compression ratio..?

Question

what is the difference between compression ratio and the efective compression ratio in analyzing thermodynamic engine cycles???

Answer 1

I know of three differences when comparing terms for compression ratios:

In diesel and otto engines the compresion ratios is easily calculated by Vmax over V min

In some applications, the compression ratio is V max theoretical air over V min. (at least that is what the mechanical engineers handbook says- but I'm not at all familiar with this one)

In compressors (air, refrigerant) the compression ratio is calculated as the ratio between inlet and outlet pressures.

Answer 2

You can't get the compression ratio of an engine without knowing the internal geometry. The reason is the cylinder pressure does not return to ambient when the valves open because the valves don't remain open long enough. That's why when you run a compression test you must crank the engine for about five seconds, it allows the residual cylinder pressure to stabilize. A reading of 130 indicates a compression ratio of about 8.5:1 based on 15 psi ambient pressure but an 8.5:1 compression ratio should read above 180 because the normal residual pressure should be 18-19 psi.

Answer 3

The compression ratio is a single number that can be used to predict the performance of any engine (such as an internal-combustion engine or a Stirling Engine). It is a ratio between the volume of a combustion chamber and cylinder, when the piston is at the bottom of its stroke and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. Literally, high ratios place increased oxygen and fuel molecules into a reduced space, thus allowing for increased power at the moment of ignition. Higher compression ratios, however, also make detonation more likely.

The geometric compression—or “simple”—ratio is determined by comparing the volume of the cylinder and cylinder head when the piston is at bottom dead center (V1) with the volume of the cylinder and cylinder head when the piston is at top dead center (V2). Therefore, compression ratio (CR) = V1 ÷ V2. A more realistic compression ratio can be measured from exhaust-port closure to top dead center, which decreases V1 and, therefore, the overall CR. This is known as the “effective compression ratio.”

For more, click:
http://www.rccaraction.com/MA/click_trips/oct04/max46ax_1.asp
http://en.wikipedia.org/wiki/Compression_ratio

Answer 4

Compression ratio assumes no heat transfer to the compressor body walls. Effective assumes heat transfer, which lowers the efficiency (cooling air lowers its pressure).

Answer 5

for the formulas u have to use comprission ratio and must know type of process is it isentropic or no , actual is not isotropic for sure !
so can apply depends on where u want to use it