Heat engines are really interesting because they change heat energy into moving energy, and we can find them in many places in our everyday lives. Here are some examples:

• Cars and Vehicles: Most cars have engines that burn gasoline. This process turns the heat from the fuel into movement, making the car go.

• Power Plants: These places use big machines called steam turbines. They heat up water by burning fuel. The hot water turns into steam, which then spins the turbines to create electricity.

• Refrigerators: Refrigerators work like heat engines but in the opposite way. They use energy to take heat out from inside and push it outside, keeping our food cold.

To better understand how well these machines work, we look at something called efficiency. Efficiency tells us how good they are at their job. It can be shown with this simple formula:

$\eta = \frac{W}{Q_{in}}$

In this formula, $W$ means the work done, and $Q_{in}$ is the heat that goes in.

Understanding this helps us appreciate how these engines make our lives easier!