Short Answer:

The Otto cycle is the ideal cycle used to model petrol (spark ignition) engines, while the Diesel cycle models diesel (compression ignition) engines. The main difference lies in the way heat is added: constant volume in the Otto cycle and constant pressure in the Diesel cycle.

In the Otto cycle, the fuel-air mixture is compressed and then ignited by a spark. In the Diesel cycle, only air is compressed, and fuel is injected afterward, which ignites due to high temperature. These differences lead to variations in efficiency, compression ratios, and engine design.

Detailed Explanation:

Difference between Otto cycle and Diesel cycle

Both the Otto cycle and Diesel cycle are theoretical thermodynamic cycles that represent the working principles of two common types of internal combustion engines. While both cycles consist of four processes (two adiabatic and two heat-transfer processes), the method of heat addition and ignition is what sets them apart.

These cycles help engineers understand engine performance and efficiency in petrol and diesel engines. Let’s explore the differences in detail.

1. Type of Engine Modeled

• Otto Cycle:
  Models spark ignition engines (used in petrol cars and bikes).
• Diesel Cycle:
  Models compression ignition engines (used in trucks, buses, and generators).

2. Heat Addition Process

• Otto Cycle:
  Heat is added at constant volume.
  This occurs after the air-fuel mixture is compressed and the spark plug ignites it.
• Diesel Cycle:
  Heat is added at constant pressure.
  Fuel is injected into hot compressed air, and combustion starts automatically.

3. Compression Ratio

• Otto Cycle:
  Has a lower compression ratio (typically 8:1 to 12:1) to avoid knocking, as the fuel-air mixture is compressed.
• Diesel Cycle:
  Uses a higher compression ratio (typically 14:1 to 22:1) since only air is compressed, which increases efficiency.

4. Efficiency Comparison

The efficiency of both cycles is influenced by compression ratio and heat addition method.

• Otto Cycle Efficiency:
  Depends only on compression ratio.
  Efficiency formula:
  η_Otto = 1 – (1 / r^(γ – 1))
• Diesel Cycle Efficiency:
  Depends on compression ratio and cut-off ratio (volume ratio during heat addition).
  Generally, more efficient at high compression ratios.

However, at equal compression ratios, the Otto cycle is more efficient than the Diesel cycle because constant volume heat addition is more effective.

5. Fuel and Ignition Method

• Otto Cycle:
  Uses a pre-mixed fuel-air mixture and is ignited using a spark plug.
• Diesel Cycle:
  Uses direct fuel injection into compressed hot air and ignites automatically without a spark.

6. Applications

• Otto Cycle Engines:
  Used in light vehicles, motorcycles, small generators.
• Diesel Cycle Engines:
  Used in heavy-duty vehicles, trains, ships, and power plants.

7. Work Output and Noise

• Otto Cycle Engines:
  Produce smoother and quieter operation.
• Diesel Cycle Engines:
  Produce more torque and are noisier due to higher compression and fuel injection pressure.

Conclusion

The Otto cycle and Diesel cycle are both important thermodynamic cycles that represent the working of petrol and diesel engines, respectively. The Otto cycle adds heat at constant volume using a spark, while the Diesel cycle adds heat at constant pressure using compression ignition. Diesel engines are more efficient at high compression ratios and suitable for heavy-duty applications, while Otto engines are simpler, quieter, and better suited for light vehicles. Understanding these differences helps in selecting the right engine type for specific applications.