Posted by T.O.O. on May 19, 1998 at 21:10:52:
The World According To T.O.O. #2 Or What’s Any Of This Got To Do With El Nino?
Combustion….Having read the first in the series again, I realize
that I’m getting somewhat out of the original sequence, but we’re going
to go in several circles before this is over anyway.
Combustion is "the process of burning" according to the dictionary.
This process of burning and its characteristics are what causes the crankshaft
to rotate and "power" your vehicle. Engines are frequently compared to
"air pumps", and they are. However, the ability to flow air is not the
great contributor to "power" that many of us have, and do think.
Air flow is important simply because without it, you can’t support
the combustion process. The thousands of hours spent porting heads is something
that began from the need to achieve greater amounts air for consumption
as engines were run at higher rpm than originally designed, and also to
"clean up" irregularities in the cylinder head ports due to casting / machining
mismatches, which is common with sand cast components. Since the piston
/ rotary engines are of a positive displacement nature (the cylinder volume
is a known), one can calculate how much air the engine requires by multiplying
total displacement by .5 (for four stroke engines) / 1,728ci.(there are
1,728 cubic inches per cubic foot) = required cubic feet of air flow per
revolution x RPM. As an example we’ll use a 1.8 ltr. Honda. 1.8 ltr.= 109.8
cubic inches x .5 = 54.9 cubic inches per revolution. 54.9 / 1,728 = .0317.
Now we’ll assume that the maximum RPM will be 8,000, so 8,000 x .0317 =
253.6 cfm.(cubic feet / minute). Now we’re assuming that the engine is
100% efficient, and that’s all the air it would inhale at 8,000 RPM. Well
designed engines can achieve 100%, and then some, in terms of (volumetric
efficiency). We’re going to forget the term volumetric efficiency, and
simply discuss efficiency because volumetric efficiency is a term that
tends to place a limit on "power", and in my opinion the VE term is absolutely
meaningless. So now we know how to determine "how much flow at (x) RPM."
is required for our application. Armed with that knowledge we can determine
the necessary flow rates for the inlet port and exhaust port, in order
to properly fill and evacuate the cylinders.
5-19-98 I’ve just now realized that this paper is far from finished,
and it certainly relates to the "Pressure Of Beans" article, so I plan
on finishing this next week. You guys are good at asking questions that
get me "off track". Please remember in the future that as I grow older
my mind goes farther and farther away, and I