speed of sound in air, ATTN Athol/Bernd

[Ext User(John McKenzie)]

OK, I've been attempting to read up on the speed of sound in air.

As far as I can make out, it varies with altitude, more specifically
temperature.

Mention was made (in the handful of sites I read) that this was partly
because in the 'real' world air pressure and altitute and pressure have
a self cancelling effect of some sort (I know I'm wording this terribly)

If we artificially raised the air pressure (say to 30psi absolute for
arguments sake) but maintained 21 C (or whatever room temperature is)
would sound travel through it any quicker/slower?

If we raised presure (again to 30psi absolute) but also raised
temperature (since I'm not in possession of the efficiency of a
particular compressor, could we arbitrarily say for example that it
raised it to 150 C) - would it have a different speed of sound vs
atmospheric pressure and an air temp of 150C?

It's probably fairly obvious here than I'm curious as to whether or not
(via temp if nothing else) a particular intake runner size could be
sorted to provide decent sonic tuning around the onset of boost, and
with a particular boost curve, could it be made to work at higher and
higher rpm with the same engine. I'm curious if I could match the
temp/rpm (and pressure if relevant) with any currently available options
(probably obvious I'm thinking of belt driven superchargers here, and
wondering if a rootes type (which is arguably less efficient, but won't
have such a spiralling boost level) might be the ticker, or it a
centrifugal one could.

All in all, I'm just not sure, but I'd like to have a better foundation
before I did any sort of specific calculations based on a particular
engine combo/size/rpm let alone cam up with a suitable length (or
lengths) to try and test it out.


--
John McKenzie

tosspam@aol.com abuse@aol.com abuse@yahoo.com abuse@hotmail.com
abuse@msn.com abuse@sprint.com abuse@earthlink.com fraudinfo@psinet.com
sweep.day@accc.gov.au
uce@ftc.gov I'm sick of using smilies, work it out yerself
admin@loopback $LOGIN@localhost
$LOGNAME@localhost $USER@localhost $USER@$HOST -h1024@localhost
root@mailloop.com
president@whitehouse.gov vice.president@whitehouse.gov
abuse@iprimus.com.au abuse@cia.gov abuse@fbi.gov abuse@asio.gov.au
abuse@federalpolice.gov.au

[Ext User(mackeb@cen.prendiville.wa.edu.au)]

Hi john,

Check out this site:
http://en.wikipedia.org/wiki/Speed_of_sound

It states about a quarter of the way down the page that in an ideal
gas, the speed of sound is depends on temp only, not pressure. It also
states that air is almost an ideal gas.

This is, however, not to say that an emulsified mix of air/fuel is
anywhere near a perfect gas.

Hope this helps (a bit)

[Ext User(DalienX)]

John McKenzie wrote:

I know your question IS car related, but you might get a better answer
at aus.science

[Ext User(John_H)]

John McKenzie wrote:

>OK, I've been attempting to read up on the speed of sound in air.
>
>As far as I can make out, it varies with altitude, more specifically
>temperature.

Velocity of sound increases with both density and temperature.
Increasing the pressure will increase the density and thus the
velocity.

Try this one.... http://www.rane.com/pdf/eespeed.pdf

Considering the complexity of the physics involved it's about as
practical as you're likely to find.

You might also find the historical background as interesting as I did
-- especially since Newton doesn't rate a mention. According to my
pro-Brit education Newton came up with the original (and incorrect)
formula and Laplace corrected it.

--
John H

[Ext User(Bernd Felsche)]

John McKenzie <jmac@alphalink.com.au> writes:

>OK, I've been attempting to read up on the speed of sound in air.

>As far as I can make out, it varies with altitude, more specifically
>temperature.

What about humidity and recirculated exhaust gases?

>Mention was made (in the handful of sites I read) that this was
>partly because in the 'real' world air pressure and altitute and
>pressure have a self cancelling effect of some sort (I know I'm
>wording this terribly)

>If we artificially raised the air pressure (say to 30psi absolute for
>arguments sake) but maintained 21 C (or whatever room temperature is)
>would sound travel through it any quicker/slower?

Higher pressure in gas means that molecules are closer together so
the wave propagates more quickly.

>If we raised presure (again to 30psi absolute) but also raised
>temperature (since I'm not in possession of the efficiency of a
>particular compressor, could we arbitrarily say for example that it
>raised it to 150 C) - would it have a different speed of sound vs
>atmospheric pressure and an air temp of 150C?

Higher temperature means that the gas molecules have a higher
"mobility" so from that alone, one would expect higher (pressure
wave) speeds.

>It's probably fairly obvious here than I'm curious as to whether or
>not (via temp if nothing else) a particular intake runner size
>could be sorted to provide decent sonic tuning around the onset of
>boost, and with a particular boost curve, could it be made to work
>at higher and higher rpm with the same engine. I'm curious if I
>could match the temp/rpm (and pressure if relevant) with any
>currently available options (probably obvious I'm thinking of belt
>driven superchargers here, and wondering if a rootes type (which is
>arguably less efficient, but won't have such a spiralling boost
>level) might be the ticker, or it a centrifugal one could.

