A railroad locomotive includes a naturally-aspirated reciprocating internal combustion engine, and a traction generator driven by the engine. A throttle position sensor produces a signal corresponding to the throttle position selected by the locomotive's operator. A load regulator receives a speed signal derived from the throttle position signal and outputs an excitation signal for the traction generator which is modified by a controller in response to air availability so that engine speed and load are controlled independently of the selected throttle position, so as to limit the exhaust smoke output of the engine.