AIR STANDARD CYCLE

1. Definition of Air Standard Cycle

An air standard cycle is an idealized thermodynamic cycle used to analyze the performance of internal combustion engines. In this concept, air is assumed to be the working fluid, and its behavior is analyzed using simplified assumptions to make engine analysis easy and theoretical.

Air standard cycles do not represent actual engine operation, but they provide a basis for comparison, performance analysis, and understanding of real engines.

4. Types of Air Standard Cycles

Based on the method of heat addition, air standard cycles are classified as:

1. Otto Cycle
2. Diesel Cycle
3. Dual (Mixed) Cycle
4. Brayton Cycle (Gas turbine)

5. Otto Cycle (Air Standard Otto Cycle)

5.1 Definition

The Otto cycle is the ideal cycle for spark ignition (petrol) engines, where heat is added at constant volume.

5.2 Processes of Otto Cycle

The Otto cycle consists of four processes:

5.3 Thermal Efficiency of Otto Cycle

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

Where:

• $r$r = compression ratio
• $\gamma$γ = ratio of specific heats

5.4 Important Observations

• Efficiency depends only on compression ratio
• Higher compression ratio → higher efficiency

6. Diesel Cycle (Air Standard Diesel Cycle)

6.1 Definition

The Diesel cycle is the ideal cycle for compression ignition (diesel) engines, where heat is added at constant pressure.

6.2 Processes of Diesel Cycle

6.3 Cut-off Ratio

$$\rho = \frac{V_3}{V_2}$$

6.4 Thermal Efficiency of Diesel Cycle

$$\eta_{diesel} = 1 – \frac{1}{r^{\gamma-1}} \cdot \frac{\rho^{\gamma} – 1}{\gamma(\rho – 1)}$$

6.5 Observations

• Efficiency depends on compression ratio and cut-off ratio
• Higher cut-off ratio reduces efficiency

7. Dual Cycle (Mixed Cycle)

7.1 Definition

The Dual cycle combines features of both Otto and Diesel cycles. Heat is added:

• Partly at constant volume
• Partly at constant pressure

This cycle represents modern high-speed diesel engines more accurately.

7.2 Processes of Dual Cycle

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

8. Comparison of Air Standard Cycles

Feature	Otto Cycle	Diesel Cycle	Dual Cycle
Heat addition	Constant volume	Constant pressure	Both
Engine type	Petrol	Diesel	Modern diesel
Efficiency (same r)	Highest	Lowest	Intermediate
Compression ratio	Lower	Higher	High

9. Air Standard Efficiency

Air standard efficiency is the thermal efficiency calculated under air standard assumptions. It represents the maximum possible efficiency of an engine cycle.

10. Limitations of Air Standard Cycle

• Does not consider real combustion
• Ignores heat losses
• Assumes constant specific heats
• No friction or pumping losses
• Not applicable for exact real engine performance

11. Significance of Air Standard Cycle

• Helps understand engine thermodynamics
• Useful for academic analysis
• Basis for real engine comparison
• Simplifies numerical problem solving

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