The model formulation, development process, and experimental validation of a new vehicle powertrain simulator called LFM (Light, Fast, and Modifiable) are presented. The existing powertrain simulators were reviewed and it was concluded that there is a need for a new, easily modifiable simulation platform that will be flexible and sufficiently robust to address a variety of hybrid vehicle platforms. First, the structure and operating principle of the LFM simulator are presented, followed by a discussion of the subsystems and input/output parameters. Finally, a validation exercise is presented in which the simulator's inputs were specified to represent the University of Delaware's fuel cell hybrid transit vehicle and “driven” using an actual drive cycle acquired from it. Good agreement between the output of the simulator and the physical data acquired by the vehicle's on-board sensors indicates that the simulator constitutes a powerful and reliable design tool.