One application of thermodynamics is the transfer of thermal energy into work in an engine. In general, this cyclic process involves an exchange of heat with two reservoirs, heat in at a high temperature and heat out at a low temperature, resulting in net positive work from the process. We use PV diagrams to describe the work done and heat exchange in each step of the cycle. There is a limit to the work you can get out of an engine compared to the net thermal energy you put in. Conservation of energy (first law of thermodynamics) says that you cannot get more energy out than you put in, i.e., you can only break even. But the second law of thermodynamics says that you cannot even break even, you can only lose. The second law of thermodynamics says that, as time goes forward, entropy (disorder) increases. It takes energy to decrease entropy. In this chapter we will pull together the ideas of work, heat, and entropy to show how everything fits together in simplified engines.