From: ntaib@silver.ucs.indiana.edu (Iskandar Taib)
Newsgroups: rec.models.rc
Subject: Re: FAQ? (questions about engines)
Date: 9 Sep 92 17:44:19 GMT
Organization: Indiana University

In article <1992Sep8.085235.2282@inland.com> bloom@inland.com writes:
>In article <Bu31B3.CBM@usenet.ucs.indiana.edu|, ntaib@silver.ucs.indiana.edu (Iskandar Taib) writes:
|| In article <1992Aug31.111533.2265@inland.com| bloom@inland.com writes:

|| When the C/L speed fliers were told to limit their nitro (they were 
|| running 50-60% nitro, and the new rules said they had to run somewhat
|| less - 40%?) they discovered that the engines ran hotter. Why? Well, 
|| with more nitro, you actually have to feed more fuel in per unit time,
|| since nitro contains less energy (joules, or btu, or whatever unit 
|| you'd like to use) both per unit mass and per unit volume, since it 
|| releases oxygen upon burning, which is why it it used to augment power 
|| in the first place (it doesn't "contain" more power; gasoline has 
|| far more btu/cc than methanol or nitromethane does - it releases oxygen 
|| which would give the same effect as a turbocharger would). 
|| 
|
|Wrong Iskandar!  Nitro provides more heat per power stroke than an engine 
|running methanol or gasoline.  You are correct in saying that nitro has 
|lower btu per mass or volume, BUT and this is the important point, 
|the power of the internal combustion engine is determined by the volume 
|of oxygen that can be pumped thru it.  Nitro requires less oxygen than 
|either methanol or gasoline for stoichiometric burning.  This means that 
|for a fixed amount of oxygen you burn more nitromethane than alcohol 
|or gasoline and thus the BTUs of the nitro burn are significantly higher. 

What I think a lot of people forget here is that the nitromethane
is there both as a fuel and an oxygen source, not simply as a fuel
"with more power". I agree with what you say above, (after all, the
added power that comes from using more nitro can be traced to the
added oxygen that comes from the nitro's NO2 groups) but remember 
that a LOT of the cooling in our engines comes from the vaporization
of the fuel. If a fuel requires less oxygen for stoichiometric bur-
ning, you will require more of it if you feed a fixed amount of air
through the ports. This added amount of fuel will serve to cool the
engine when it vaporizes. 

Take gasoline engines as an example. Gasoline has a much higher 
specific energy content than methanol does (in this case, since 
methanol is partially oxidized, and gasoline also has a higher
C:H ratio than methanol does). So a gasoline fueled motor can run
longer on a fixed volume or weight of fuel compared to an alco-
hol fueled motor. You'd set the needle a lot "leaner" on a gaso-
line motor. But the gasoline motor runs hotter, even though it 
does not produce nearly as much power as the alcohol motor. Why?
Since you reduce drastically the amount of fuel ingested, you 
take away a lot less heat per unit time. I am, of course, ignoring 
specific heats of vaporization, but I'm sure that common experience
would tell us that gasoline engines run hotter (I _don't_ think 
anyone will dispute this.. see Aldrich's writings for example)
so even if gasoline had the higher heat of vaporization it doesn't
make up for the much decreased volume of fuel ingested per unit time.

Now why _does_ an alcohol engine produce more power? I've read that
it also has to do with the cooling effects of vaporization. When 
you vaporize the fuel in the carb, you drastically lower the tem-
perature of the incoming charge. This allows you, in turn, to stuff
more air + fuel into the cylinder when combustion takes place. 
More air = more oxygen = more power.



-- 
-------------------------------------------------------------------------------
Iskandar Taib                          | The only thing worse than Peach ala
Internet: NTAIB@SILVER.UCS.INDIANA.EDU |    Frog is Frog ala Peach
Bitnet:   NTAIB@IUBACS                 !
Newsgroups: rec.models.rc