NetNews Usenet Archive 1992 #19/NN_1992_19.iso/spool/rec/models/rc/3586

Text File | 1992-09-02 | 4.5 KB | 91 lines

Newsgroups: rec.models.rc
Path: sparky!uunet!ftpbox!mothost!white!rtsg.mot.com!svoboda
From: [e-mail] (David Svoboda)
Subject: Re: FAQ? (questions about engines)
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Date: Wed, 2 Sep 1992 21:59:12 GMT
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Well, I hate to get into this one, but...
In article <[e-mail]> [e-mail] (Bill Chidester) writes:
|
|:>     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.

PV=nRT.  That's the thermo law you are violating.

The only way for a fuel additive, in the absence of engine mods, to produce
more power is for the burning reaction to produce more pressure, and the above
equation says that with that pressure comes temperature, in proportion.

|  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.

The bottom line is that if a fuel combination produces more heat, then the engine
will produce more power, and the engine will run hotter.  That's just what nitro
does.  And that's just why high octane in your car (burns cooler) produces less
power than low octane (unless you modify your engine to take advantage).

Look at it this way: if nitro cooled the engine down, then 50% nitro would be
the answer for tightly cowled engines.  It's not.

|   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.

Most sport flyers go all season on a single cheap plug.  I recommend changing
the plug every season, just in case.  Nevertheless, my record for a K&B idle
bar plug is four years (on an old OS Max 35).

There is no sense in spending more money than necessary.  But don't hesitate to
spend as much as *is* necessary.

|   How do you figure out when a plug is going bad?

Well, most of the time (on sport engines), the engine doesn't run, and the
glow panel shows an open circuit.

|   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.

Other way.  You need a hotter plug if it won't idle without a battery.  Go
with the hottest plug that will last in your particular engine.

            Dave Svoboda, Palatine, IL