factors affecting exhaust pulse duration

[XR Scooter]

Yeah Gray, you caught that too? That is a tough one to counter. I wonder if my dad can beat up his dad? LOL!

[jaguar57]

Please forgive my previous curt response but what else could naturally result from someone (XR) making a long entry that just regurgitates all the common knowledge about expansion chambers when the post topic was asking about an aspect that has absolutely no info about it on the internet or in any 2 stroke technology book?

I know all that “common” stuff and then some but I wanted some input concerning something that is never discussed.

Why is it important? Because if the full exhaust pulse creates the sonic wave that comes back as the baffles return pressure wave and the diffusers vacuum wave then it is important. (length of returns waves affect how the pipe design is or isn’t in harmony with the engine porting within the desired powerband of RPM))

[jaguar57]

The thought occurred to me that the sonic wave may not be equal in time to the exhaust pulse. (It was just an assumption. All assumptions should be proven.) So I used my spreadsheet to see that a .9 millisecond exhaust pulse creates the same diffuser wave length (in time) that is shown on Blairs pressure trace. That same time also gives the right baffle wave time in accord with the pressure trace for a Rotax 494 (two 247cc cylinders). So I have my answer. Although those two engines have very different exhaust pulse times they have the same sonic wave time which is what should be used in calculations for the diffuser and baffle wave lengths (on my spreadsheet). But I will still use my formula for calculating exhaust pulse time for use in including it in the combined waves grapher.

[pcnsd]

The thought occurred to me that the sonic wave may not be equal in time to the exhaust pulse. (It was just an assumption. All assumptions should be proven.) So I used my spreadsheet to see that a .9 millisecond exhaust pulse creates the same diffuser wave length (in time) that is shown on Blairs pressure trace. That same time also gives the right baffle wave time in accord with the pressure trace for a Rotax 494 (two 247cc cylinders). So I have my answer. Although those two engines have very different exhaust pulse times they have the same sonic wave time which is what should be used in calculations for the diffuser and baffle wave lengths (on my spreadsheet). But I will still use my formula for calculating exhaust pulse time for use in including it in the combined waves grapher.

Which are you looking for? The exhaust pulse description or Sonic wave in pipe description? They are related but not the same event in physics. The Exhaust pulse is a pressure event described in Duration/Time. The Sonic wave in pipe is a pressure event described by Distance/Time.

[jaguar57]

I used to think that the sonic wave which creates the pipes return waves had the same time duration as the exhaust pulse but with the help of a pressure trace for a Rotax 494 I saw that was not the case. So I'm no longer "looking for an answer" but of course any intelligent commentary from 2 stroke experts is welcome. (Unfortunately most riders that have a basic understanding think they are experts)

[jaguar57]

Now it turns out my original assumption was correct. Reviewing again the pressure trace from the 2 cylinder Rotax I had forgotten that there were extra pressure waves from the other cylinder since they are connected to one expansion chamber via a Y manifold. So the extra wave caused the original exhaust pressure pulse to extend in time (unexpectedly) as well as height. How do I know? The race shop that sent me that trace sent me another one where they used a test setup which separated the two exhausts. That setup revealed the exhaust lasted .92ms, the same time as the sonic wave. I feel like a stumbling rookie but I'm happy to of gotten to the bottom of this. (If you stumble long enough you will stumble onto the truth)

So the one cylinder 100cc engine of Blair had an exhaust duration of .84ms and the 247cc Rotax cylinder had a duration of .92ms. Both engines were of a single exhaust port design. I expect the small difference is because the larger the cylinder, the smaller the percentage of the combustion area that the exhaust port has. The smaller that percentage is, the longer it takes the exhaust gasses to exit. Having a large bridged exhaust port, or a port with two auxiliary ports would bring that percentage back up.

Edited February 13, 2016 by jaguar57