Compression Ratio Calculator
Calculate the static compression ratio and total engine volume for a combustion engine. Enter engine characteristics, select units, and view results in standard, step-by-step, or chart views.
Result
Calculation Steps
Volume Visualization
Formulas
Compression Ratio Calculations
The compression ratio (CR) is calculated as follows:
Where:
- V_d: Displacement volume (cc)
- V_c: Compressed volume (cc)
Displacement Volume
Where:
- b: Bore diameter (cm)
- s: Piston stroke (cm)
Compressed Volume
Components:
- V_chamber: Combustion chamber volume (cc)
- V_piston: Piston volume (cc, negative for dished, positive for domed)
- V_gasket = ¼ × g² × t × π (g: gasket bore, t: gasket thickness, in cm)
- V_clearance = ¼ × b² × c × π (c: deck clearance, in cm)
Usage Notes:
- All length inputs are converted to cm, volumes to cc.
- Results are rounded to 3 decimal places.
- Piston volume is positive for domed pistons, negative for dished.
How to Use the Compression Ratio Calculator
- Enter Engine Specs: Input number of cylinders and measurements (bore, stroke, etc.).
- Select Units: Choose units for lengths (mm/cm/in) and volumes (cc/liters/cu in).
- Choose Display: Select standard, step-by-step, or chart view.
- Calculate: Click “Calculate” to process inputs.
- View Results: Review the compression ratio and engine volume.
- Reset: Click “Reset” to clear fields.
Frequently Asked Questions (FAQs)
What is the compression ratio?
The compression ratio (CR) is the ratio of the total volume (displacement + compressed volume) to the compressed volume when the piston is at top dead center, indicating how much the air-fuel mixture is compressed.
How is the compression ratio calculated?
CR = (V_d + V_c) / V_c, where V_d is the displacement volume (¼ × b² × s × π) and V_c is the sum of combustion chamber, piston, gasket, and clearance volumes.
What is a good compression ratio?
Higher compression ratios improve engine performance but are limited by fuel octane. Typical static CR ranges from 10:1 to 12:1 for conventional engines, corresponding to 8:1 to 8.5:1 dynamic CR.
What’s the difference between static and dynamic compression ratio?
Static CR is based on total volumes, while dynamic CR accounts for the intake valve closing point, affecting actual compression. This calculator computes static CR.
Why does piston volume vary?
Piston volume is positive for domed pistons (reducing V_c) and negative for dished pistons (increasing V_c), affecting the compressed volume.