i sent the following to the list yesterday but never got it back, did
anyone else see it?
(snip)
>>What determines the compression rating is the distance of the top
>>compression ring below the top of the piston.
(snip)
i think i must have missed the start of this, but the answer above seems
curious- given the volume of the "compression/combustion chamber" bounded
by the piston top, the cylinder walls, and whatever space there is in the
the head, the compression ratio is the ratio of this volume at bottom dead
center to the volume at top dead center- on the theoretical side, and going
back to thermodynamics (otto cycle for you fellow chemistry types), higher
compression ratio means more efficiency, on the more practical side, higher
compression means greater likelihood to knock at a given fuel octane rating
and increased oxides of nitrogen in the exhaust- anyway, given typical
compression ratios in the 8:1-10.5:1 range and atmospheric pressure of
14.7psi (760 torr, average at sea level), and neglecting adjustments for
compression of gas at constant energy (tends to increase temperature and,
therefore, pressure in the compression chamber during compression stroke)
and the fact that intake is not totally efficient and also that intake
valve usually still partially open as the compression stroke starts, you
should expect cylinder compression to be about 14.7*(8.0-10.5), or
120-160psi- it's hard for me to believe that the space between the piston
and cylinder wall but above the top ring is anything but negligible, which
brings me around to my first comment about the "curious" allusion to the
compression ring above
___ ___
/xxx\ /xxx\ terrible ascii "art", try to imagine / & \ as valves
|xxxxx| || P || (wow, it's a hemi!) and P as piston
|xxxxx| || o || compression ratio comparable to number of x @BDC
|xxxxx| | | divided by number of x @TDC
|| P || | |
|| o || | |
BDC TDC
thanks
joe
Joseph R Schneider
"You have a Lucas type 4 generator on a 12 volt system, and you know the
British, they'd rather spend time glueing a piece of wood to the dashboard
than get the electrical system right." Tim Robbins as Ed Walters in "I.Q."
|