Abstract: There is disclosed a heat-engine/battery-electric hybrid power system for use in a low-pollution, low-fuel consumption vehicle. The engine shaft is coupled by a fixed mechanical ratio to the shaft of a dynamotor, the field current of the dynamotor being controlled by the accelerator pedal for drawing power from a battery to aid the engine in the turning of the driveshaft or for drawing power from the driveshaft to recharge the battery. A feedback loop is employed for continuously adjusting the throttle so that the manifold vacuum remains constant. In this manner, the ratio of the engine exhaust to the driveshaft speed remains substantially constant. The engine exhaust is directed to a thermal reactor, secondary air for which is derived from an air pump driven by the engine shaft. The ratio of the exhaust gas to the secondary air is thus substantially constant independent of the shaft speed so that the thermal reactor is maximally effective at all vehicle speeds.