Expansion chamber type exhaust for rotaties?

[j79guy]

Here is a question that is nagging me; Why not expansion chamber type exhaust on a rotary engine, especially those with a peripheral exhaust port?
I was reading my old "Two Stroke Tuner's Handbook" by Gordon Jennings, and the chapter on exhaust pipe design. With a properly designed di/con type exhaust, a two stroke engines' output can be effectively doubled. (Assuming the engine can withstand the stresses, and attendant intake, scavenge & compression capacity is built in to support the output.) Rotary engines, especially those with peripheral ports, have significant port timing overlap similar to two stroke engines, thus my pondering about a di/con exhaust for these engines. The exhaust on rotaries have to be constructed out of high temperature materials as the exhaust temperature on these engines is quite high, so obviously, a di/con pipe couldn't be built out of thin wall mild steel, but for now this is a mental exercise.
The guys at Racing Beat, have always recommended straight primary header pipe, "as long as possible". (I've seen some rotary exhaust primary pipes over 50" long.) So there doesn't seem to be any real tuning going on via length.
What say you?

Robin Sipe.

 

[dgallup]

Two stroke exhaust timing is symmetrical, therefore it is impossible to individually optimize the opening & closing timings. The diverging/converging expansion helps evacuate the cylinder with a negative pressure pulse & refill the cylinder with a positive pressure pulse since neither opening nor closing can be at the optimum point in the cycle. Rotaries do not suffer from this deficiency.

 

[globi5]

Also, the Mazda RX-8 is probably the only commercially available car, which has an engine without overlap (as opposed to all piston engines).

 

[j79guy]

dgallup: True, the exhaust port timing is asymmetrical, however it too is fixed in this asymetrical timing, as is the overlap between the intake/exhaust port timing. (Which is made more pronounced by the traditional opening up of the intake and exhaust ports in the quest for more power at a higher RPM.) I am thinking along the lines of an di/con exhaust system, with port timing more optimised for this type exhaust, rather than a simple header. Thanks, I'll ponder your point some more.

globi5: Yes, I am aware of the newer rotaries having the intake and exhaust ports on the side plates, I was thinking more of the racing rotaries with pure peripheral ports for the intake and exhaust, and the vast majority of on-road rotaries that have a peripheral exhaust port, with significant intake/exhaust port timing overlap.

I have a couple 4-port and 6-port engines to play with. Short of coupling one to the water brake and spending time on poking around various exhaust designs, I'm hoping this question had been asked and answered before. If not, then maybe it would make for an interesting techincal diversion. The racing engines with peripheral intake and exhaust ports have high port opening overlap. (Depending upon how agressive the timing is.) Some of these highly strung engines will not even start unless wound up to 1000+ rpm. They make fantastic power for the cubic inch displacement, but at high rpm and a razor thin powerband. Turbocharging the rotaries has allowed this timing to be made more conservative, but I'm curious as to why di/con exhaust was never applied to these engines in naturally aspirated racing applications. An answer to a question no one is asking?

Robin Sipe

 

[j79guy]

dgallup: I realised on my drive home tonight, after thinking about assymetrical port timing, that in a typical loop port two stroke, on the piston upstroke the transfer ports close before the exhaust port. In a rotary, obviously this is not the case, with the intake port closing well after being mechanically separated from the exhaust, timing-wise. Therefore, the two stroke di/con exhaust theory will not work to artifically supercharge a rotary.

Robin Sipe.

 

[izzmus]

There have been many piston engines without valve overlap. One of them is sitting on the stand right now waiting to be mounted to my VW.

Regardless, there HAS been some work with expansion chambers. Most of it is kept secret, unfortunately.

Racing Beat's recommendations are old-think. The current fashion is primary lengths in the 13" range. Not too many people are talking about the collector shape, which is critical.