Source: Construct Handbook, pg(s). 51
Hell engines are mindless constructs animated by an infernal contract instead of normal magic, and are usually created by inept artificers willing to sell their souls to Hell for the chance to see their creations brought to life.
Being mindless, hell engines lack a devil's cunning but are not vulnerable to chaotic or holy attacks, and can banish hostile outsiders, redirecting their planar energy to summon devils. Because of this, hell engines are invaluable as Hell's shock troops and vault guardians. Various loopholes exist in the infernal contracts that give power to hell engines: for example, if it is destroyed before completing its contracted task, the contract will be nullified, so most devils include clauses that prevent signees from destroying their own hell engines.