What is meant by reversible engine? Explain, why the efficiency of a reversible engine is maximum.

Answer

### Step-by-Step Solution:

1. **Definition of an Engine**:
An engine is a device that converts heat energy into work. In this process, heat serves as the input, and work is the output.

**Hint**: Remember that the core function of an engine is to transform energy from one form to another.

2. **Cyclic Process**:
An engine operates in a cyclic manner, meaning it goes through a series of processes that eventually return to the initial state. This allows the engine to continuously convert heat into work.

**Hint**: Think of a cycle as a loop where the starting and ending points are the same.

3. **Reversible Engine**:
A reversible engine is one that can operate in both forward and reverse directions. This means that the engine can retrace its steps precisely without any loss of energy. For example, if the engine moves from state A to state B, it can return from state B to state A following the same path.

**Hint**: Focus on the concept of retracing steps and the ability to reverse processes without energy loss.

4. **Boundary Conditions**:
The reversibility of an engine is achieved by reversing the boundary conditions of the system. This means that the engine can adjust its operation based on the external conditions to maintain reversibility.

**Hint**: Consider how changing the environment or conditions can affect the operation of the engine.

5. **Efficiency of Reversible Engines**:
The efficiency of an engine is defined as the ratio of the work output to the heat input. For reversible engines, particularly the Carnot engine, it is assumed that there is no friction or other forms of energy loss. This absence of friction leads to minimal heat loss, maximizing the efficiency of the engine.

**Hint**: Recall that efficiency is all about minimizing losses; the less heat lost, the more efficient the engine.

6. **Carnot Engine**:
The Carnot engine is a theoretical model of a reversible engine that operates between two heat reservoirs. It is considered the most efficient engine possible because it operates under ideal conditions (no friction, no energy loss).

**Hint**: Think of the Carnot engine as the benchmark for efficiency in thermodynamic systems.

7. **Conclusion**:
Therefore, the efficiency of a reversible engine is maximum because it minimizes energy losses, particularly heat losses, during the conversion of heat into work. This is why reversible engines, like the Carnot engine, are considered ideal.

**Hint**: Summarize the key points: no energy loss = maximum efficiency.