A motorjet is a rudimentary type of jet engine which is sometimes referred to as thermojet, a term now commonly used to describe a particular and completely unrelated pulsejet design. At the heart the motorjet is always an ordinary piston engine (hence, the term motor), but instead of this driving a propeller, it drives a compressor. The compressed air is channeled into a combustion chamber, where fuel is injected and ignited. The high temperatures generated by the combustion cause the gases in the chamber to expand and escape at high pressure from the exhaust, creating a thermal reactive force that drives the engine. Motorjet engines provide greater thrust than a propeller mounted on a piston engine; this has been successfully demonstrated in a number of different aircraft.

Motorjet research was nearly abandoned at the end of World War II as the turbojet was a more practical solution to jet power as it used the jet exhaust to drive a gas turbine, providing the power to drive the compressor without the additional weight of a piston engine that generated no thrust.

• In 1908, Rene Lorin first proposed the motorjet design.

• The first application of the motorjet principle was by Henri Coandă in his Coanda-1910 aeroplane (1910).

• In 1917, O. Morize of Chateaudun, France, proposed the Morize ejector scheme in which a reciprocating engine drove a compressor supplying air to a liquid fueled combustion chamber which discharged into a convergent-divergent tube and ultimately out into the atmosphere.

• The term "motor jet" was established in a patent filed in Britain by J.H. Harris of Esher, U.K., in 1917.

• It was next explored by Secondo Campini in the early 1930s, although it was not until 1940 that an aircraft (the Campini Caproni CC.2) would fly powered by his engine. Campini established the misnomer "thermojet" at this time to describe his motorjet.

• NACA engineer Eastman Jacobs was actively pursuing thermojet research in the early 1940s for a project that came to be known as Jake's jeep but which was never completed as turbojet technology overtook it.

• Japanese engineers developed the Tsu-11 motorjet engine to power Ohka kamikaze aircraft as an alternative to the rocket engines that these aircraft were then using.

• The Soviet Mikoyan-Gurevich I-250 (N) designed in 1944 used a piston engine to drive both a propeller at the nose of the plane, and a motorjet compressor leading to a jet exhaust at the tail. Between 10 and 50 I-250 (a.k.a. MiG-13) aircraft were produced, serviced, and flown by the Soviet Navy through 1950.