Electric hybrid vehicles change the global design of the electrical power on board, but also of the thermal engine. The concept presented is based on recovery and conversion of thermal energy from the engine cooling circuit and the exhaust pipe by a Rankine cycle. The paper presents the choices of thermodynamic fluids permitting to reach appropriate energy efficiency for conversion of heat in electricity. Water, R-245ca, isopentane are compared and different technical options are studied. Potential for improving the net fuel consumption of hybrid vehicles is estimated to be as high as 32%.