A study was made to determine the effect of hybrid operation on the fuel economy and emissions of a vehicle using a gas turbine engine, a continuously variable transmission, and an electric power storage system. Both series and parallel hybrids were considered in a 1815 kg vehicle.To facilitate this study, a computer program was written which modeled the vehicle and, using experimental data, computed its fuel consumption and emissions over the 1972 FTP driving cycle, starting with a fully warmed up engine.This study indicates that, under certain conditions, the fuel consumption or emissions of the hybrid vehicle may be reduced as compared to its non-hybrid counterpart, but under other conditions, they may be increased. It is not possible to reduce fuel consumption and all of the emissions simultaneously. The reduction of one pollutant is usually accompanied by an increase in one of the others. The extent of the reduction or increase experienced with hybrid operation depends on the particular type of hybrid and on the engine operating conditions. The following table shows the relative effects of minimizing fuel consumption or any of the various pollutants. The table also shows which type of hybrid is best as well as the operating schedule and fuel control parameter. The fuel consumption and emissions are normalized with respect to the standard vehicle as defined in Assumption 8. This table indicates that the hybrid offers a small decrease in fuel consumption as compared to the standard car and at the expense of increased NOx emissions. Minimizing unburned hydrocarbons reduces fuel consumption but at the expense of increased NOx. Both fuel consumption and NOx emissions are increased when carbon monoxide is minimized. Very large increases in fuel consumption and in unburned hydrocarbon and carbon monoxide emissions result from minimizing oxides of nitrogen emissions.It must be concluded that the hybrid vehicle does not automatically guarantee lower fuel consumption or emissions.