From: bill@c3177208.ssr.hp.com (Bill Chidester)
Subject: Re: FAQ? (questions about engines)
Date: Wed, 2 Sep 1992 04:05:08 GMT
Organization: Hewlett-Packard, Orlando Fl.
X-Newsreader: Tin 1.1scd1 PL4


:Hot and cold mean the same thing in model engines as in car engines.  A 
:hotter plug means exactly that: the plug element stays hotter period.  
:There is more to heat range than the diameter of the coil.  The gap between 
:the coil and the body of the plug also determines the heat range.  Bigger 
:gap means slower heat transfer which means a hotter element, everything 
:else the same.  A thicker element also means a hotter plug, more mass 
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                                     |
                                     |
                                   Nope!

:at high temperature takes longer to cool between power strokes.
:

  You are confusing temperature (hot/cold) with heat capacity (energy).

  There's a world of difference between the way a spark plug works and
  a glow plug works.  A glow plug cools primarily by conduction: one end
  of it is tied to a big heat sink.  Look at your glow plugs -- the thicker
  elements are on the colder plugs, not the hotter plugs.  A hot glow plug
  uses a smaller mass to attain a higher temperature during the power portion
  of the stroke (trust me, they charge more for the cold plugs because it
  takes more platinum to plate the increased surface area of the larger
  element).
 
  To demonstrate:  Place a 17 caliber steel shot on an asbestos sheet
  (observing all relevant OSHA, NIH, FBI and FDA precautions) next to a
  sixteen pound steel shotput.  Apply an equal amount of heat to both
  masses by heating for three seconds with a blowtorch.  Immediately pick
  up the two shots.  The one with the smaller mass will cause smoke to be 
  emitted from your fingers, indicating high temperature.  The big mass will
  still seem to feel quite comfortable to you.  Same thing happens in the
  engine -- the smaller mass of the glow element will get hotter than the
  larger mass.  Hot plugs use skinny wires.

  Now, bandage your fingers and we'll move on to the heat capacity experi-
  ment.  Heat both masses to the same temperature.  For engineering purposes,
  they'll be at the same temperature when they're both cherry red (do the big
  one first).  Now, apply an equal amount of coolant to each mass (spit on 'em).
  and pick both of them up.  The one that we have classified as "hot" didn't
  cause the same reaction as it did in the first experiment.  Smoke is now
  being emitted from your other hand...  This is "heat capacity", the bigger
  mass is slower to heat up and slower to loose it's energy.  Cold plugs use
  fat wires and retain more heat.

  Of course, I'm being facetious.  No sensible engineer would do such a silly
  experiment.  

  He'd use a co-op student.

:> 
:>     Actually, adding nitromethane to fuel will make an engine run cooler, not
:>     hotter (allowing lower temperature for a given RPM, or higher RPM for a
:>     given temperature).  From the engine's point of view, "cold" = "good", so
:>     it will run faster and more efficiently.  I agree with the water contami-
:>     nation, especially if you leave fuel sitting in an unsealed container in
:>     your flight box.
:
:I don't think the laws of thermodynamics can be violated!  Nitro gives you 
:more power thru more heat per power stroke.  Have you measured the 
:head temperature of an engine running more nitro???  I'd bet a big dinner 
:on this one.
:

  You're on!
 
  I'm not sure what law of thermodynamics is being violated.  This ain't
a steam engine we're talking about here -- an internal combustion engine is
not an adiabatic engine!  The object of an internal combustion engine is to 
produce mechanical power, not heat.  The heat energy is a byproduct that is 
lost during the process, it isn't stored anywhere to later be put back into
the cycle:  Hence all those ugly fins we have glued onto the cases.  If
you are delivering the same RPM into a prop at a higher engine temperature,
your engine is running LESS efficient.  The addition of nitro is to make the
engine more efficient, i.e., run cooler at the same RPM, or ultimately, more
RPM (power, when twirling a prop) at the same temperature. 

  On to the next experiment.  After carefully reassuring your co-op student  
that this one won't hurt.  No, really.  It won't!  Use a reliable, ABC engine
as a combination test device and temperature indicator.  An ABC engine will
quit running at the about same internal temperature due to the expansion of
the liner around the piston.  It will heat up and loose compression, shutting
down when the engine reaches a certain temperature.

  Get a tank of FAI, flip it over (have the co-op use one of his more heavily
bandaged fingers to start the engine) and tach it out to it's maximum RPM.
Now do the same thing with 5%, 10% and 15% fuel.  You'll note that the engine
will gain a few hundred RPM per each additional 5%.  The reason?  You added
energy, but you also increased the efficiency of the engine so it ran a little
cooler.  The end result is more RPM into the prop (up to the point where the
ABC engine will shut down), indicating more horsepower on the shaft of the
engine.

  As to the why of nitro running cooler, I don't know.  It may be that it
because it is more volatile, it produces more gas volume per unit of heat
during detonation.  My buddies, the engine wizards, tell me there's more to
it than that; they say the gas produced is a better heat conductor and it
draws more heat out of the engine.  I guess this would translate into a hotter
exhaust for the same head temperature.  If this is the case, the exhaust will
be hotter, but the engine won't be.

  I understand the Chop House has great steaks...

:
:Yes, it's true that the Rossi and Enya plugs are better quality than a Fox 
:plug.  For most sport flyers though, they usually don't need to spend the 
:extra money.  Most of these engines run fine on Fox plugs.
:

   Time for a "quality is worth the money" pitch!  Let's assume that an
inexpensive plug cost $1.50 and and expensive one goes for $5.00.  Arguably,
you may get three times the lifetime out of the dead cold plug (it has a
thicker element and two to three times the platinum, that's why it costs
$5.00), so you pay $4.50 for three cheap plugs or $5.00 for one not so cheap
plug, lasting the same amount of time.  

   How do you figure out when a plug is going bad?  Most flyers will take
off, go dead stick, hopefully land it safely, diddle the engine, and try
again.  The really attentive RC jockey will replace the plug after the 
third dead stick.  Us dumber ones may do five or six.  At our field, if you
go dead on the end of a cross wind take off run, you have to carefully 
fly between the barbed wire strands on the fence at the end of the field.

   What did you get for fifty cents?  Ten to fifteen dead stick landings.

   Anyhow, your instinct was right -- if you are having idle problems on new 
plugs and you're loosing RPM after you take the battery off, go to a plug
with a bigger element.  Those are most definitely "cold" plugs.         
   
--------------------------
Bill Chidester 
bill@c3177208.ssr.hp.com