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PISTON DISPLACEMENT
The formula for finding piston displacement can be expressed as:
D = B × R
2
× S × N
D = Piston displacement
B = 3.14159265359 (a constant)
S = Piston stroke
N = Number of cylinders
R = Radius of one cylinder (one-half of bore)
(R
2
, read “R squared”, means R multiplied by itself)
Example 1
A single cylinder engine has a bore of 70 millimeters, and a stroke of 62 millimeters. What is its
displacement?
First convert 70.0 millimeters (bore) and 62.0 millimeters (stroke) to centimeters by dividing each by 10,
which is equivalent to multiplying each by 0.10, This step is necessary so that our answer will come out in
cubic centimeters. Then using bore and stroke figures expressed in centimeters in the formula:
R = one-half of bore = 3.50
S = 6.20
N = 1
D = B × R
2
× S × N
D = 3.1416 × (3.50)
2
× 6.20 × 1
D = 3.1416 × 12.25 × 6.20
D = 238.6 cubic centimeters
Example 2
A 3-cylinder engine has a bore of 60.0 millimeters and a stroke of 58.8 millimeters. What is its piston
displacement?
First convert both bore and stroke into centimeters by multiplying by 0.10:
R = one-half of bore = 3.0 centimeters
S = 5.88 centimeters
N = 3
D = B × R
2
× S × N
D = 3.1416 × (3.0)
2
× 5.88 × 3
D = 3.1416 × 9.0 × 5.88 × 3
D = 498.75 cubic centimeters
Note that the formula will work equally well if bore and stroke are expressed in inches or millimeters. If
they are expressed in inches, the answer will come out in cubic inches. The answer will come out in cubic
millimeters if millimeters are used in the formula.
COMPRESSION RATIO
To determine compression ratio of an engine, it is first necessary to know piston displacement (of one
cylinder) and combustion chamber volume. The formula can be expressed as:
Compression ratio = Piston displacement + Combustion chamber vol.
Combustion chamber volume
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