Compression Ratio

The compression ratio is the ratio of the total volume in a cylinder to the clearance volume. The clearance volume is the volume of the combustion chamber when the piston is at TDC, Figure 5.7. The total volume is the clearance volume plus the displacement.

The compression ratio is an engine characteristic related to engine efficiency, that is, the ability of the engine to convert energy in the fuel to useful mechanical energy. The greater the compression ratio, the greater the potential efficiency of the engine. The maximum compression ratio that can be obtained is a function of the type of fuel and the physical strength of the engine components. A compression ratio of 7 to 8:1 is common for gasoline engines and a ratio of 15 to 22:1 is common for diesel engines.

Expressed mathematically compression ratio is:

FIGURE 5.7. Cylinder clearance volume.

where CR = Compression ratio; PD = Piston displacement (in3); CV = Clearance volume (in3).

Problem: What is the compression ratio for a single cylinder engine with a B x S of 3.25 x 3.75 and a clearance volume of 6.20 in3?

Solution: The first step is to calculate the displacement:

PD =-x S x n =---- x 3.75 x 1 = 31.093 ... or 31.1 in3

Then the compression ratio is:

6.2 in3

Notice that the answer has no units. To have a ratio the units in the numerator and the denominator must be the same; thus the units cancel each other. A compression ratio of 6.0 would be expressed as 6.0:1.