Indicator diagram

An indicator diagram is a chart used to measure the thermal, or cylinder, performance of reciprocating steam and internal combustion engines and compressors.

The indicator diagram was developed by James Watt and his employee John Southern to help understand how to improve the efficiency of steam engines.

[6] Watt used the diagram to make radical improvements to steam engine performance and long kept it a trade secret.

Though it was made public in a letter to the Quarterly Journal of Science in 1822,[7] it remained somewhat obscure, John Farey, Jr. only learned of it on seeing it used, probably by Watt's men, when he visited Russia in 1826.

In 1834, Émile Clapeyron used a diagram of pressure against volume to illustrate and elucidate the Carnot cycle, elevating it to a central position in the study of thermodynamics.