A 4-cylinder aircraft engine has a cylinder bore of 3.78 inches, and is 8.5 inches deep. With the piston on BDC, the top of the piston is 4 inches from the bottom. What is the approximate total piston displacement of this engine?

• A. 180
• B. 210
• C. 200
• D. 230

Answer: (C)

Total piston displacement (engine displacement) is found by multiplying the piston’s swept volume per cylinder by the number of cylinders. The swept volume equals the bore area times the stroke.

Bore radius = 3.78 / 2 = 1.89 inches. Area = π r^2 ≈ 3.1416 × (1.89)^2 ≈ 11.22 square inches.

The stroke is the distance the piston travels from BDC to TDC. With the piston at BDC, its top is 4 inches from the bottom of the cylinder, and the cylinder depth is 8.5 inches, so stroke = 8.5 − 4 = 4.5 inches.

Displacement per cylinder ≈ 11.22 × 4.5 ≈ 50.5 cubic inches. With four cylinders, total displacement ≈ 4 × 50.5 ≈ 202 cubic inches, which is typically rounded to about 200 cubic inches.

So the approximate total piston displacement is 200 cubic inches.