Sankey diagrams show what happens to the energy used during a chemical or physical process. The amount of energy input or used up beach part of the process is represented by a bar with an arrow indicated the direction of the process, with the thickness of the bar indicating the amount or proportion of energy that part of the process is responsible for.
This Sankey diagram illustrates the changes changes in a car engine.
Most of the energy supplied to the car engine is lost as heat of some sort – through the exhaust, as friction, and to heat the engine coolant. These heat losses total about 75% of the energy supplied as fuel, so the engine is only about 25% efficient.
Sankey diagrams can be very complicated, involving energy transfers between different process and feedback mechanisms.