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Title: Compression Ratio
Describing the Principle: During engine operation, the air and fuel mixture in an engine must be compressed. This compression helps squeeze and mix the air and fuel molecules together for better combustion. The more the air and fuel are compressed, the more efficient the combustion in the engine. Compression ratio is a measure of how much the air and fuel has been compressed. Compression ratio is further defined as the ratio of the volume in the cylinder above the piston when the piston is at BDC (Bottom Dead Center), as compared to the volume in the cylinder above the piston when the piston is at TDC (Top Dead Center). The compression ratio is graphically shown in the diagram above. The mathematical formula for calculating compression ratio is:
Compression ratio = volume above the piston at BDC divided by volume above the piston at TDC
For example, if the volume at BDC were 55 cubic inches, and the volume at TDC were 5 cubic inches, the compression ratio would be 11-1. Depending upon the street rod engine, compression ratios may be as low as 8.5-1, to as high as 10 or 11-1 in high performance engines.
Back to Mechanical Principles
Describing the Principle: During engine operation, the air and fuel mixture in an engine must be compressed. This compression helps squeeze and mix the air and fuel molecules together for better combustion. The more the air and fuel are compressed, the more efficient the combustion in the engine. Compression ratio is a measure of how much the air and fuel has been compressed. Compression ratio is further defined as the ratio of the volume in the cylinder above the piston when the piston is at BDC (Bottom Dead Center), as compared to the volume in the cylinder above the piston when the piston is at TDC (Top Dead Center). The compression ratio is graphically shown in the diagram above. The mathematical formula for calculating compression ratio is:
Compression ratio = volume above the piston at BDC divided by volume above the piston at TDC
For example, if the volume at BDC were 55 cubic inches, and the volume at TDC were 5 cubic inches, the compression ratio would be 11-1. Depending upon the street rod engine, compression ratios may be as low as 8.5-1, to as high as 10 or 11-1 in high performance engines.
Back to Mechanical Principles
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