Name the factors considered by an engine designer in selecting the compression ratio for a compression-ignition engine. Give examples of compression-ratio values. How does compression ratio affect NOx emissions?

 Answer

Compression ratio (CR) is the ratio of the cylinder volume when the piston is at BDC to the clearance volume when the piston is at TDC.

[
CR=\frac{V_s+V_c}{V_c}
]

Where:
(V_s) = Swept volume
(V_c) = Clearance volume

Factors considered in selecting compression ratio

An engine designer considers the following factors:

1. Thermal efficiency and fuel economy – Higher compression ratio generally gives higher thermal efficiency and better fuel economy.
2. Compression temperature – Sufficient temperature is required for reliable ignition of fuel, especially during cold starting.
3. Maximum cylinder pressure – A higher compression ratio increases compression and firing pressure.
4. Mechanical strength – Cylinder, piston, connecting rod, crankshaft and bearings must withstand the increased loads.
5. Thermal loading – Higher firing temperature increases the thermal stress on engine components.
6. Engine size and design – Bore, stroke, combustion-chamber design and piston design influence the suitable compression ratio.
7. Emissions – The compression ratio and combustion conditions must satisfy the required emission limits, particularly NOx.
8. Valve overlap and scavenging – Compression ratio may influence piston and valve design and the amount of residual exhaust gas in the cylinder.

Typical compression-ratio values

• Petrol/Spark-ignition engine: approximately 9:1 to 10:1
• Large slow-speed diesel engine: approximately 18:1 to 20:1
• Compression temperature in a typical diesel engine: approximately 650°C

Effect of compression ratio on NOx emission

Increasing the compression ratio can reduce NOx emissions under suitable engine operating conditions.

The reasons are:

Higher compression ratio
↓
Higher density of intake air
↓
Reduced valve overlap may be required
↓
More residual exhaust gas remains in cylinder
↓
Internal EGR effect
↓
Lower oxygen concentration and combustion temperature
↓
Lower NOx formation

However, increasing the compression ratio also increases maximum cylinder pressure and mechanical and thermal loads. Therefore, there is a practical limit to increasing the compression ratio.

Conclusion

A high compression ratio is desirable for good thermal efficiency, fuel economy and reliable ignition, but it is limited by maximum cylinder pressure, mechanical strength, thermal loading, engine design and emission requirements. Under appropriate combustion conditions, increasing the compression ratio can help reduce NOx emissions.