Compression Ratio

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.

 

compression ratio formula
\( CR \;=\; 1 + ( \pi\;/\;4 ) \; ( BORE^2 \; STROKE \;/\; CCV + HGV + PDV ) \)
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 formula
\( CR \;=\; V_d + V_c \;/\; V_c \)     (Compression Ratio) \( V_d \;=\; CR \; V_c \;/\; CR - 1 \) \( V_c \;=\;  V_d  \;/\; CR - 1 \)
Symbol	English	Metric
\( CR \) = Compression Ratio	\(dimensionless\)	\(dimensionless\)
\( V_d \) = Displacement Volume	\(in^3\)	\(mm^3\)
\( V_c \) = Clearance Volume	\(in^3\)	\(mm^3\)