English: Diagram of a Franchot hot air engine invented in 1854 by French engineer Charles-Louis-Félix Franchot. The engine is similar to the earlier Stirling engine but with two double acting cylinders which minimised thermal loses. The hot cylinder is on the left and the cold cylinder is on the right.

The pistons move with a 180 degree phase difference. Air is the working fluid. The left hand cylinder (C) is surrounded by flue gasses from a furnace. The right hand cylinder (C1) has a jacket of flowing cold water. As the pistons move hot air is pushed into the cold cylinder via the U shaped pipes above and below the cylinders and back again. These pipes contain a 'regenerator' material like steel wool that retains the heat temporarily outside of the working fluid. The air expands and contracts as it experiences different temperatures in the two cylinders. This expansion increases the pressure and exerts a force on the piston face which turns the crank. The contraction reduces the pressure in that end of the cylinder.

Franchot's innovation was to make the cylinders 'double acting' so pressure was alternatively applied to the upper and lower surfaces of each piston head.