The Otto cycle is an ideal thermodynamic cycle that describes the working process of a four-stroke petrol engine.

It was developed by Nikolaus Otto, who invented the first successful four-stroke internal combustion engine.

• The Otto cycle converts the chemical energy of fuel into mechanical work.

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Applications of Otto Cycle

The Otto cycle is used in:

• Petrol engines
• Motorcycles
• Cars and light vehicles
• Small generators
• Lawn mowers
• Portable machines

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Main Processes of Otto Cycle

The Otto cycle consists of four thermodynamic processes:

1️⃣ Isentropic Compression
2️⃣ Constant Volume Heat Addition
3️⃣ Isentropic Expansion
4️⃣ Constant Volume Heat Rejection

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1. Isentropic Compression Process (1 → 2)

During this process:

• The piston moves from Bottom Dead Center (BDC) to Top Dead Center (TDC).
• Air-fuel mixture is compressed.
• Pressure and temperature increase.
• No heat transfer occurs.

Characteristics

• Volume decreases
• Pressure increases
• Temperature increases

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2. Constant Volume Heat Addition (2 → 3)

At the end of compression:

• The spark plug ignites the compressed fuel-air mixture.
• Combustion occurs rapidly.
• Heat is added at constant volume.

Characteristics

• Sudden rise in pressure
• Temperature increases sharply
• Volume remains constant

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3. Isentropic Expansion Process (3 → 4)

This is the power stroke of the engine.

• High-pressure gases push the piston downward.
• Work is produced.
• Gas expands without heat transfer.

Characteristics

• Volume increases
• Pressure decreases
• Temperature decreases

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4. Constant Volume Heat Rejection (4 → 1)

In this process:

• Exhaust gases release heat.
• Pressure decreases.
• System returns to its initial condition.

Characteristics

• Heat rejected
• Volume constant
• Cycle repeats continuously

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Otto Cycle Diagram

The Otto cycle is represented on:

P-V and T-S Diagram of Otto cycle

• Shows heat transfer and entropy changes.

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Working of Four-Stroke Petrol Engine Based on Otto Cycle

The practical Otto cycle works in four strokes:

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1. Suction Stroke

• Intake valve opens.
• Air-fuel mixture enters cylinder.

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2. Compression Stroke

• Piston compresses mixture.
• Pressure and temperature rise.

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3. Power Stroke

• Spark plug ignites mixture.
• Explosion pushes piston downward.

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4. Exhaust Stroke

• Exhaust valve opens.
• Burnt gases leave cylinder.

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Thermal Efficiency of Otto Cycle

The efficiency of the Otto cycle depends mainly on the compression ratio.

Efficiency Formula

$$\eta = 1 – \frac{1}{r^{(\gamma -1)}}$$η=1−r(γ−1)1​

Where:

• $\eta$η = Thermal efficiency
• $r$r = Compression ratio
• $\gamma$γ = Specific heat ratio

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Factors Affecting Otto Cycle Efficiency

• Compression ratio
• Fuel quality
• Combustion efficiency
• Heat losses
• Engine speed

Higher compression ratio generally gives higher efficiency.

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Advantages of Otto Cycle

• Simple engine design
• Smooth operation
• High-speed capability
• Good power-to-weight ratio
• Widely used in automobiles

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Disadvantages of Otto Cycle

• Lower efficiency than diesel cycle
• Fuel consumption can be higher
• Limited compression ratio due to knocking
• Produces emissions

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Difference Between Otto Cycle and Diesel Cycle

Feature	Otto Cycle	Diesel Cycle
Engine Type	Petrol Engine	Diesel Engine
Ignition	Spark Ignition	Compression Ignition
Heat Addition	Constant Volume	Constant Pressure
Efficiency	Lower	Higher
Compression Ratio	Lower	Higher

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Real-Life Importance of Otto Cycle

The Otto cycle forms the foundation of modern gasoline engines and transportation systems. Engineers use it to:

• Design engines
• Improve fuel efficiency
• Reduce emissions
• Increase performance

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Conclusion

The Otto cycle is the ideal thermodynamic cycle used in spark ignition petrol engines. It consists of four important processes: compression, heat addition, expansion, and heat rejection. Understanding this cycle is essential for studying engine performance, thermodynamics, and automotive engineering.