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MG Midget and Sprite Technical - Exhaust silencer - help with maths puzzle.

I've just aquired a 'sound barrier' from demon tweaks (the extra muffler that goes on the end of the exhaust to get the sound down for track days - necessary for my maniflow), however, I'm STILL showing over 100dBA.

So I was talking over my options with my Dad, whos been big into bikes and track days his whole life and he was telling me of a device with tubes in it that goes inside the end of the exhaust that can really help get the noise down. I don't know much about it yet as he could only tell me the name in dutch! So, he's going to have a go at making something for me, but needs quite a few bits of info before he can work out the various parameters. I'm able to work out everything except the ammount of gas coming out of the exhaust - i.e. the exhaust flow rate. I'd like to calculate this at 4000 rpm as thats about where i'll need to rev the engine too when being noise tested. obviously I can work out roughtly the amount of air being drawn in but how do i work out the amount of volume post cobustion? Estimates will do!!

Nick

Nick we determine corrected gas flow as "mass Flow"
This is where we consider the effects of Flow, temperature and pressure. You could google mass flow to understand this better but from your point of view it will be essentially the same as the air in. When heated up although there seems to be more flow it is in actual fact less dense.
Bob (robert) Midget Turbo

As an example ....fill a glass with water and measure, put into frezzer.....did the ice create more water...nope!!!

still the same amount just a differant form....same as your exhaust


prop
Prop

Hi bob, if an engine were an air pump then i'd agree with you and the problem would be trivial. I understand mass balance etc (im finishing my PhD in chemical engineering this year, so it might be a tricky viva if i didn't!) However, its more the conversion of the hydrocarbon fuel + o2 --> Co2 + H2O that stumps me a little. If we assume a lambda of 1, so about 14:1 air:fuel by mass then we can work out how many moles of fuel are burnt per second etc. I can even work a little with the fact that 1 mole of an ideal gas occupies 22.4dm^3 at STP and get an estimate of the extra volume of gas created in the combustion process, but the problem is that an exhaust isnt working at STP. I suppose i could measure the gas temp ... and then pv/t = const etc.. Thinking aloud now. still not sure!
Nick

Hmm I too was having another thought because the conversion of the fuel molecules to a gas? was releasing a lot more energy, just struggling to understand that relationship myself.
Anyway I have before when I was young bought some motorbike exhaust wadding, opened up the can and repacked it with new wadding and you would not believe the difference. :-)
Bob (robert) Midget Turbo

just get a repackable silencer off Kim at Magic midget, and some spare wadding of Peter May and you will be set up for years. I use these on the 1380 race engine and it always passes noise tests here and abroad. Haven't had to repack it yet...
David Smith

whilts thats a great suggestion, I just dont have a spare £155 at the moment, plus fundementally theres nothing wrong with the manifolw, its a hell of an exhaust, its just noisey!!

Anyway back to my calcs!!:

So i've been thinking if, i'm doing say 4000rpm, then assuming a rough VE of 75% at this point, that would mean that i'd be drawing in .75*2*(1430/4)*4000 = 2145000 cm^3 min^-1 or air, or 2.145m^3 min^-1. Now as the density of air is about 1.2kg m^-3 then thats 2.57kg min^-1.

For perfect combustion i'd be adding 2.57/14.7 kg of fuel to that = 0.175kg min^-1 of fuel. Working this backwards with the dinsity of fuel and my rear axel ratio that would equate to about 21mpg at 4k. So i know this is the right ball park.

now comes the bit i can't work out:

typically petrol combusts as per:

2*(C8H20) + 25*(O2) --> 16*(CO2) + 18*(H2O), so on this basis how much C02 and H2O would i produce if i added 0.175kg of petrol to 2.57kg of Air per minute? most importantly what volume would they occupy?

Nick

This thread was discussed on 26/07/2008

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