Air-engine, an engine worked by means of the expansion of air when heated. Cold air passes through a furnace, becomes greatly heated, and thereby expands. Its expansion is made to drive a piston forwards in a cylinder; the air is then passed off to the exhaust or to a regenerator, and a fresh supply drives the piston back. This reciprocating motion of the piston is converted into a rotatory motion by means of a connecting rod and crank, and continuous motion is so produced. Great economy of heat is effected, there is no liability of expense, and management is easy. But air-engines, though theoretically efficient, have not hitherto been quite successful in practice. The high temperature of the air causes it to burn away the less durable working parts of the machine, and the constant repairs necessary diminish the practical efficiency.