Abstract: A transient driving cycle simulator test system for testing a prime mover in a laboratory environment is described, which includes an inertia wheel assembly operatively coupled to the prime mover for simulating vehicle acceleration and deceleration horsepower requirements, a dynamometer operatively coupled to the inertia wheel assembly for simulating vehicle road-load horsepower requirements, and a microcomputer control circuit for providing closed-loop control of the prime mover, the inertia wheel assembly and the dynamometer in response to a set of predetermined transient driving cycle specifications. The inertia wheel assembly features a pair of disc brakes for selectively retarding the rotation of an output shaft of the prime mover to provide forced deceleration of the vehicle being simulated.