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Compression Ratio

 

Compression Ratio Formula

CR=1+π4BORE2STROKECCV+HGV+PDV

Symbol English Metric
CR = Compression Ratio dimensionless dimensionless
\pi Pi 3.141 592 653 ... 3.141 592 653 ...
BORE = Bore in^2 mm^2
STROKE = Stroke Length in mm
CCV = Combustion Chamber Volume in^3 mm^3
HGV = Head Gasket Volume in^3 mm^3
PDV = Piston Deck Volume in^3 mm^3

Compression ratio, abbreviated as CR, a dimensionless number, is a term used in reference to internal combustion engines, and refers to the ratio of the maximum volume inside the combustion chamber of an engine (when the piston is at the bottom of its stroke) to the minimum volume inside the combustion chamber (when the piston is at the top of its stroke).  In other words, the compression ratio is the ratio of the total volume inside the combustion chamber when the piston is at the bottom of its stroke (with the piston at its lowest point), to the volume inside the combustion chamber when the piston is at the top of its stroke (with the piston at its highest point).

A higher compression ratio means that the air/fuel mixture inside the combustion chamber is compressed to a smaller volume before it is ignited, resulting in more energy being released during combustion.  This can lead to better fuel efficiency, more power, and improved performance.  However, high compression ratios can also lead to increased engine stress, higher operating temperatures, and a greater risk of engine knocking, which can cause damage to the engine.  The compression ratio of an engine is determined by the design of the engine, and is typically expressed as a ratio, such as 10:1 or 12:1.  A typical compression ratio for a gasoline engine is in the range of 8:1 to 12:1, while a diesel engine typically has a higher compression ratio, in the range of 15:1 to 20:1.     

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Compression Ratio Formula

CR \;=\;  \dfrac{ V_d + V_c }{ V_c }     (Compression Ratio)

V_d \;=\;  \dfrac{ CR \cdot V_c }{ CR - 1 }

V_c \;=\; \dfrac{  V_d  }{ CR - 1 }

Symbol English Metric
CR = Compression Ratio dimensionless dimensionless
V_d = Displacement Volume in^3 mm^3
V_c = Clearance Volume in^3 mm^3