Short Answer:

The efficiency limitations of real thermodynamic cycles are mainly due to friction, heat losses, non-ideal fluid behavior, and irreversibilities during processes. Unlike ideal cycles, real cycles cannot achieve 100% efficiency because of various practical and physical constraints. These losses reduce the actual output compared to theoretical values.

In real engines and power plants, factors like incomplete combustion, pressure drops, mechanical wear, and temperature differences cause performance losses. As a result, real thermodynamic cycles operate at lower efficiency than ideal cycles like the Carnot or Rankine cycles, which are only possible in theory.

Detailed Explanation:

Efficiency limitations of real thermodynamic cycles

In thermodynamics, an ideal cycle assumes perfect conditions such as no friction, perfect insulation, reversible processes, and ideal gases. These assumptions help in easy calculation and understanding but do not reflect real-world systems. In actual power plants and engines, several real-world limitations prevent these systems from achieving ideal efficiency.

The efficiency of a thermodynamic cycle is defined as the ratio of useful work output to heat input. In real cycles, due to unavoidable losses at various stages, this efficiency is always lower than the ideal case. Understanding these limitations helps in designing better systems and reducing energy losses.

Main Efficiency Limitations in Real Thermodynamic Cycles

1. Friction Losses

• In real engines, mechanical components like pistons, crankshafts, and turbines experience friction.
• Friction converts part of mechanical energy into heat, which is lost and not used for useful work.
• It also reduces the lifespan and smooth functioning of machines.

2. Heat Loss to Surroundings

• No insulation is perfect. Heat always leaks from hot components like boilers, combustion chambers, and turbines.
• These losses reduce the amount of heat that can be converted into work.
• Cooling systems, while necessary, also remove useful energy.

3. Irreversibility of Processes

• Ideal cycles assume reversible processes, meaning they can be reversed without loss.
• In reality, processes like compression, expansion, and combustion are irreversible, causing energy dissipation.
• Irreversibilities create entropy generation, which lowers efficiency.

4. Non-Ideal Gas Behavior

• Ideal gas laws assume perfect gas behavior, which is not true at high pressures and temperatures.
• Real working fluids deviate from ideal conditions, especially in steam or gas turbines.
• This affects calculations and performance of the system.

5. Incomplete Combustion (in IC engines)

• In petrol or diesel engines, fuel does not burn completely, especially under fast operating cycles.
• Unburnt fuel leads to loss of energy and increases pollutants.
• This reduces the effective power produced per unit of fuel.

6. Pressure Drops in Pipes and Valves

• As fluids move through pipes, valves, and heat exchangers, pressure drops due to friction and turbulence.
• Lower pressure reduces expansion work in turbines or output in engines.

7. Temperature Differences

• Ideal cycles assume heat transfer at constant temperature.
• In real systems, heat transfers between fluids at temperature differences, causing inefficiency.
• Larger the temperature difference, more the irreversibility and energy loss.

8. Component Limitations

• Real systems are limited by material strength, temperature limits, and cost.
• High-efficiency designs often require expensive materials or complex systems, which are not practical for all applications.

Real vs. Ideal Efficiency

• Carnot efficiency is the maximum theoretical efficiency based on temperature limits.
• Real cycles (like Otto, Diesel, Brayton, Rankine) operate well below Carnot efficiency due to above limitations.
• For example, if Carnot efficiency is 60%, a real Rankine cycle might operate at 35–40%.

Attempts to Improve Real Efficiency

Engineers use several techniques to reduce losses:

• Regeneration: Using exhaust heat to preheat fluids.
• Reheating: Reheating steam between turbine stages.
• Intercooling: Reducing compressor work in gas turbines.
• Using better materials to allow higher temperature and pressure.

Even with these improvements, perfect efficiency remains impossible.

Conclusion

The efficiency limitations of real thermodynamic cycles arise from practical problems such as friction, heat losses, irreversibilities, and component constraints. These factors make it impossible for real engines and power plants to match the ideal or Carnot efficiency. However, understanding these limitations helps engineers develop more efficient and reliable systems by minimizing energy losses through smart design and better materials.