Short Answer:

The Otto, Diesel, and Dual cycles are three main thermodynamic cycles used to describe the operation of internal combustion engines. The Otto cycle represents spark-ignition engines, the Diesel cycle represents compression-ignition engines, and the Dual cycle combines features of both. The main difference lies in how heat is added during combustion — at constant volume in Otto, at constant pressure in Diesel, and partly at both in the Dual cycle.

Among these, the Otto cycle provides the highest efficiency for the same compression ratio, while the Diesel cycle is more practical for heavy engines. The Dual cycle represents real engine behavior more closely by balancing efficiency and pressure rise, making it ideal for modern compression ignition engines.

Detailed Explanation :

Comparison of Otto, Diesel, and Dual cycles

The Otto, Diesel, and Dual cycles are the three important air-standard thermodynamic cycles that describe how internal combustion engines convert heat energy into mechanical work. Although all three cycles consist of the same main processes — compression, heat addition, expansion, and heat rejection — they differ in how the heat is supplied during the combustion phase.

The Otto cycle is also known as the constant volume cycle, the Diesel cycle as the constant pressure cycle, and the Dual cycle as the limited pressure cycle or mixed cycle. Each has distinct characteristics, efficiency, and application based on the combustion process and engine type.

1. Otto Cycle

The Otto cycle is used in spark ignition (SI) engines, such as petrol engines. In this cycle, the heat addition process takes place at constant volume, meaning the piston remains momentarily stationary during combustion. The air-fuel mixture is compressed and ignited by a spark plug, causing rapid pressure rise.

• Processes involved:
  1–2: Isentropic compression
  2–3: Constant volume heat addition
  3–4: Isentropic expansion (power stroke)
  4–1: Constant volume heat rejection

Efficiency:
The efficiency of the Otto cycle depends only on the compression ratio. Higher compression ratios provide better efficiency but are limited by knocking in petrol engines.

Applications:
Used in petrol engines, motorcycles, scooters, and small automobiles.

2. Diesel Cycle

The Diesel cycle is used in compression ignition (CI) engines. In this cycle, only air is compressed during the compression stroke. Fuel is injected directly into the high-temperature compressed air, and combustion occurs at constant pressure.

• Processes involved:
  1–2: Isentropic compression
  2–3: Constant pressure heat addition
  3–4: Isentropic expansion
  4–1: Constant volume heat rejection

Efficiency:
The efficiency of the Diesel cycle depends on both the compression ratio and cut-off ratio (the ratio of volumes before and after constant pressure heat addition).

Here,  is the cut-off ratio.

Applications:
Used in heavy-duty engines such as trucks, buses, tractors, and industrial machinery.

3. Dual Cycle

The Dual cycle combines features of both the Otto and Diesel cycles. It is also called the limited pressure cycle. In this cycle, heat addition occurs partly at constant volume and partly at constant pressure. This makes it a more realistic representation of actual engine behavior.

• Processes involved:
  1–2: Isentropic compression
  2–3: Constant volume heat addition
  3–4: Constant pressure heat addition
  4–5: Isentropic expansion
  5–1: Constant volume heat rejection

Efficiency:
The Dual cycle efficiency depends on compression ratio (r), cut-off ratio (β), and pressure ratio (α). It is usually between the efficiencies of the Otto and Diesel cycles.

Applications:
Used in modern high-speed diesel engines and heavy vehicles where controlled combustion is required.

4. Comparison of the Three Cycles

Parameter	Otto Cycle	Diesel Cycle	Dual Cycle
Type of engine	Spark Ignition	Compression Ignition	Compression Ignition
Heat addition	Constant volume	Constant pressure	Partly at volume & pressure
Efficiency	Highest for same compression ratio	Lower than Otto	Between Otto and Diesel
Pressure rise	Highest	Lowest	Moderate
Fuel used	Petrol	Diesel	Diesel
Knocking tendency	High	Low	Moderate
Practical application	Petrol engines	Diesel engines	Modern diesel engines

(Note: Description kept simple for understanding; table here only for summarizing key points.)

5. Efficiency Comparison

For the same compression ratio:

However, in practice, Diesel engines often achieve better efficiency than petrol engines because they operate at much higher compression ratios.

The Dual cycle provides a balanced approach — moderate peak pressure, practical heat addition, and realistic efficiency. It is more suitable for engines where fuel burns over a period instead of instantaneously.

Conclusion

The Otto, Diesel, and Dual cycles are fundamental thermodynamic models representing different internal combustion engine operations. The Otto cycle is ideal for spark-ignition engines with constant volume combustion, the Diesel cycle is suitable for compression-ignition engines with constant pressure combustion, and the Dual cycle combines both processes for practical representation.

Among the three, the Otto cycle offers the highest theoretical efficiency, the Diesel cycle provides reliability for heavy-duty use, and the Dual cycle offers the best compromise between efficiency and realistic operation, making it the most accurate model for modern engines.