Our compression ratio calculator helps you determine the compression ratio of an internal combustion engine.
The compression ratio is a fundamental parameter that tells us how much the air–fuel mixture is compressed before ignition.
Do you want to know the formula for compression ratio, understand the meaning of swept volume and clearance volume, and see how they affect engine performance?
Are you curious about why sports cars often have higher ratios compared to everyday commuter vehicles? Look no further, because you’ve come to the right place!
Compression Ratio Calculator
Compression Ratio Formula & Explanation
📘 Main Formulas
Swept Volume (Vswept):
Vswept = π × (Bore / 2)² × Stroke
Compression Ratio (CR):
CR = (Vswept + Vclearance) ÷ Vclearance
📖 Meaning of Terms
- Bore (B) — Cylinder diameter (in mm). Convert to cm when calculating volumes.
- Stroke (S) — Piston travel length (in mm). Convert to cm for calculations.
- Vswept — Swept (displacement) volume per cylinder, in cc.
- Vclearance (Vc) — Clearance volume at top-dead-center, in cc.
- CR — Compression Ratio, expressed like
10.5 : 1. - π — Pi, approx 3.14159.
📝 Worked Problem Example
Problem: An engine cylinder has Bore = 86 mm, Stroke = 86 mm, and Clearance Volume = 50 cc. Find the Swept Volume and the Compression Ratio.
Step-by-step Solution
Bore = 86 mm = 8.6 cm,
Stroke = 86 mm = 8.6 cm.
Radius = Bore ÷ 2 = 8.6 ÷ 2 = 4.3 cm.
Area = π × radius² = π × (4.3)² ≈ 58.09 cm².
Swept Volume = Area × Stroke = 58.09 × 8.6 ≈ 499.56 cc.
CR = (Vswept + Vc) ÷ Vc
= (499.56 + 50) ÷ 50 ≈ 10.99 : 1.
📊 Reference Table for Compression Ratio Calculation
| Term | Description | Typical Value / Range | Units |
|---|---|---|---|
| Bore (B) | Cylinder internal diameter | 70 – 100 | mm |
| Stroke (S) | Distance piston travels | 70 – 120 | mm |
| Clearance Volume (Vc) | Volume above piston at TDC | 30 – 70 | cc |
| Swept Volume (Vswept) | Volume displaced by piston movement | 200 – 600 | cc (per cylinder) |
| Compression Ratio (CR) | Ratio of total cylinder volume to clearance volume | Petrol: 8 – 12 Diesel: 14 – 22 | — |
FAQs
What is compression ratio in an engine?
The compression ratio is the ratio of the maximum cylinder volume to the minimum cylinder volume. In simple terms, it shows how much the air–fuel mixture is squeezed before ignition.
Why is compression ratio important?
A higher compression ratio generally means better fuel efficiency and more power output. However, it also increases the risk of knocking, which is why performance engines need higher-octane fuel.
What is a good compression ratio for my engine?
For petrol engines, a compression ratio between 8:1 and 12:1 is considered normal. Diesel engines usually run much higher, between 14:1 and 22:1. The “good” range depends on your fuel type, engine design, and performance goals.