FUEL AIR CYCLE

1. Introduction

Fuel air cycle In real internal combustion (IC) engines, the working medium is not pure air as assumed in air-standard cycles. Instead, it is a mixture of fuel vapour, air, and combustion products.
To analyze engine performance more realistically, Fuel–Air Cycles are used. These cycles consider the actual chemical composition, combustion process, and variable specific heats of the working fluid.

Fuel–air cycle analysis bridges the gap between:

• Air-standard cycles (ideal, simplified)
• Actual engine cycles (very complex)

2. Limitations of Air-Standard Cycle

Air-standard cycle assumptions:

1. Working medium is pure air.
2. Combustion is replaced by external heat addition.
3. Specific heats are constant.
4. Exhaust process is replaced by heat rejection.

These assumptions lead to errors because:

• Real engines involve fuel combustion
• Specific heats vary with temperature
• Dissociation of gases occurs at high temperature
• Actual exhaust gases are different from intake air

Hence, Fuel–Air Cycles are introduced for better accuracy.

3. Definition of Fuel–Air Cycle

A Fuel–Air Cycle is a thermodynamic cycle in which:

• The working fluid is a fuel–air mixture and combustion products
• Heat addition occurs by actual combustion
• Chemical reactions and variable specific heats are considered

4. Assumptions of Fuel–Air Cycle

Compared to air-standard cycles, fewer idealizations are made:

1. Fuel and air are mixed before combustion.
2. Combustion is complete (usually assumed).
3. Working fluid composition changes during the cycle.
4. Specific heats vary with temperature.
5. No heat loss to surroundings.
6. Friction and pumping losses are neglected.

5. Processes in Fuel–Air Cycle

A typical fuel–air cycle consists of the following processes:

1–2: Compression Process

• Isentropic compression of fuel–air mixture
• Temperature and pressure increase
• Specific heats vary with temperature

2–3: Combustion Process

• Actual combustion replaces heat addition
• Occurs at:
  • Constant volume (SI engines – Otto type)
  • Constant pressure (CI engines – Diesel type)
• Chemical energy of fuel converts into thermal energy

3–4: Expansion Process

• Isentropic expansion of combustion products
• Produces useful work
• Temperature and pressure decrease

4–1: Exhaust and Intake

• Products are exhausted
• Fresh fuel–air mixture enters
• Cycle repeats

6. Fuel–Air Cycle vs Air-Standard Cycle

Aspect	Air-Standard Cycle	Fuel–Air Cycle
Working medium	Pure air	Fuel + air + gases
Combustion	Replaced by heat addition	Actual combustion
Specific heats	Constant	Variable
Accuracy	Low	Higher
Complexity	Simple	More complex
Practical relevance	Theoretical	Closer to real engines

7. Effects Considered in Fuel–Air Cycle

a) Variable Specific Heats

• Specific heats increase with temperature
• Leads to lower efficiency compared to air-standard cycle

b) Dissociation

• At high temperatures, gases like CO₂ and H₂O dissociate
• Reduces peak temperature and pressure
• Lowers engine efficiency

c) Fuel–Air Ratio

• Rich mixture → higher power, lower efficiency
• Lean mixture → lower power, higher efficiency

8. Types of Fuel–Air Cycles

1. Fuel–Air Otto Cycle

• Applicable to spark ignition engines
• Combustion at constant volume
• More realistic than air-standard Otto cycle

2. Fuel–Air Diesel Cycle

• Applicable to compression ignition engines
• Combustion at constant pressure
• Includes effects of actual fuel injection

3. Fuel–Air Dual Cycle

• Combination of constant volume and constant pressure combustion
• Closely represents modern high-speed engines

9. Advantages of Fuel–Air Cycle Analysis

• Better prediction of engine performance
• Considers chemical reactions
• More accurate efficiency calculation
• Useful for advanced engine research

10. Disadvantages

• Complex mathematical analysis
• Requires thermodynamic property data
• Not suitable for quick calculations

11. Applications

• IC engine design and development
• Performance prediction
• Combustion analysis
• Advanced thermodynamic studies

12. Conclusion

Fuel–air cycles provide a more realistic representation of IC engine operation compared to air-standard cycles. Although complex, they are essential for accurate performance evaluation, especially in modern high-efficiency engines.