The use of energy storage in an automobile provides two important fuel-saving advantages: first the engine can be run at or close to its condition of maximum efficiency, and secondly the energy of braking can be recovered regeneratively. This paper examines how these factors are linked to the size of the energy storage unit and describes a control policy which has the purpose of minimizing the capacity whilst retaining the fuel-saving benefits. Both flywheel and accumulator storage units are considered; results are obtained for three standard test cycles. Some preliminary attempts are made to assess the effect of road gradient. It is concluded that a capacity of the order of 0.15 to 0.20 MJ for a 1360 kg automobile would provide most of the fuel-saving benefits for typical driving patterns where grade is not an important consideration.

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