What is the Otto cycle?

Short Answer:

The Otto cycle is a type of air-standard cycle that explains how a four-stroke petrol engine works. It consists of four main processes—two are isentropic (compression and expansion), and two are constant volume (heat addition and heat rejection). This cycle represents how fuel is converted into mechanical energy in spark ignition (SI) engines.

The Otto cycle is used in most petrol engines, where combustion occurs at constant volume. It helps engineers understand engine performance, efficiency, and fuel consumption. The efficiency of the Otto cycle mainly depends on the compression ratio and the properties of the working gas (air-fuel mixture).

Detailed Explanation :

Otto Cycle

The Otto cycle is the idealized thermodynamic cycle used to describe the working process of a spark ignition (SI) internal combustion engine, which is commonly used in petrol engines. It was developed by Nikolaus A. Otto in 1876, who also built the first successful four-stroke engine based on this principle. The cycle provides a clear understanding of how energy is transformed from fuel into mechanical work inside the cylinder.

In the Otto cycle, air (or an air-fuel mixture) is considered the working substance. The process involves compression of the mixture, ignition by a spark, combustion at constant volume, and then expansion to produce power. Finally, the exhaust gases are released, completing one full cycle. The Otto cycle assumes that the engine operates under ideal conditions, meaning there are no friction losses and the working medium behaves as a perfect gas.

Processes in Otto Cycle

The Otto cycle consists of four thermodynamic processes which form a closed loop on the pressure-volume (P–V) and temperature-entropy (T–S) diagrams. These processes are:

  1. Isentropic Compression (Process 1–2):
    During this process, the piston moves upward, compressing the air-fuel mixture inside the cylinder. Since the process is adiabatic (no heat transfer), the temperature and pressure of the mixture increase while the volume decreases. This step stores energy in the form of compressed air-fuel mixture, preparing it for combustion.
  2. Constant Volume Heat Addition (Process 2–3):
    At the end of the compression stroke, a spark ignites the compressed mixture. The combustion process is assumed to occur at constant volume because the piston is momentarily stationary at the top dead center (TDC). As a result, the pressure and temperature rise sharply due to the release of heat energy from the burning fuel.
  3. Isentropic Expansion or Power Stroke (Process 3–4):
    After combustion, the high-pressure gases expand and push the piston downward. This process is also adiabatic (no heat transfer). The gas does work on the piston, which turns the crankshaft and produces mechanical energy. During expansion, both pressure and temperature drop as the gas performs work. This stroke is the power-producing stage of the engine.
  4. Constant Volume Heat Rejection (Process 4–1):
    Once the expansion is complete, the exhaust valve opens, and heat is rejected at constant volume. The gas returns to its initial state, completing one full cycle. The remaining exhaust gases are then expelled from the cylinder, and the cycle starts again.

Representation on P–V and T–S Diagrams

  • On the P–V (Pressure-Volume) diagram, the Otto cycle appears as a closed loop with two vertical (constant volume) and two curved (isentropic) lines.
  • On the T–S (Temperature-Entropy) diagram, the two isentropic processes are shown as vertical lines, while the constant volume processes are shown as horizontal lines.
    These diagrams help in visualizing how pressure, volume, temperature, and entropy change throughout the cycle.

Efficiency of Otto Cycle

The efficiency of the Otto cycle depends primarily on the compression ratio (r), which is the ratio of the maximum volume to the minimum volume in the cylinder. The efficiency is given by the formula:

Where:

  •  = Efficiency of the cycle
  •  = Compression ratio
  •  = Ratio of specific heats (Cp/Cv)

From this equation, it can be seen that higher compression ratios result in greater efficiency. However, in real engines, very high compression ratios may cause knocking, which limits the practical efficiency.

Applications of Otto Cycle

The Otto cycle is mainly used in:

  • Petrol engines in cars, motorcycles, and small generators
  • Small aircraft piston engines
  • Power tools and garden machinery like lawnmowers
  • Portable engine systems where lightweight and compact designs are needed

It remains the most common cycle for engines that operate on spark ignition and gasoline fuel.

Advantages of Otto Cycle

  • Simple design and easy operation
  • Provides good speed control and performance
  • Suitable for light vehicles and small machines
  • Relatively less vibration and noise compared to diesel engines

Limitations of Otto Cycle

  • Lower efficiency compared to Diesel cycle at the same compression ratio
  • Limited by knocking in petrol engines
  • Not suitable for very large or heavy-duty engines
  • High fuel consumption at low loads
Conclusion :

The Otto cycle is a fundamental thermodynamic cycle that explains the working of spark ignition petrol engines. It includes four key processes—isentropic compression, constant volume heat addition, isentropic expansion, and constant volume heat rejection. The efficiency of this cycle depends mainly on the compression ratio. The Otto cycle remains the base model for understanding and designing petrol engines, making it one of the most important cycles in mechanical and automotive engineering.