Formulae
Engine Capacity/Displacement
| Information required: |
1. Cylinder bore, 2. Crankshaft stroke, 3. Number of cylinders |
| Calculation: |
0.7854 x bore x bore x stroke x number of cylinders. |
| Example: |
Bore = 81mm (8.1cm), Stroke = 77.6mm (7.76cm), cylinders = 4 |
| Solution: |
0.7854 x 8.1 x 8.1 x 7.76 x 4 = 1599.5cc |
In the above example we have used centimetres to calculate capacity in cubic centimetres.
To find the displacement in cubic inches simply substitute measurements in inches:
8.1cm = 3.189" & 7.76cm = 3.055" giving 0.7854 x 3.189" x 3.189" x 3.055" x 4 = 97.6 cu.in.
To quickly convert from cc's to cu.ins. divide by 16.387, and from cu.in. to cc. Multiply by 16.387
Crankshaft Stroke
| Information required: |
1. Bore size, 2. Engine capacity, 3. Number of cylinders |
| Calculation: |
Divide engine capacity by: (0.7854 x bore x bore x no. of cylinders) |
| Example: |
1600cc 4 cylinder engine with an 81mm bore size |
| Solution: |
1600 ÷ (0.7854 x 8.1 x 8.1 x 4) = 1600 ÷ 206.12 = 7.76cm (77.6mm) |
Compression Ratio
Information required:
1) Capacity of ONE cylinder
2) Compressed volume
Cylinder capacity is the total engine displacement divided by the number of cylinders.
E.g. 4 cylinder 1600cc engine = 400cc per cylinder.
Compressed volume is the area above the piston crown when the piston is at its highest point or top dead centre (TDC).
This area comprises:
1) The combustion chamber (usually in the cylinder head as shown, but can also be in the piston crown),
2) The head gasket thickness,
3) The area between the cylinder block face and the piston crown, commonly referred to as the 'deck height'.
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