Exhaust port inserts

[Bob Monkhouse]

Just throw a quick one off. I\'ve seen on a thread somewhere discussion about the exhaust port inserts and what they may be for. Using my knowledge of two strokes and as I was just having a look at our Grants casings/rotor housing thingys it struck me that there could be another reason for them as well as heat transfer and noise reduction. Two srokes \'supercharge\' using a resonant exhaust system which in its basic form is/are a pipe and two cones, these use the sound waves reflecting down the exhaust pipe to both suck and then blow(to firstly scavenge exhaust gases and aid transfer pressure then  to \'blow\' to ensure the fresh charge isn\'t sucked out the ex.port. If you put a positive wave down a pipe-especially a tapered one, the wave changes its sign and is reflected back as a negative one. To stop boring everyone(what do you mean you fell asleep 10 minutes ago)!!, the tapered insert would aid scavenging as the rotor tip passes over the exhaust port. the positive pressure wave provided by the \'baffle cone\' is not needed on a rotary as the induction cycle is not dependant on crankcase pressure as in the two stroke. Thats:WaveBye  my theory and I\'m sticking to it!!!

[leon]

Okay bob im goign to be a thicket here: Sounds fancy, i know bits about waves etc from radio coms but i dont understand about the combustion cycle all that well other than the fundimentals. What exactly does this theory mean in real world terms and where does it take you?

[Bob Monkhouse]

All types of engines need some kind of cylinder scavenging to ensure the fresh incoming charge isn\'t polluted with spent gases(lets not go into egr valves and emissions crap,after all you don\'t see race motors with them on-not to mention cats etc). Four strokes(boingy things) are scavenged using tuned tubular manifolds where one cylinder exhausting creats a vacuum in another thus pulling the spent gases out and the fresh charge in. Two strokes do a fairly similar thing but with another(positive this time) sound wave arriving at the exhaust port a milli second or two afterwards.I wouldn\'t think rotarys are any different in needing exhaust scavenging and the taper in the port would help this. Its just a conclusion that I came up with after seeing previous discussion on it. I\'m sure if you\'ve ever noticed that bikes used to be regularly fitted with \'mega\' exhausts to aid performance(though I don\'t think the thick greasers who fitted them would have a clue why they worked)!However(and here endeth the lesson), a taper on the exhaust-aforesaid \'mega\', wouldn\'t work on a turbot charged rotary due to the sound waves not being able to pass the turbot(theres that fish thing again)hence the possible use of the taper in the port. Er, have you seen Ann  Elks theory on dinosaurs? Its a bit less boring than this and more concise!!!!!!!!!!!!!!!!!

[leon]

Bob, thank you :mmmm  learn somthing new everyday.

[Carl]

Hi,
You are right about the scavenging effect,the taper helps with this but also the inserts keep heat away from the rotor housing in the exhaust port area by transfering it to the exhaust manifold too.
Regards
Carl