Question

Why is carnot cycle most efficient?

Answer 1

Carnot theorem:

1. According to this, “No other engine that works between two given temperatures can be more efficient than a reversible engine that works within the same two temperatures”.
2. A reversible engine will always have greater efficiency than an irreversible engine.

Carnot cycle

Carnot cycle: It is an ideal reversible thermodynamic process that includes the following four processes:

1. Isothermal expansion: No other heating is more efficient than this one, as there is no finite temperature difference between the heat source and heat receiver. Thus, it is the most efficient reversible process.
2. Adiabatic expansion: This is the most efficient expansion, as there is no heat loss and full energy is utilized only for expansion and thus for doing work.
3. Isothermal compression: This is the most efficient heat rejection process to a heat sink at the same temperature.
4. Adiabatic compression: Compressed (without friction) with no heat lost to surroundings. Power input is used only for increasing the pressure and temperature.

Graphical Representation of Carnot cycle

The efficiency of the heat engine cannot be $100\%$. The efficiency is given as

Efficiency =$1–\frac{T_2}{T_1}$

$T_2$ is the temperature of the sink

$T_1$ is the temperature of the source

The efficiency is $100\%$ only when $T_2$ is zero or $T_1$ is infinite. This is not possible since some amount of heat gets rejected to the sink.

Since no other process can be more efficient than the processes involved in the Carnot cycle, this cycle is the most efficient.

The addition of heat in the system at a large temperature in the case of the Carnot engine is one of the causes of the large efficiency.

Hence, the Carnot cycle is the most efficient.