Re: Exhaust flow


"Albert T Cone" <albertcone@xxxxxxxxxxx> wrote in message
news:3u6j53F1030mjU1@xxxxxxxxxxxxxxxxx
> Burgerman wrote:
>> "Albert T Cone" <albertcone@xxxxxxxxxxx> wrote in message
>> news:3u6e26Fuh7f4U1@xxxxxxxxxxxxxxxxx
>>>
>>>Yup. If you optimise the system to give impedance matching at ~75% of
>>>peak-power rpm, then you get a benefit across almost the whole rev-range,
>>>which is cool.
>>>
>>
>> How does it give less back pressure than no exhaust???
>> I don't understand the term impedance when used for exhausts! The flow
>> from a turbo is all but pulse free and still expanding at full power as
>> the exhaust valve opens before the end of the powerstroke. How can any
>> lump of tubing give less pressure than an open / no / huge exhaust?
>>
>> Please explain!
>
> AIUI, most fixed-geometry turbos can be modelled as single-pole
> restrictive orificies, meaning basically that they transmit high-frequency
> pulses and give you essentially a static pressure drop.

Pulses I am not so sure about. A turbo alone makes a very efficient
silencer! My open exhaust bike was loads quieter than the other "silenced"
bikes that I was beating!


> As such, the output is not pulse-free, but the pulse amplitude is reduced
> cf the exhaust output, by an amount dependent on the pulse freq (i.e.
> rpm...) (which explains why turbod engines make less noise, all else being
> equal, than a non turbo, even though they exhaust the same amount of gas)

Yep thats what I said earlier?


> The exhaust is then designed basically as a resonant pipe, giving a
> standing-wave pressure distribution, with an anti-node at the turbo and
> normally a node at either the open end or at the silencer.

But there is bugger all pulse left? Cant do much...

> At an anti-node, the pressure varies from positive-negative with the
> pulse-period, and the system naturally synchs so that the negative
> pressure-peak occurs at the same time as the pulse occurs.
>
> So whilst it's true that the *average* pressure isn't lower than with an
> open pipe, it is at the points in the cycle where gas-flow is highest.

Well since the turbo and its nozzle is so restrictive and therefore quietens
most of the noise I doubt you would ever see any advantage over an open pipe
or no pipe.


>
> This is all from talking to a very clever chap who worked for Ford on
> finite-element exhaust modelling software - it's not my field at all.
> Last I heard he'd packed it in and got a job designing prosthetic knees!
> still, i'm pretty convinced he knew what he was on about... :-)

He may do but any gains would be so minimal that its not worth the effort.
Much better to just have a huge or no exhaust to maximise pressure
differential at all revs! At least I think so...


.

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