>All in all, I'm just not sure, but I'd like to have a better
>foundation before I did any sort of specific calculations based on
>a particular engine combo/size/rpm let alone cam up with a suitable
>length (or lengths) to try and test it out.

Somebody else has already posted a reference to a paper on acoustics
that seems to have enough to give you a fair approximation of the
speed of sound in air.

You need computational fluid dynamics to be able to calculate
something even moderately useful beforehand. Those results will have
to be taken with a pinch of salt. It's not a steady-state problem.
Tweaking is inevitably required in practice.

The only way that I can think of; after decades of thinking about
the problem, albeit not full-time "professional"; is to measure the
desired output(s) and then adapt the inputs accordingly.

Although this is throwing my potential fortunes to the wind; one
method that you could use would be to measure the pressure at an
inlet port and the pressure at the throttle body. The basic idea is
to get the peak inlet pressure over the time that inlet valve is
opening (note: not all the time that it's open) for given lift.

As you probably know, the timing of the pressure wave is based on
the inlet air column hitting a closing valve at speed, resulting in
a peak pressure at the inlet, which bounces back along the inlet
tract and usually bounces back, unless the tract is effectively
"open".

The controllable variables are at the first glance; valve opening
and closing times and the length of the inlet tract.

The relative timing of valve opening and closing is usually fixed by
a cam profile, but some variation is possible by say direct
electromechanical valve actuation or by using a desmodromic valve
system with two, independently adjustable camshafts; one for opening
and the other for closing. You can then open the intake valve at
"any" time to take advantage of the pressure waves, restricted only
by the amount of exhaust overlap is appropriate; and close the valve
to time the return pulse (or even to eliminate the throttle valve!).

Various means of adjusting inlet tract length are widely known;
using flaps or slide valves to switch between tracts; rotating
"drums" to alter the length of a spiral manifold, etc.

If you can vary boost pressure, then you have another controllable
input. The benefit of passive-dynamic supercharging on top of active
supercharging is small.

You should IME, optimise for passive-dynamic supercharging up to
on-boost engine speeds without trying too hard to also make it good
when on boost.
--
/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
\ / ASCII ribbon campaign | "Laws do not persuade just because
X against HTML mail | they threaten."
/ \ and postings | Lucius Annaeus Seneca, c. 4BC - 65AD.

[Ext User(The Raven)]

Been done on just about every engine design since the introduction of
computers to engine design etc.

For Mazda rotaries: Look up the DEI (Dynamic Effect Intake). Mazda claimed
it provide a small boost equivalent to 5psi at certain engine speeds. Also
look at the R26B (race engine) and note the variable length intake runners
(IIRC varied from 24 to 36 inches in operation).

Ford SHO engine was also a relatively early adopter of variable intake
lengths.

--
The Raven
http://www.80scartoons.co.uk/batfinkquote.mp3
** Now I will bring chaos to the world!

"John McKenzie" <jmac@alphalink.com.au> wrote in message
news:43DB4CC7.3851@alphalink.com.au...
> OK, I've been attempting to read up on the speed of sound in air.
>
> As far as I can make out, it varies with altitude, more specifically
> temperature.
>
> Mention was made (in the handful of sites I read) that this was partly
> because in the 'real' world air pressure and altitute and pressure have
> a self cancelling effect of some sort (I know I'm wording this terribly)
>
> If we artificially raised the air pressure (say to 30psi absolute for
> arguments sake) but maintained 21 C (or whatever room temperature is)
> would sound travel through it any quicker/slower?
>
> If we raised presure (again to 30psi absolute) but also raised
> temperature (since I'm not in possession of the efficiency of a
> particular compressor, could we arbitrarily say for example that it
> raised it to 150 C) - would it have a different speed of sound vs
> atmospheric pressure and an air temp of 150C?
>
> It's probably fairly obvious here than I'm curious as to whether or not
> (via temp if nothing else) a particular intake runner size could be
> sorted to provide decent sonic tuning around the onset of boost, and
> with a particular boost curve, could it be made to work at higher and
> higher rpm with the same engine. I'm curious if I could match the
> temp/rpm (and pressure if relevant) with any currently available options
> (probably obvious I'm thinking of belt driven superchargers here, and
> wondering if a rootes type (which is arguably less efficient, but won't
> have such a spiralling boost level) might be the ticker, or it a
> centrifugal one could.
>
> All in all, I'm just not sure, but I'd like to have a better foundation
> before I did any sort of specific calculations based on a particular
> engine combo/size/rpm let alone cam up with a suitable length (or
> lengths) to try and test it out.
>
>
> --
> John McKenzie
>
> tosspam@aol.com abuse@aol.com abuse@yahoo.com abuse@hotmail.com
> abuse@msn.com abuse@sprint.com abuse@earthlink.com fraudinfo@psinet.com
> sweep.day@accc.gov.au
> uce@ftc.gov I'm sick of using smilies, work it out yerself
> admin@loopback $LOGIN@localhost
> $LOGNAME@localhost $USER@localhost $USER@$HOST -h1024@localhost
> root@mailloop.com
> president@whitehouse.gov vice.president@whitehouse.gov
> abuse@iprimus.com.au abuse@cia.gov abuse@fbi.gov abuse@asio.gov.au
> abuse@federalpolice.gov.au

