From: bill@c3177208.ssr.hp.com (Bill Chidester)
Subject: Re: FAQ? (questions about engines)
Date: Thu, 10 Sep 1992 02:50:01 GMT
Organization: Hewlett-Packard, Orlando Fl.
X-Newsreader: Tin 1.1scd1 PL4


   Whew!  Just when Iskandar and I thought this string was cooling off,
   we get a reprieve!

bloom@inland.com writes:
: If you took a thermodynamics class, you would know that there is not and 
: never will be an ideal engine.  Internal combustion engines operate 
: in the range of 30-40% efficiency.  This means that about 60-70% of 
: the chemical energy of the fuel is wasted as heat and is not converted 
: into mechanical energy of turning the prop.  To make your prop turn 
: faster you add more heat (nitro).  Adding nitro does not increase the 
: efficiency of your engine!
: 
   Well, I only took three thermodynamics classes, but I enjoyed every
   hour of sleep I got in them.  During one of my lucid periods it was
   explained that steam engines were good to study since they produced 
   almost all of their power by converting heat to mechanical work.  It
   was noted that other engines produced mechanical work by using pressure
   instead of heat; the simpliest being a gas turbine engine.  Unfortunately,
   pressure was about all a turbine engine used to produce work and the heat 
   effects could be ignored so it had no real application in a thermo-
   dynamics course.

   The instructor also dismissed internal combustion engines, since
   most of the work produced in them was due to the volume expansion
   of the liquid fuel reacting with the oxygen in air after ignition. 
   There is a good deal of heat involved in the production of the pressure
   but the thermodynamics course I had dealing with the heat energy due to
   combustion started out with gases (ignoring the state change and
   volume change from liquid to gas) and had a piston mounted free
   standing in the cylinder so they could conveniently ignore the volume
   changes after reaction (which is what makes you engine go 'round and 
   'round).  

   I'm also suspicious of making the assumption that the energy produced
   by the reaction of oxygen and nitromethane plus the energy produced
   by the reaction of oxygen and ethyl alcohol forms a linear system (i.e.
   it is the same as the reaction of ethyl alcohol, oxygen and nitromethane 
   combined at the same time).  It would seem that different compounds would
   be formed in the two cases and probably provide different amounts of energy.

: 
: 
: >>
: >>|>  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.
: 
: Good story, wrong answer!  I'll provide you with the correct analogy. 
: Heat a copper BB and a sixteen pound steel shot to 1000 F in a furnace.
: Remove them from the furnace and set them down on a copper slab.  Wait 
: 1 minute and pick them both up.  Which one is gonna burn you now?
: 
   Well, let's see.  You modified the thermal properties of the smaller   
   mass by changing from iron to copper, modified the heat loss by going
   from asbestos to a copper slab, changed the energy supplied to the
   two masses by sticking them in an oven that supplies unlimited energy
   (for practical purposes) and then heated both masses to thermal
   equilibrium.

   Somewhere, I seem to have lost control of my experiment.
   
   You've changed the experiment from a temperature rise demonstration
   to a heat capacity demonstration (the point of the experiment that
   followed).  Hot and cold are relative indicators and are used to 
   confuse us.  They don't have any reliable correlation to the amount of
   energy in a system.  A big, cold mass can very easily contain much 
   more energy than a small, hot one.

   If you subject a smaller mass (of the same compound) to the same amount
   of energy, it will get hotter than a larger mass!  The second part of the
   experiment resembles your proposed modifications without changing the
   compound, and using a measured amount of coolant (spit).  I agree with
   you 100%:  If you heat a larger mass to the same temperature as a 
   smaller mass, it will retain more heat.  It also takes more energy to
   heat the larger mass.  Cold plugs still use fat wires!                     

: 
: 
   At any rate, when I put FAI fuel in my cowled Cherokee and run an O/S
   .45 up at 14,000 RPM, it runs too hot.  When I put in 10% nitro and
   run at 14,000 RPM, it runs cooler -- no popping, no burnt oil odor, no 
   skin sizzling head temperature.  Same RPM, same prop = same horsepower. 
   It may well be that I'm running richer (not sure, may have to pay more
   attention next time), but the nitro definitely gives me a cooler running
   engine.  Now, we can extend this to leaning the mixture to the point that
   we're now running the same temperature as the FAI, but we will then be
   running more RPM.  The addition of the nitro is yielding more power at 
   the same temperature.  Same power, lower temperature:  Same temperature,
   more power.  In neither case did the addition of the nitro cause the
   engine to run hotter!  
    
   The only time I really get interested in a problem is when it collides
   with what intuition tells me must be right.  Hot does not mean "more 
   power" than cold:  It seems right, but it isn't.

   Other fights I like to start are with statements like:  "Dry air cools
   better than humid air" (not an easy one to put over on pilots, but 
   almost anyone else will bite at it) and "An electric arc is a great heat
   conductor".  Both of these statements confound our intuition (everyone
   knows water weighs more than air, right?) but they are easily demonstrated
   to be true.

--------------------------------
Bill Chidester    KE4WS
bill@c3177208.ssr.hp.com
Newsgroups: rec.models.rc