Re: [GTALUG] motorcycle exhaust systems [was slide rules; was Re: Chromebook death dates]

[Stewart C. Russell via talk]

On 02/06/2023 17.49, Dave Collier-Brown via talk wrote:


The diffuser cone should actually be a catenoid, with a diameter and 
length based on the RPM range you're after.


There was a lovely little demo two-stroke at the National Engineering 
Laboratory in Scotland that had its diffuser shaped such that the sonic 
wave provided enough reflected back pressure that it didn't need valves. 
It was (of course) single speed and fearsomely inefficient, but it could 
run (kinda)


 Stewart
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Re: [GTALUG] motorcycle exhaust systems [was slide rules; was Re: Chromebook death dates]

[Dave Collier-Brown via talk]

In a racing two-stroke, the exhaust starts out as a simple pipe, bells out at a 
certain rate, continues as a a much larger pipe, then narrows again to quit a 
small diameter, somewhat like this illustration

[cid:part1.7eRQwO40.k4xp302Y@indexexchange.com]

The diffuser cone should actually be a catenoid, with a diameter and length 
based on the RPM range you're after.

I used to make them in three parts, a long shallow cone, a short steeper cone and a quite 
short cone taking me to the full size of the main "expansion" chamber. I fed 
the rpm in and calculated the endpoint of the first cone, rolled it out of cardboard and 
did the rest by eye with a pair of scissors and tape, so as to fit on the bike without 
burning the rider. I usually had to wiggle the pipes a lot on trail and motocross bikes, 
so I was always recalculating what RPM I would peak at if I made it a little shorter or 
longer.

There  actually were a bunch of calculations, and they were in the front pages of a 
workbook where I recorded my various experiments and how well they worked. Only one got 
used constantly, and got scratched into my "calculator".

--dave


On 6/2/23 10:32, D. Hugh Redelmeier via talk wrote:

| From: Dave Collier-Brown via talk 

| When I was a motorcycle mechanic I had a circular slide rule, with a
| permanent mark at the coefficient for computing a catenoid, as I did a
| lot of 2-stroke exhaust systems. Some hilariously wrong, some which got
| me a reputation as a wizard.

I know what a catenary curve is.  The standard example is of a chain
hanging between two posts (that's where the name comes from).  Or a hydro wire.


I understand that a catenoid is a surface of revolution of a catenary
curve:


I don't know what the coefficient for computing it would be.  I don't
even know what the inputs to this computation would be.

How is this applied to 2-stroke engine exhaust systems?
Is this related to back pressure or resonance?

Maybe you are a wizard.

[I hate 2-stroke engines because they pollute so much.  I'm only
exposed to them running lawn maintenance equipment.]
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--
David Collier-Brown, | Always do right. This will gratify
System Programmer and Author | some people and astonish the rest
dave.collier-br...@indexexchange.com
 |  -- Mark Twain


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[GTALUG] motorcycle exhaust systems [was slide rules; was Re: Chromebook death dates]

[D. Hugh Redelmeier via talk]

| From: Dave Collier-Brown via talk 

| When I was a motorcycle mechanic I had a circular slide rule, with a
| permanent mark at the coefficient for computing a catenoid, as I did a
| lot of 2-stroke exhaust systems. Some hilariously wrong, some which got
| me a reputation as a wizard.

I know what a catenary curve is.  The standard example is of a chain
hanging between two posts (that's where the name comes from).  Or a hydro wire.


I understand that a catenoid is a surface of revolution of a catenary
curve:


I don't know what the coefficient for computing it would be.  I don't
even know what the inputs to this computation would be.

How is this applied to 2-stroke engine exhaust systems?
Is this related to back pressure or resonance?

Maybe you are a wizard.

[I hate 2-stroke engines because they pollute so much.  I'm only
exposed to them running lawn maintenance equipment.]
---
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