Fig. 11 is a scatter plot of the efficiency of the 65kWeFCS for warm start on federal urban drive schedule (FUDS)[1]. The efficiency curve has two branches that are especiallywide apart at low FCS power. The lower branch correspondsto FCS performance during periods of vehicle accelerationfrom zero or slow speeds with the CEM shaft spinning nearits idle speed. In order to meet the sudden surge in powerdemand, our dynamic simulation model attempts to rapidlyincrease the cathode air flow rate by increasing the shaftspeed. This requires the CEM motor/controller to draw largepower from the PEFC stack degrading the efficiency of theFCS. During an acceleration event the dynamic efficiency ofthe FCS is lower than the steady-state efficiency at the samepower. Conversely, the dynamic efficiency can be greater thanthe steady-state efficiency during the deceleration events becausesome of the CEM parasitic power can be supplied bythe inertial power stored in shaft, compressor, expander andmotor. The upper branch of the efficiency curve correspondsto FCS performance during periods of deceleration and constantpower demand.