An internal combustion engine/hydropneumatic accumulator energy storage automobile can provide for regenerative braking and more efficient engine operation. A configuration is considered which requires only a single hydrostatic pump-motor (P/M) unit. The engine drives through a gearbox to a final reduction unit at the drive axle. The variable-displacement P/M unit is geared to the gearbox input shaft and controlled to alternately put energy into or take energy out of the accumulator. Computer simulations covering a wide range of parameter variations show that, with a proper control strategy, the concept has the potential of providing significant fuel savings.