# What moves the piston in a Carnot heat engine?

• trigger701
In summary, the conversation discusses Carnot heat engines and their role in understanding entropy. The main question is why the piston in the engine moves, and the answer is that it is due to the expanding gas and external control such as a flywheel or crank shaft. The conversation also delves into the concept of a reversible process and how it can be achieved through gradual changes in force. Ultimately, it is concluded that a net amount of work is done in the Carnot cycle.

#### trigger701

Hi everyone,

I have been looking at Carnot heat engines in a bid to better understand entropy, and I can't figure out how it actually does work. Why does the piston move?

In some diagrams I have seen weights being removed from the piston, reducing the pressure at constant temperature and therefore causing expansion. In other diagrams however, the piston is attached to a wheel and the expansion seems to just happen.

Can anybody help me understand why the piston is actually moving? And if it is due to external control of the piston, then surely this would require work- does this not defeat the object?

Trigger

trigger701 said:
Why does the piston move?

Expanding gas.

In the high pressure phase, the expanding gas moves the piston. In the low pressure phase, something else must move the piston back. That is often done with inertia from a flywheel or with a crank shaft connection to another piston that is in the high pressure phase. The greater the pressure difference between the high and low pressure phases, the more work can be extracted from the machine.

Last edited:
you should realize that a Carnot cycle is a theoretical, ideal cycle. There are no cycles that achieve the Carnot predictions.

The ideal Carnot cycle consists of reversible expansions and compressions, both isothermal and adiabatic. To do work reversibly during the expansion phases, the force applied by the piston has to be decreased very gradually. Your example of removing tiny weights from the piston is one way of doing this. In the case of removing tiny weights, the piston is rising, and the weights are thus being lifted and delivered to a series of higher elevations. This work on the weights increases their potential energy. But adding and removing tiny weights is not the only way that reversible work can be done by the working fluid. In the case of a piston attached to a wheel, the force applied by the wheel must also be decreasing gradually in order for the work being done on it to be reversible. So the examples you have seen in books are just different ways of making the work done in the cycle to be reversible. Remember that a reversible process is one in which the system is only slightly removed from being in thermodynamic equilibrium throughout the change. So, a reversible process can be regarded as a continuous sequence of thermodynamic equilibrium states of the system. This can be achieved by making sure that the force applied by the system is never more than slightly different from that of the entity that it is doing work on or receiving work from.

trigger701
Chestermiller said:
In the case of removing tiny weights, the piston is rising, and the weights are thus being lifted and delivered to a series of higher elevations. This work on the weights increases their potential energy.

Thank you very much Chestermiller, a very helpful response.

So would you say that the weights lifted can in theory just be put back on to the piston to do the work necessary for the compression phases, and this means that you are offsetting the work required to lift the weights in the first place? You are still, therefore, doing overall useful work during the expansion.

trigger701 said:
Thank you very much Chestermiller, a very helpful response.

So would you say that the weights lifted can in theory just be put back on to the piston to do the work necessary for the compression phases, and this means that you are offsetting the work required to lift the weights in the first place? You are still, therefore, doing overall useful work during the expansion.
The amounts that you put on at the various elevations during the compression phases do not match the amounts you take off during the expansion phases. We know this because a net amount of work is done.

trigger701
Chestermiller said:
The amounts that you put on at the various elevations during the compression phases do not match the amounts you take off during the expansion phases. We know this because a net amount of work is done.

Thank you, this alleviates my confusion. I'm glad I asked.

## 1. What is a Carnot heat engine?

A Carnot heat engine is a theoretical thermodynamic system that operates on the principle of converting heat energy into mechanical work. It is named after the French physicist Sadi Carnot and is often used as a benchmark for measuring the efficiency of real-world heat engines.

## 2. How does a Carnot heat engine work?

A Carnot heat engine works by taking in heat energy from a high-temperature source, converting some of it into mechanical work, and then releasing the remaining energy to a low-temperature sink. This process is repeated in a cyclic manner to produce continuous work output.

## 3. What moves the piston in a Carnot heat engine?

In a Carnot heat engine, the piston is moved by the expansion and contraction of a working gas. When the gas is heated, it expands and pushes the piston outward, and when it is cooled, it contracts and pulls the piston back in. This movement of the piston is what drives the engine's mechanical work.

## 4. What factors affect the efficiency of a Carnot heat engine?

The efficiency of a Carnot heat engine is determined by the temperature difference between the high-temperature source and the low-temperature sink. The larger the temperature difference, the higher the efficiency. Additionally, the type of working gas and the design of the engine can also impact its efficiency.

## 5. Is a Carnot heat engine a realistic model?

No, a Carnot heat engine is a theoretical model and is not physically realizable. It serves as an idealized benchmark for comparing the efficiency of real-world heat engines. However, some real-world engines, such as steam turbines, can approach the efficiency of a Carnot heat engine under certain conditions.