[Ext User(TheTaipan)]

But sound will not travel in a vacuum eg. space, irrespective of the
temperature. Not that I have ever tried this one, so I am taking the word
of others here...

Also of interest to the OP, due to extreme HOT weather, AND unusual weather
conditions - there was an incident north of Brisbane where an F111 broke the
sound-barrier many many km's away and people were reporting "earthquakes"
and "explosions", and "aliens" up and down the Brisbane/D-bay area...


<mackeb@cen.prendiville.wa.edu.au> wrote in message
news:1138452704.584239.227460@o13g2000cwo.googlegr oups.com...
> Hi john,
>
> Check out this site:
> http://en.wikipedia.org/wiki/Speed_of_sound
>
> It states about a quarter of the way down the page that in an ideal
> gas, the speed of sound is depends on temp only, not pressure. It also
> states that air is almost an ideal gas.
>
> This is, however, not to say that an emulsified mix of air/fuel is
> anywhere near a perfect gas.
>
> Hope this helps (a bit)
>

[Ext User(John McKenzie)]

Thanks one and all for taking the time to reply, including the links
posted. I'm still working on it.


Bernd Felsche wrote:
>
> John McKenzie <jmac@alphalink.com.au> writes:
>
> >OK, I've been attempting to read up on the speed of sound in air.
>
> >As far as I can make out, it varies with altitude, more specifically
> >temperature.
>
> What about humidity and recirculated exhaust gases?

the latter isn't likely (though I am aware of it's potential in terms of
boosting dynamic compression for part throttle economy stuff) to be a
huge concern with the sort of cam timing I'm looking at (and indeed the
exhaust design. It's nothing extroadinary) for full power at any rate.


>
> Somebody else has already posted a reference to a paper on acoustics
> that seems to have enough to give you a fair approximation of the
> speed of sound in air.

Reading through it atm.


> You need computational fluid dynamics to be able to calculate
> something even moderately useful beforehand. Those results will have
> to be taken with a pinch of salt. It's not a steady-state problem.
> Tweaking is inevitably required in practice.
>
> The only way that I can think of; after decades of thinking about
> the problem, albeit not full-time "professional"; is to measure the
> desired output(s) and then adapt the inputs accordingly.
>
> Although this is throwing my potential fortunes to the wind; one
> method that you could use would be to measure the pressure at an
> inlet port and the pressure at the throttle body. The basic idea is
> to get the peak inlet pressure over the time that inlet valve is
> opening (note: not all the time that it's open) for given lift.

yep - I guess I was just wondering more broadly if there's even a
snowballs chance in hell that it could somehow be designed with such an
inlet runner length as to 'work' as boost and rpm climbed. In a sense
looking to vary the speed of sound rather than the runner length (which
has been done in a number of cars to date).

> The relative timing of valve opening and closing is usually fixed by
> a cam profile, but some variation is possible by say direct
> electromechanical valve actuation or by using a desmodromic valve
> system with two, independently adjustable camshafts; one for opening
> and the other for closing. You can then open the intake valve at
> "any" time to take advantage of the pressure waves, restricted only
> by the amount of exhaust overlap is appropriate; and close the valve
> to time the return pulse (or even to eliminate the throttle valve!).

All no doubt true, but beyond any means I'll be able to employ, hence
the questions.

>
> Various means of adjusting inlet tract length are widely known;
> using flaps or slide valves to switch between tracts; rotating
> "drums" to alter the length of a spiral manifold, etc.
>
> If you can vary boost pressure, then you have another controllable
> input. The benefit of passive-dynamic supercharging on top of active
> supercharging is small.

no doubt, but it's just one of those icing on the cake things I've
wondered about from time to time. I suspect that something like this
might have been employed in f1 years back, though it's just as likely
they just designed them to work best at a particular boost level and rpm
range, just after the upshift.

> You should IME, optimise for passive-dynamic supercharging up to
> on-boost engine speeds without trying too hard to also make it good
> when on boost.

Within reason or practicality that's where I've looked. On the holden 6,
the vk efi inlet manifold works very well for this very reason. On some
other stuff I've had to look toward whatever produced the most
favourable mixture distribution (blowthrough carbed dinosaur that I am)
as the priority.



--
John McKenzie