This node is fictional. Refer to Continental Class Space Battleships.
These are the reaction engines that accelerate or decelerate a Continental Class battleship in normal space. There are 12 of them, aligned at the stern end of the ship, six on the belly side, six on the back side.
A main engine is essentially a fusion reactor optimised for producing hot plasma, not for producing energy. Starting a main engine takes huge amounts of power, which is why they are started one by one and not all at once when the ship starts accelerating or decelerating; starting more than three engines at once would cause the ship's power grid to black out. A main engine cannot power itself from the fusion reaction inside, all the electricity it needs comes from the ship's reactor blocks.
The fusion reactor is connected to a long tube consisting of various different facilities. This tube is known as the "tunnel". The tunnel comprises electromagnetic constrictor segments, accelerator segments, an injection chamber and an electromagnetic nozzle at the end. This allows the engine to operate in many different modes which differ in thrust and fuel efficiency. High-thrust modes usually expel high amounts of gaseous matter at low speeds and consume enormous amounts of fuel, power and reaction mass, while low-thrust modes with very high exhaust speeds expel only tiny amounts of ions and subnuclear particles and can be sustained over months if necessary. At maximum thrust, each engine can consume more than six tons of reaction mass per second. All the components of the engine are integrated and connected by a "thrust frame" that in turn pushes against the pusher plate. The frame and the plate thus transmit the thrust to the ship's spaceframe.
Deceleration is normally done by turning the ship around and then firing the engines. However, the middle four engines (3, 4, 9 and 10) have tunnels reaching forward from their reaction chambers as well as tunnels pointing astern. Those "retro sections" are specifically designed to allow only modes with extremely high thrust. In combat, the ability to suddenly decelerate without changing the ship's orientation can prove vital.
The engines are mounted on the outside of the hull rather than in it to make for better cooling and easier maintenance and replacement. However, about a quarter of an engine's cross section is inside the hull and can thus be maintained from inside. All the equipment (mostly large, superconducting magnets) in an engine's tubular containment is arranged in rings; most of these rings can be rotated slowly to make the entire equipment be accessible through a maintenance hatch. TV has led many people to believe that the huge blocks of machinery in a battleship's "engine rooms" are the engines. They aren't. The "engine rooms" are actually the D/T injector rooms, and these blocks contain the machinery that injects deuterium, tritium and carrier gases into the reaction chambers as well as controls the reaction. Similar, much less impressive rooms containing the reaction mass injectors can be found further astern.