FORMULA FOR COMPRESSION RATIO (CR)
| CR = CYL. VOLUME @ BDC ÷ CYLINDER VOLUME @ TDC |
CV = CLEARANCE VOLUME
CR = (CV + SWEPT VOLUME) ÷ CV
CR = 1+ SWEPT VOLUME ÷ CV
CR = 1+ (SWEPT VOLUME ÷ VOL @ TDC)
CR = 1+ (0.7854 x BORE x BORE x STROKE) ÷ (CCV + HGV + PDV)
CCV = Combustion Chamber Volume, in cubic inches
Note: if volume is given in cc’s then ÷ 16.4 to get cubic inches.
HGV = Head Gasket Volume, in cubic inches,
HGV = Head gasket compressed thickness x 0.7854 x bore x bore
PDV = (Piston Deck Volume) + (Piston Dome Effective Volume)
PDV = (0.7854 x bore x bore x deck to piston distance) +
(volume of piston depressions - volume of piston bumps)
CV = CCV + HCV + PDV
Example: What is CR of the engine in #9 if heads have 72 cc chamber, head gasket is compressed to 0.040 inch and flat top pistons give 0.025 deck clearance at TDC?
Please Note: The above equations and rules apply only to four-cycle engines. The equations have been simplified for ease of understanding. Answers will be approximate but generally will be close enough for use as a guideline.
|
