Mechanics:
Mana Engines work on the principle of mana to energy conversion. It can be thought of as an automatic Evocation machine in that respect. At the core four free-floating, runeworked rings of four different mithril alloys are spun toward a central point by magnetic fields (much like the drive turbines on a skimmer). The interplay of fields is such that more energy is drawn from the aether than is required to maintain the motion of the rings. A lot more. Several variations on the engine have been built for conversions of mana to different types of energy; electrical, heat, etc. No mana engine yet built allows for a perfect conversion so regardless of the engine's focus there will be by-products of other energy types. This energy release is not typically allowed to form at the focal point of the rings as it would risk misaligning the fields (see the section on field misalignment) at best, or damaging the weaker metals at worst. The mana is usually shunted along an axis etending from the shell to more distant points.
It should be noted that, since the engine is actually just converting naturally present mana into energy, mana engines are unable to function in space (where ambient mana is of a very low level) or in the holy land of Ainador.
Modern Context:
The most common use of mana engines today is in modern aerofighters, whose particular configuration of the engine is designed to produce kinetic energy. Secondary emissions consist largely of heat as well as some minor ionic discharge (providing for the blue color).
Some fringe environmental groups object to the technology on the grounds that it does not provide a path for the energy to return to Aether. This concerns them on the grounds that it could deplete the level of mana in the universe. More mainstream groups concede that the amount consumed by today's level of usage is only a drop in the universal bucket.
Field Misalignment:
The field alignment of the engine's rings is crucial to establishing the stability needed for the mana draw. Interfering fields, variations in the rotation speed, or relative positioning of the rings can all cause the engine to shutdown. Of particular concern to pilots is a spell easy enough for a first year apprentice to cast which misaligns the fields in such a way that the engine reverses its energy draw. In an aerofighter this means that all thrust is lost and the engine cannot be restarted because all kinetic energy directed at the rings is drained off. The only option is to wait for the residual mana powering the field to dissipate, usually requiring about half an hour.
While such a misalignment spell is easily cast, it is fortunately also easy to counter and its strength is directly proportional to one's distance from an engine's core. This means that a malefactor has to be awfully close, or that a pilot must awfully distracted for such situations to arise.