Ramjet engines are one of the more rare forms of propulsion employed in the world of aviation. They are a form of airbreathing jet engine which do not employ a fan to push air into the engine; instead, the speed of the aircraft is itself used to force, or 'ram' air into the engine, hence the name. Ramjets also go by other names such as flying stovepipes or athodyds (an abbreviation of aero thermodynamic duct).
Since ramjets use the speed of the aircraft itself to push air into the engine, they cannot start a plane up from a standstill. Indeed, they are extremely inefficient at low speeds, and attain maximum efficiency only at speeds exceeding Mach 3. Thus, another form of propulsion is required to get the craft to that speed before the ramjet can take over. The jets can work at speeds of up to Mach 6.
A ramjet uses the high pressure generated in front of a craft moving through the air at very high speeds to force air into the engine. A spiked protrusion at the front of the engine compresses the air, heating it and slowing it down to subsonic speeds.
Once inside, some of that air is combined with fuel and burned, producing exhaust gases. These exhaust gases are channeled into a narrow stream by a exhaust nozzle, accelerating them to supersonic speeds, which pushes the ramjet forward, courtesy of Newton's Laws. (Above-Right): A diagram of a ramjet engine, with the speeds of airflow in different section given in Machs.
Parts of a conventional Ramjet
A ramjet engine consists of three main parts: the inlet/diffuser, the combustor, and the nozzle/s.
The inlet/diffuser's job is to slow down incoming supersonic air to subsonic speeds so that it can be burned with fuel. The spiked tip of the ramjet (known as the innerbody) creates a conical shock wave in front of it - remember, it is always travelling faster than sound - and this forces more air into the space in between the innerbody and the tube, compressing it and slowing it down, although not to subsonic speeds. Once past the innerbody, the less-supersonic air again forms a shock wave inside the engine which finally slows it down to subsonic speeds. (Note that subsonic ramjets do not need such fancy equipment and have a simple hole as the inlet (without any innerbody), as the air is already subsonic.)
Once inside and slowed-down, the air is mixed with fuel, sprayed by pumps, and burned. A structure known as a flame holder prevents the flame from being blown out by the still-very-fast air. The structure of the flame holder may vary, and some designs use other methods to achieve flame stabilization.
The nozzle is the final part of a ramjet. Its purpose is to increase the velocity of the exhaust gases produced by the combustion of fuel. For subsonic ramjets, a simple convergent nozzle will do the trick, whereas for supersonic ones, a more complicated convergent-divergent nozzle is required. (Right): A diagram of a convergent and convergent-divergent nozzle.
Uses of Ramjets
Ramjets are used by a few aircraft and a good number of missiles. The SR-71 Blackbird is one of the more famous planes which employs turbojet-ramjet engines, which act like normal turbojets at subsonic speeds but then convert to ramjets at speeds exceeding Mach 1. You can learn more about the SR-71 here: http://aeronautics-for-all.blogspot.in/2014/12/iconic-planes-sr-71-blackbird.html.
(Below): A SR-71 Blackbird. One can clearly see the spiked innerbody of the turbojet-ramjet engines which propel this aircraft.