Monkeyleg, the problem there is that you don't know the final volume above the piston in your calculation, thus you don't have a divisor in order to get a ratio. If one assumes that the volume remaining above the piston is cylindrical and "flat" on top, you could do it, but you'd need to know the distance from the top of the piston to the top of the cylinder at the end of the stroke to calculate that final remaining volume.
The volume of a cylinder is pi x the radius squared, times the height. In your question, the radius is half the bore, and the height is the stroke. That's the volume "swept" by the piston. Then you calculate the volume remaining
above the piston at TDC (if you can), add that to the swept volume of the piston, and divide that total by the volume remaining above the piston at TDC, to obtain a compression ratio.
Unfortunately, the top of the cylinder is not usually flat, but complex in shape, like in the "hemi" combustion engines. You could come pretty close in the old "flathead" engines, but not with present-day engines. That's why I added "if you can" in the above. To complicate this further, the tops of the pistons can have complex shapes to swirl the intake charge --or whatever.
Fortunately, the engine manufacturers have done all this for you, and publish this compression ratio.
You
could do it by "brute force" by filling the cylinder with oil and measuring the two volumes of oil, though. Yuch.
280plus:
I'm thinking gauge.
I usually don't. I like real zero points in pressures and temperatures, so one can have a cardinal number scale from which to work.
For example, going from 10°C to 20°C does not "double" the temperature. It only increases the temperature by "let's call it" 3.5 % in real terms. This, since the real zero point in temperature is 273.15°C below the so-called"zero" of the centigrade scale. ("Absolute Zero.") You are really only going from 283.15 to 293.15 in this example.
That's why the Kelvin scale of temperature*, with a real zero, is so valuable in thermodynamics. You can't do calculations in "Centigrade.*"
And going from "zero" gauge pressure to 14.7 psig (gauge) is not an "infinite" increase in pressure (14.7 divided by zero), but only a doubling of the real, or absolute pressure. I do not live, here at 6000 feet above sea level (Colorado), in a negative pressure of 2.5 psi. I live in air whose pressure is 12.2 psi.
This sometimes leads to problems when you're taking compression readings on engine cylinders. A cylinder which reads low may not actually be bad.
Let's face it. If you took readings of cylinder compression in outer space, they'd all be "bad."
(A hint: "gauge" is spelled alphabetically. That is, "a" comes before "u," not the other way around. "Gage," however, is perfectly acceptable, if unusual.)
Terry, 230RN
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*
I point out, before others do, that there is no such thing as "degrees Kelvin." "Kelvin" is a unit all by itself. Thus, the freezing point of water is not 273.15 "degrees Kelvin" but simply 273.15 Kelvin, or K.
Thus, water freezes at 273.15K.
