Explain the working of an Otto cycle.

Short Answer:

The Otto cycle is a thermodynamic cycle that describes how a petrol (spark ignition) engine works. It consists of four ideal processes: isentropic compression, constant volume heat addition, isentropic expansion, and constant volume heat rejection. This cycle is used in most gasoline-powered vehicles.

In the Otto cycle, air-fuel mixture is compressed, ignited by a spark, and then expanded to produce power. After expansion, the exhaust gases are released, and the cycle repeats. The cycle is idealized but helps understand how internal combustion engines generate power from fuel.

Detailed Explanation:

Working of an Otto cycle

The Otto cycle is named after Nikolaus Otto, who developed the first successful four-stroke internal combustion engine. It is an ideal thermodynamic model used to understand the working of petrol engines that use spark ignition. The cycle describes the conversion of chemical energy from fuel into mechanical work using air (or air-fuel mixture) as the working fluid.

This cycle is commonly used in cars, bikes, and small generators, where the engine operates in four strokes: intake, compression, power (expansion), and exhaust. The theoretical Otto cycle is represented by four processes on P-V (pressure-volume) and T-S (temperature-entropy) diagrams.

Processes in Otto Cycle

The Otto cycle includes two adiabatic (isentropic) and two constant volume processes:

  1. Process 1–2: Isentropic Compression (Adiabatic Compression)
    • The piston moves upward, compressing the air-fuel mixture in the cylinder.
    • No heat is exchanged with surroundings.
    • Pressure and temperature increase, volume decreases.
    • Work is done on the gas.
  2. Process 2–3: Constant Volume Heat Addition
    • At the end of compression, a spark ignites the fuel.
    • Heat is added at constant volume (no change in piston position).
    • Temperature and pressure rise sharply.
    • This is where combustion happens in real engines.
  3. Process 3–4: Isentropic Expansion (Adiabatic Expansion or Power Stroke)
    • The high-pressure gases expand and push the piston downward.
    • No heat is exchanged.
    • Volume increases, pressure and temperature drop.
    • Work is done by the gas (this is the useful work output).
  4. Process 4–1: Constant Volume Heat Rejection
    • The exhaust valve opens.
    • Heat is rejected to the environment at constant volume.
    • Pressure and temperature decrease to original state.
    • The cycle is ready to begin again.

Additional Strokes in Real Engines

Although the Otto cycle describes only four thermodynamic processes, a real petrol engine works in four mechanical strokes:

  1. Intake – air-fuel mixture enters the cylinder.
  2. Compression – piston compresses the mixture (process 1–2).
  3. Power – ignition and expansion (process 3–4).
  4. Exhaust – gases are released (part of process 4–1).

The theoretical model helps explain the energy transformation, but real engines include other factors like valve timing, heat loss, and friction.

Efficiency of Otto Cycle

The thermal efficiency of the Otto cycle depends on the compression ratio (r) and the specific heat ratio (γ):

η = 1 – (1 / r^(γ – 1))

Where:

  • r = V₁/V₂ (compression ratio)
  • γ = Cp/Cv (usually 1.4 for air)

Higher compression ratio = higher efficiency, but in real engines, very high compression can cause knocking (uncontrolled combustion), which must be avoided.

Importance of Otto Cycle

  • Basic model for petrol engines used in most vehicles.
  • Helps calculate engine performance, power, and efficiency.
  • Shows relationship between compression ratio and fuel economy.
  • Basis for understanding engine tuning and combustion improvement.
Conclusion

The Otto cycle is an ideal thermodynamic cycle that explains the working of spark ignition (petrol) engines. It includes two adiabatic and two constant volume processes to convert heat energy into mechanical work. This cycle forms the backbone of modern automobile engines and helps in analyzing their performance and efficiency. While real engines have many complexities, the Otto cycle provides a clear and simple understanding of how fuel power is used to drive machines.