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Introduction to Marine Engineering

Massachusetts Maritime Academy

Subsection 6.3.5 Compression Ratio and Clearance Volume

Clearance volume is the volume remaining in the cylinder when the piston is at TDC. Because of the irregular shape of the combustion chamber (volume in the head) the clearance volume is determined empirically measuring the amount of fluid required to fill the chamber while the piston is at TDC. This volume is then added to the displacement volume in the cylinder to obtain the cylinder’s total volume.
An engine’s compression ratio is the ratio of the volume of the cylinder when the piston is at the bottom of its stroke (BDC) to the volume when the piston is at the top of its stroke (TDC), as shown in Figure 6.3.5.
Figure 6.3.5. Compression Ratio
\begin{equation*} \text{Compression Ratio (CR)} = \frac{V_c + V_d}{V_c} \end{equation*}
Where:
  • \(V_c\) is the clearance volume (the volume of the combustion chamber when the piston is at TDC), and
  • \(V_d\) is the engine displacement (the volume swept by the piston as it moves from BDC to TDC).

Example 6.3.6. Compression Ratio.

Determine the compression ratio of an engine with the following specifications:
  • Bore = 100 mm
  • Stroke = 80 mm
  • Clearance Volume = 50 cc
Answer.
This engine has a compression ratio of approximately 13.57:1
Solution.
Calculate the engine displacement (Swept Volume):
\begin{equation*} V_d = \pi \times \left(\frac{100}{2}\right)^2 \times 80 = 628,318.53 \text{ mm}^3 = 628.3 \text{ cc} \end{equation*}
Calculate Compression Ratio:
\begin{equation*} CR = \frac{V_c + V_d}{V_c} = \frac{50 + 628.3}{50} = \frac{678.3}{50} \approx 13.57:1 \end{equation*}
This engine would have a compression ratio of approximately 13.57:1.
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