9. Gas power cycle free study note

1. Introduction to Gas power cycle

A gas power cycle is a thermodynamic cycle in which a gas is used as the working fluid to produce mechanical power. In these cycles, the working fluid usually remains in the gaseous phase throughout the cycle.

Gas power cycles are widely used in:

• Internal combustion engines
• Gas turbines
• Jet engines
• Power generation systems

In these cycles, heat is supplied to the gas, which expands and produces work. After expansion, the gas is cooled or exhausted and the cycle repeats.

Basic Characteristics

1. Working fluid is gas (air, combustion gases).
2. Heat energy is converted into mechanical energy.
3. Processes involve compression, heat addition, expansion, and heat rejection.

2. Types of Gas Power Cycles

The most important gas power cycles are:

1. Otto Cycle
2. Diesel Cycle
3. Dual Combustion Cycle
4. Brayton Cycle

1. Otto Cycle

Definition

The Otto cycle is the ideal thermodynamic cycle for spark ignition (SI) engines, such as petrol engines.

Processes in Otto Cycle

The cycle consists of four processes:

1. Process 1–2: Isentropic Compression
   • Air is compressed in the cylinder.
   • Pressure and temperature increase.
2. Process 2–3: Constant Volume Heat Addition
   • Fuel-air mixture burns.
   • Heat is added at constant volume.
3. Process 3–4: Isentropic Expansion
   • High-pressure gas expands.
   • Work is produced.
4. Process 4–1: Constant Volume Heat Rejection
   • Heat is rejected to surroundings.

Efficiency of Otto Cycle

$$\eta = 1 – \frac{1}{r^{\gamma -1}}$$

Where:

• r = Compression ratio
• γ = Specific heat ratio

Applications

• Petrol engines
• Motorcycles
• Small generators

2. Diesel Cycle

Definition

The Diesel cycle is the ideal cycle for compression ignition engines (diesel engines).

Processes of Diesel Cycle

1. Process 1–2: Isentropic Compression
   • Air is compressed in the cylinder.
2. Process 2–3: Constant Pressure Heat Addition
   • Fuel is injected and combustion occurs.
3. Process 3–4: Isentropic Expansion
   • High pressure gases expand producing work.
4. Process 4–1: Constant Volume Heat Rejection

Efficiency

Efficiency depends on:

• Compression ratio
• Cut-off ratio

Applications

• Trucks
• Buses
• Heavy machinery
• Diesel generators

3. Dual Combustion Cycle

Definition

The dual combustion cycle is a combination of Otto cycle and Diesel cycle.

Heat is added partly at:

• Constant volume
• Constant pressure

Processes

1. Isentropic compression
2. Constant volume heat addition
3. Constant pressure heat addition
4. Isentropic expansion
5. Constant volume heat rejection

Advantages

• More practical representation of real engines.
• Used in modern diesel engines.

4. Brayton Cycle (Gas Turbine Cycle)

The Brayton Cycle, also known as the Gas Turbine Cycle or Joule Cycle, is the ideal thermodynamic cycle used to describe the operation of gas turbine engines. It is widely used in aircraft jet engines, gas turbine power plants, and combined-cycle power stations.

In the Brayton cycle, air is compressed, heated at constant pressure, expanded through a turbine to produce work, and then exhausted. The turbine generates enough power to drive the compressor, while the remaining power is available as useful output.

Thermal Efficiency

For the ideal Brayton cycle,$$\eta = 1-\frac{1}{r_p^{(\gamma-1)/\gamma}}$$where:

• $r_p$rp​ = Pressure ratio
• $\gamma$γ = Ratio of specific heats ($C_p/C_v$Cp​/Cv​)

The thermal efficiency increases as the pressure ratio increases (within practical limits).

Net Work Output

$$W_{net}=W_T-W_C$$

where:

• $W_T$WT​ = Turbine work
• $W_C$WC​ = Compressor work

Heat Added

$$Q_{in}=C_p(T_3-T_2)$$

Heat Rejected

$$Q_{out}=C_p(T_4-T_1)$$

Applications

• Gas turbine power plants
• Aircraft jet engines
• Industrial turbines

5. Comparison of Gas Power Cycles

Cycle	Heat Addition	Engine Type
Otto Cycle	Constant Volume	Petrol Engine
Diesel Cycle	Constant Pressure	Diesel Engine
Dual Cycle	Volume + Pressure	Modern Diesel Engine
Brayton Cycle	Constant Pressure	Gas Turbine

6. Advantages of Gas Power Cycles

1. High power output
2. Simple design
3. Quick start and stop
4. Used widely in transportation and power generation

7. Limitations

1. Lower efficiency compared to steam power cycles
2. High fuel consumption
3. High operating temperatures

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External References

1. NPTEL Course