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