Adamant Mod uses the Quasi-Newtonian Propulsion model. Additionally, engines do not store any supplies. Instead, you must use reactor components to store supplies. This allows for several interesting design decisions that are not present in the more traditional engine models.
The following is borrowed from the SE4 Modding 101 Tutorial, and was originally written by Suicide Junkie:
The effect of this is that any two ship hulls (save the Colony Ship) will require roughly the same percentage of hull space to have the same movement rate. The choice of how fast your ships move is left up to you, rather than an arbitrary limit as seen in stock SE4 and many mods that do not utilize Quasi-Newtonian Propulsion.
The major difference beteen stock and Adamant propulsion mechanics is that you have more choices when designing your ships. Instead of just sticking on as many engines as you want, you now have the choice of how fast and how long ranged your ships will be. In stock SE4, you have this decision to make, but to a rather limited extent. The number of engines on any given ship hull is a no-brainer, just put on the max. You can add some Supply Storage or Solar Panels to increase range, but this requires using more hull space than you normally would. With movement generation and supply storage forcibly separated into two separate component branches, you can have several different ship designs that have different velocitys and ranges while using the exact same amount of hull space. This is accomplished simply by removing a few engines and replacing them with reactors.
Range vs. Velocity
The most important consequence of the Adamant propulsion model is that you must now consider range versus velocity. The faster your ship moves per turn, the less distance it will be able to cover in the long run. So, you must now choose whether you want the ship to be fast or long ranged. Generally, defensive ships can be made faster and shorter-ranged because they will not have to stray too far from your Resupply Depots. Attack ships, on the other hand, might have to travel long distances without any resupplying, so they might need to be slower and carry more supplies. Exploration ships should generally be slow and carry a lot of supplies, so that they can explore more systems. It will take longer to explore new systems, but the ship will be able to cover much more territory. But, as always, how you design your ships is completely up to you, not up to some arbitrary engine limits imposed by stock SE4 and many mods.