Newcomen's engine was so inefficient because the heat put into the cylinder as it filled with steam was immediately lost when the working vacuum was created by injecting cold water.

This continual heating and cooling meant more coal was burned in generating wasted heat than was burned in performing useful work.

James Watt was the first person to realise this when, in 1763/64, while working as a scientific instrument maker at Glasgow University, he received a model Newcomen engine needing repairs.