In thermal engineering, power generation and heat engine analysis rely heavily on ideal and practical thermodynamic cycles. Among them, the Carnot cycle and the Rankine cycle are two fundamental cycles used to understand heat engine performance and steam power plants respectively.
This article explains the difference between Carnot cycle and Rankine cycle with definitions, processes, diagrams, comparison table, advantages, limitations, and applications.
What is Carnot Cycle?
The Carnot cycle is a theoretical and ideal thermodynamic cycle that represents the maximum possible efficiency a heat engine can achieve when operating between two temperature reservoirs.
Processes of Carnot Cycle
The Carnot cycle consists of four reversible processes:
- Isothermal Expansion – Heat absorbed from high-temperature reservoir
- Adiabatic Expansion – Temperature drops without heat transfer
- Isothermal Compression – Heat rejected to low-temperature reservoir
- Adiabatic Compression – Working fluid returns to initial state
Key Characteristics
- Completely reversible
- Uses ideal conditions
- Sets the upper limit of thermal efficiency
- Not feasible for real power plants
What is Rankine Cycle?
The Rankine cycle is a practical thermodynamic cycle widely used in steam power plants for electricity generation.
Processes of Rankine Cycle
The Rankine cycle also consists of four processes:
- Isentropic Compression – Pump compresses liquid water
- Constant Pressure Heat Addition – Boiler converts water into steam
- Isentropic Expansion – Steam expands in turbine producing work
- Constant Pressure Heat Rejection – Condenser rejects heat
Key Characteristics
- Practically achievable
- Accounts for real fluid behavior
- Foundation of thermal power plants
- Efficiency lower than Carnot but realistic
Difference Between Carnot Cycle and Rankine Cycle
Basis of Comparison | Carnot Cycle | Rankine Cycle |
Type of Cycle | Ideal and theoretical | Practical and real |
Nature | Completely reversible | Partially irreversible |
Heat Addition | Isothermal | Constant pressure |
Heat Rejection | Isothermal | Constant pressure |
Compression | Gas or vapor compression | Liquid compression |
Efficiency | Maximum possible | Lower but realistic |
Feasibility | Not practical | Widely used |
Application | Benchmark for efficiency | Steam power plants |
Working Fluid State | Vapor throughout | Liquid → vapor → liquid |
Equipment Complexity | Not realistic | Industrially feasible |
Thermal Efficiency Comparison
- Carnot Efficiency:
η=1−TLTHeta = 1 – frac{T_L}{T_H}η=1−THTL
(Depends only on temperature limits) - Rankine Efficiency:
Depends on boiler pressure, condenser pressure, superheating, reheating, and real losses.
Advantages and Limitations
Carnot Cycle
Advantages
- Highest possible efficiency
- Ideal reference cycle
Limitations
- Not practical
- Difficult compression of wet vapor
- No real engine follows Carnot cycle
Rankine Cycle
Advantages
- Simple design
- Suitable for large-scale power generation
- Easily modified (reheat, regeneration)
Limitations
- Lower efficiency than Carnot
- Energy losses due to irreversibility
Applications
- Carnot Cycle
- Thermodynamic analysis
- Efficiency benchmarking
- Academic and theoretical studies
- Rankine Cycle
Conclusion
The Carnot cycle defines the theoretical efficiency limit, while the Rankine cycle provides a practical framework for real-world steam power generation. Engineers use Carnot efficiency as a benchmark and Rankine cycle as the actual working model.
FAQs
The main difference is that the Carnot cycle is a theoretical ideal cycle, while the Rankine cycle is a practical cycle used in real steam power plants.
The Carnot cycle requires isothermal heat addition and rejection, which is not practically achievable in steam turbines and boilers, making it unsuitable for real power plants.
The Carnot cycle has maximum theoretical efficiency, but the Rankine cycle has lower efficiency because of real-world losses and practical limitations.
In the Carnot cycle, expansion occurs isothermally and adiabatically, whereas in the Rankine cycle, steam expands adiabatically in a turbine.
The Rankine cycle is used in thermal power plants because it is practical, reliable, and economically feasible.
Yes, the Carnot cycle is a fully reversible cycle, while the Rankine cycle is not completely reversible due to friction and heat losses.
Both cycles typically use steam (water) as the working fluid, but the Rankine cycle is specifically designed to operate with liquid-vapor phase change efficiently.
Pump work is considered in the Rankine cycle because liquid water is pumped to high pressure, while the Carnot cycle is a theoretical model where such practical considerations are ignored.
The Carnot cycle is easier for understanding basic thermodynamic concepts, while the Rankine cycle is better for learning real power plant operation.
No, the Rankine cycle cannot achieve Carnot efficiency because of irreversibilities and practical constraints in real systems.
The Carnot cycle is mainly used as a benchmark for maximum efficiency in thermodynamics, not for practical power generation.
Both are important, but the Rankine cycle is more important for practical and numerical problems, while the Carnot cycle is crucial for theoretical understanding.
Why shouldn’t we use carnot cycle in place of rankine cycle as carnot has more efficiency???
Can you read. It is theoretical
Since it’s theoretical why should it be study
because adding heat at constant temperature is not practically possible due to system limitations , likewise the same in the case of Heat rejection ,
so when you try to use Carnot principle then you implement some of things on it and then it is practically possible and that modified system in Rankine cycle
What is the essence of studying it since it can’t be practicalize?
Studying the ideal cycle creates a standard that all power cycles aim to achieve. It is important to study the carnot cycle to adapt whatever cycle you work with to be as efficient as possible.
So for the rankine cycle, there are modifications like superheating, reheating, and regeneration (or a combination of them) which all improves the efficiency of the cycle.
Aren’t isothermal processes in Carnot cycle also isobaric?
no , just because of ideal gas equation pv = nrt