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NOISE REDUCTION IN TWO STROKE ENGINE BY CONTROLLING THE VELOCITY OF EXHAUST GAS


admin (Simon W)
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I found this very interesting indeed :-) 

Link to full document below. 

Technical description: 

This invention particularly deals with reducing the velocity with which the exhaust gases are released 
from the combustion chamber to the atmosphere, thereby reducing the noise created by sudden 
expansion of exhaust gases. 

The time for which the exhaust port remains open is around 135 degrees of the crank angle. At this 
stage of 135 degrees of crank angle the exhaust gases are forced out of the combustion chamber into 
the atmosphere creating sudden expansion of gases. The so proposed invention uses the rest of the 
time (i.e. the remaining 225 degrees of the crank angle) in addition to the available 135 degrees to 
control, regulate the exhaust gases flow. This is done with the help of an ‘exhaust gas reservoir’ and a 
series of ‘valve arrangement’ to control, regulate the flow of the exhaust gases. 

Referring to port timing diagram of a common two stroke engine

https://archive.org/stream/10I34IJAET0933526V9Iss4Pp507512/10I34-IJAET0933526-v9-iss4-pp507-512_djvu.txt

SW :D

 

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Interesting article but from reading it the chamber will go between the exhaust port and the expansion chamber. The problem I can see with this is it will severely hinder effectiveness of the expansion pipe. From my book leanings the expansion pipe is designed to produce the high gas velocity/low presure entering the widest part which in turn creates high backpressure front as the pipe narrows again to push some of the unburnt charge back into the cylinder.

If there is a large void between the two what would stop working surely?

 

Ref:  Tuning for speed. Phil Irvine (1960)

 

Great book for getting right back to basics. When I was racing 2 strokes and building my own pipes it was my reference as it contains all the calculations needed.

https://www.abebooks.co.uk/book-search/title/tuning-for-speed/author/irving/pics/

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Really really interesting. I wouldn’t be surprised if there was a trade off between performance/efficiency and additional noise reduction (isn’t there always?!).

That said, I think one of the biggest issues our sport has in terms of engagement with local communities is noise pollution. The way I see it is if we can significantly reduce that (perhaps at a small cost to performance or efficiency) then there could certainly be a big benefit in being responsible pilots. 

Will take a better read of it later so my hobby mechanic skills can be overwhelmed :P 

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Virtually any interference of the exhaust gases causes loss of power due to the additional back-pressure cause by such means. This is why we often have straight through arrangements with holes through to cushioning material in a surrounding silencer. 

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To me it depends on the cost both literally - what it would cost to modify existing engines and what performance loss against noise reduction gain. 

Silent Paramotors are obviously the snake oil as like you said, the prop will make noise but if there is a quieter engine that doesn’t sound like, as someone put it - a dirt bike in the sky for a performance loss I can deal with. I’d be happy. 

Would love to jump onboard with electric Paramotors but I feel it’s the same lifecycle as electric cars - early gen will be/are low distance and high recharge time which will both improve with tech.  

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