[modeleng] Re: Tr: Re: Steam engine RPM ?

  • From: "Phill Smith" <steam@xxxxxxxxxxxxxx>
  • To: <modeleng@xxxxxxxxxxxxx>
  • Date: Sat, 24 Sep 2005 23:09:45 +0800

While cleaning out my "inbox" I came across this one that I was going to
reply to.

Engine speed is governed by one factor, piston speed. For a given pressure
can only drive a piston at determinable maximum speed. The faster you try to
get to go, the greater the frictional losses for the flowing steam and
piston, and therefore resulting in a lower pressure within the cylinder
trying to push a piston that requires more force to accelerate it to the
high speed. All this is calculatable. But believe me, it's not worth the
time and effort.
The speed of a model, or should I say RPM of it's flywheel/driving wheel, is
directly related to the scale of the engine, and pressure it is operated at.
Assuming (which isn't true) that the ports and cylinder don't condense the
steam any more than the full size, and the frictional resistance is also
proportionately reduced.
In other words, if the full sized loco did 90 mph with it's 6' drivers, then
your 1/8 model can do the same speed with it's 9" drivers.... The critical
factor, piston speed, is the same. But, taking a reality check, we would
find that the condensation rate and increased friction to the steam flow,
would retard the engine back to say 22.5 mph. This also explains why it is
so hard to get a model to operate at full sized RPM. The piston speed
required is 1/8 of the full sized engine, but this equates to a very much
lower pressure when operating under a "no load" situation (which is normal
for display purposes). You must remember that the full sized engine was
operating under it's designed load, and therefore was using all the power of
the pressure being fed to it. Now the theoretically correct load for a 1/8
model, operating at the same steam pressure, is "the full sized load divided
by 8 cubed. Or 1/512 of the full sized load. If this load is placed on the
display model, it will run at around the correct speed. If you are operating
your boiler at 1/2 the original pressure, then that 1/512 becomes 1/1024 the
load. In reality, the increased frictional losses in the steam passages, the
hugely magnified condensation losses in the passages and cylinder, both
contribute to a reduction to approximately another 1/4 of the applicable
load. This then takes the load required for a correct operating speed down
to 1/4096 of the full sized load.

All this boils down to the following;

Load up your engines when displaying them, and they'll run at the right
speed.

Please excuse the lecture.

Cheers,

Phill.


----- Original Message ----- 
From: "alan Stepney" <alan@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
To: <modeleng@xxxxxxxxxxxxx>
Sent: Wednesday, September 07, 2005 4:53 AM
Subject: [modeleng] Re: Tr: Re: Steam engine RPM ?


> As far as I am aware, there is no absolute formula for speed against
engine
> size.
> There are many rough guides, with the actual limiting factors being, the
> valve gear, inertia in moving parts, and general inefficiency, plus the
> normal design criteria.
>
> At a guess, that size of engine would probably turn at around 500-600 rpm
> without too much trouble.
> It is quite a long stroke for the bore, which will limit speed.
>
> Alan
>
>
>
> ----- Original Message ----- 
> From: "Pcx" <pierre.courbouleix@xxxxxxxxxx>
> To: <modeleng@xxxxxxxxxxxxx>
> Sent: Tuesday, September 06, 2005 7:24 PM
> Subject: [modeleng] Tr: Re: Steam engine RPM ?
>
> > Hi All,
> When discussing with a friend we had this question ( may seem crazy to the
> most experienced of You), for which we found no reliable answer, how to
> calculate (estimate?) the running speed of a steam engine (RPM) knowing
the
> type of engine (single/double acting, condensing/non condensing), the
> stroke,the bore and the inlet steam pressure ? For your info we are
building
> an horizontal slide-valve steam engine 1 1/2 bore, 2 1/4 stoke , of a type
> inspired from an old design .
> > I thank you for your help and the infos you can forward ....
> >  Regards
> >
> > pierre.courbouleix@xxxxxxxxxx
> > Pierre
>
>
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