An inlet duct for use with an engine is presented. The invention includes a duct structure, at least one spike disposed along an interior surface of the duct structure, and an inlet throat formed by one or more apexes disposed along an equal number of spikes. The inlet throat corresponds to the minimum cross-sectional area through which airflow passes as otherwise allowed by the maximal obstruction formed by the apex(es) within the duct structure. Each spike is bounded by a longitudinal ridge and a lateral ridge along an upper end and a base along a lower end. The longitudinal ridge and the lateral ridge intersect at the apex. In preferred embodiments, the longitudinal ridge is at least partially non-linear so as to properly conform to the interior surface of the duct structure. The portion of each spike upstream of the inlet throat functions primarily as a supersonic diffuser. The portion of each spike downstream of the inlet throat functions primarily as a subsonic diffuser. Airflow is isentropically compressed and then expanded within the inlet duct so that greater-than-subsonic flow at an input end is reduced to subsonic flow at an output end.