the critical time.
So exhaust would be resonating back and forth in that pipe. We'd be using an actual pulse, rather than the reflection of a pulse, to do the work. In the middle of the U, there would need to be a Tee that takes the pressure off into a silencer/muffler.
To keep this pipe from being too long, it could expand to a larger diameter. It would be conical, like most of our pipes are now, so it ends up fat where it goes around the U bend. So it could be tuned with its length and/or diameter. To picture it, there would be 90 degree bends pointing down at the exhaust ports, feeding twin conical pipes going down just past the bottom of the engine, connecting in a big, fat U, that has a Tee that feeds into a muffler.
If this system works, the engine would need to be designed for it. If we simply added it to an existing engine, the extra heat from the increased power could create a problem. And detonation could be a problem if we don't adjust the compression ratio of the engine.
But if we provide for those things, the advantages are:
1. Improved fuel economy from recovering most of that lost charge in the exhaust.
2. Lighter weight, because we can use smaller displacement to get the same power.
3. Less vibration, because the smaller pistons and connecting rods have less reciprocating mass.
Remember that a smaller displacement engine needs smaller, lighter crankshaft flywheels and a smaller, lighter crankcase. So the whole engine can be significantly smaller and lighter, to produce the same power. And a smaller displacement engine can operate safely at higher RPMs, allowing a smaller exhaust system.
My MZ201 is a good example of a relatively untuned engine. It uses brute displacement (626 cc) to get decent power from high torque, but has a fairly light weight exhaust system. A CorsAir M25Y is just the opposite. It uses a tiny 173 cc displacement with a bulky, heavy, tuned, exhaust system to get its power at high RPMs. Both engines work fine, but the CorsAir engine gets better fuel economy and vibrates less.
It would be interesting to find out how long or fat the U pipe would need to be, and if it could be designed to have a reasonable range of RPMs where it would be efficient. A good engine to test this on would be a Rotax 377. It's already designed to take the heat and power of the 447.