From: bloom@inland.com
Newsgroups: rec.models.rc
Subject: Re: FAQ? (questions about engines)
Date: 8 Sep 92 08:42:15 CST
Organization: Inland Steel Company; East Chicago, IN

In article <1992Sep4.160330.21836@dcdwest.uucp>, plb@dcdwest.uucp (Phil Blais) writes:
>>
>>
>>|>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.
>>|>
>>|>
>>The extra heat you get is just energy you can't use.  The ideal engine doesn't
>>get hot.  The exhaust should be cold.  This way, all energy is mechanical and none is used to heat up the exhaust gases.
>>

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!



>>
>>|>  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?




>>|>
>>|>  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.
>>
> 
> If they gave an award for the year's best posting I'd vote for this one. I just
> wish I would have seen the original article to give the author my heart felt
> congratulations. I sincerely hope he/she is a teacher. The combination
> of humor with subject material is priceless when it comes to getting kid's
> attention. Bravo!
> -- 
> =========================================================================
> phil blais  N6KXD AMA 12662      ITT    Aerospace Communications Division
> dcdwest!plb@ucsd.edu           DEFENSE    10060 Carroll Canyon Road
>  (619) 578-3080					San Diego, CA 92131
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