Calculating the Compression Ratio
Posted: Mon Jun 30, 2008 5:54 pm
The ratio is calculated by the following formula:
, where
b = cylinder bore (diameter)
s = piston stroke length
Vc = volume of the combustion chamber (including head gasket). This is the minimum volume of the space into which the fuel and air is compressed, prior to ignition. Because of the complex shape of this space, it usually is measured directly rather than calculated.
If the nominal compression ratio of an engine is given, the pre-ignition cylinder pressure can be estimated using the following relationship:
where p0 is the cylinder pressure at bottom dead center (BDC) which is usually at 1 atm, Cr is the compression ratio, and γ is the ratio of specific heats of the working fluid, which is about 1.4 for air, and 1.3 for methane-air mixture.
For example, if an engine running on gasoline has a compression ratio is 10:1, the cylinder pressure at top dead center (TDC) is
, where b = cylinder bore (diameter)
s = piston stroke length
Vc = volume of the combustion chamber (including head gasket). This is the minimum volume of the space into which the fuel and air is compressed, prior to ignition. Because of the complex shape of this space, it usually is measured directly rather than calculated.
If the nominal compression ratio of an engine is given, the pre-ignition cylinder pressure can be estimated using the following relationship:
where p0 is the cylinder pressure at bottom dead center (BDC) which is usually at 1 atm, Cr is the compression ratio, and γ is the ratio of specific heats of the working fluid, which is about 1.4 for air, and 1.3 for methane-air mixture.
For example, if an engine running on gasoline has a compression ratio is 10:1, the cylinder pressure at top dead center (TDC) is