What is compression ratio?

What is compression ratio?

• A. A compression of a cylinder volume between BDC and TDC
• B. The ratio of bore to stroke
• C. The ratio of intake to exhaust pressure
• D. The ratio of air to fuel

Answer: (A)

Compression ratio measures how much the engine squeezes the air–fuel mixture inside a cylinder. It’s the ratio of the cylinder volume when the piston is at bottom dead center (the largest volume) to the volume when the piston is at top dead center (the smallest volume, called the clearance volume). In other words, compression happens from the larger volume at BDC down to the smaller volume at TDC, and the ratio is essentially (swept volume plus clearance volume) divided by the clearance volume. Higher compression generally improves thermal efficiency but can require higher-octane fuel to avoid knocking. For example, if the piston moves from 450 cc at BDC to a 50 cc clearance volume at TDC, the compression ratio is (450 + 50) / 50 = 10:1. Other choices describe different concepts: bore-to-stroke is a geometric relationship that influences displacement, not compression; intake-to-exhaust pressure or air-to-fuel ratio are unrelated to the cylinder’s compression ratio.

Compression ratio measures how much the engine squeezes the air–fuel mixture inside a cylinder. It’s the ratio of the cylinder volume when the piston is at bottom dead center (the largest volume) to the volume when the piston is at top dead center (the smallest volume, called the clearance volume). In other words, compression happens from the larger volume at BDC down to the smaller volume at TDC, and the ratio is essentially (swept volume plus clearance volume) divided by the clearance volume.

Higher compression generally improves thermal efficiency but can require higher-octane fuel to avoid knocking. For example, if the piston moves from 450 cc at BDC to a 50 cc clearance volume at TDC, the compression ratio is (450 + 50) / 50 = 10:1.

Other choices describe different concepts: bore-to-stroke is a geometric relationship that influences displacement, not compression; intake-to-exhaust pressure or air-to-fuel ratio are unrelated to the cylinder’s compression ratio.