Conference vmszoo::rc

1264.0. "small engines --- how tiny?" by KAY::FISHER (Stop and smell the balsa.) Fri Nov 09 1990 08:49

Moderator feel free to move this to an appropriate place.  
I just thought this was worth capturing from the usenet.

Article         3426
From: [email protected] (The Root of all Evil)
Newsgroups: rec.models.rc,alt.models
Subject: Re: Smallest diesel engine available?
Date: 7 Nov 90 04:16:44 GMT
Organization: Citicorp/TTI, Santa Monica
 
In article <[email protected]> [email protected] (Mike Smith) writes:
+Does anyone know what is the smallest true diesel (i.e. no glow plug,
+compression ignition) engine available today?
+
  Regularly, "Indy" advertises diesels that are made in England,  .049  to
..15  in size.  I would guess that a conversion kit could be made up to fit
the Cox .010, I've seen the Cox .020's converted to diesel.  All it really
takes  is  a  new  head,  with  a  tight  fitted  screw  plug  to vary the
compression ratio.  The conversion kits are made by several manufacturers,
and the typical fuel mix is 95% diesel fuel and 5% petroleum ether.
 
  You can make a common Cox .049 or .020 run on straight diesel #2 IF you
leave the glowplug on! I've done it! ..it runs pretty good but never revs
much past 7000 RPM..and unfortunately, that limits the power to the prop!
...it runs quite a long time on a tank - compared to the normal glow-fuel,
better than DOUBLE!
 
  I've forgotten most of the thermo I took...anybody up on this stuff??
(It dawns on me that I could call up a buddy of mine at a 3 letter place
that I used to work. He's a freakin' DOCTOR of THERMO! ;-) )
 
  The question  of  the  smallest  engine  is  interesting.  I've  seen  2
prototype  Cox  engines,  one  was  .005  and  the  other was .0025 - both
required heavy duty nitromethane/oil mixtures to actually  run.  Like  90%
nitro  and 10% polyoil.  They ran 30K+ RPM, had bearings on the shaft, and
were of ABC construction, not at all like your typical reed-valve Cox.
 
  Those two protos aside, the scaling laws  of  physics  would  limit  the
lower order limit on how small an engine (of a given type) can  be  built,
given  no latitude of fuel choices.  For example; the current champ in the
small-scale turbojet arena (single-shaft, Brayton cycle for  those  taking
notes..)has  an impeller diameter of just under 2 inches, a burner chamber
that's 6 inches in length, a power turbine that is 1.75 inches in diameter
and  it  won't  run on anything except acetylene gas!  As it turns out the
reason has to do with the combustion temperature of the  fuel/air  mixture
vs.  the  heat loss plus friction.  Remember that most gas turbines run up
to 300% excess air for cooling-you can't expect stoichiometric  combustion
to take place in a continuous cycle engine, except for that small fraction
of the air just below the fuel injector(s)!  It would melt the engine!
 
  In a turbine another thing works against you; the compressor efficiency!
the smaller the diameter the faster the damn thing has to turn to bring in
the required volume of air, then a fourth-to-a-third of THAT can  actually
be  used for combustion purposes...which limits the amount of energy to do
the work of turning the expander which turns the shaft and the compressor.
 
  Recipocating piston designs (2 or 4 stroke) avoid these problems due  to
the  nature  of  their  combustion cycle...the compression ratios in small
sized gas turbines is usually 2 or 3:1, in a small diesel it can  be  25:1
or  higher!  This  increases  their efficiency greatly.  Diesels can be as
good as gold for fuel efficiency,  properly  sized.  Usually  the  scaling
laws  will  dictate  the  practical  size and power ranges for the various
engines.  The turbines work best in sizes at and beyond 300 BHP...
 
  Given proper attention to avoid significant loss of  heat,  keeping  the
friction  to  a minimum and designing the engine to take advantage of more
energetic fuels, one could probably build an engine smaller than .00025  -
the question is HOW MUCH SMALLER?  It would undoubtedly be a 2-cycle, with
compression ignition, most likely  of  recipocating  piston  design  -  as
opposed  to  a  rotary type - too much surface area and too much heat loss
with those!  Maybe using an exotic fuel like propyl nitrate...I know  that
if  you  use  fuels  of  the mono-propellant types, you can actually build
turbine expanders with wheels MUCH  SMALLER  than  1.75  in.  diameter!  I
would guess that that's cheating though...
 
  Using ordinary diesel fuel #2...I dunno...it works  at  .020,  with  the
glowplug, maybe I'll try it with my .010 and report back...
 
  What would you use such a tiny engine for? INDOOR FLIGHTS?? Maybe...
  Let us know if you find the limit! ;-)
 
-Avatar-> (aka: Erik K. Sorgatz) KB6LUY           +-------------------------+
Citicorp(+)TTI                          *----------> panic trap; type = N+1 *
3100 Ocean Park Blvd. Santa Monica, CA  90405     +-------------------------+
{csun,philabs,psivax,pyramid,quad1,rdlvax,retix}!ttidca!sorgatz **
(OPINIONS EXPRESSED DO NOT REFLECT THE VIEWS OF CITICORP OR ITS MANAGEMENT!)