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The performance of battery-capacitor combinations for diesel engines starting was investigated using computer simulations. The system was comprised of a diesel engine, starting motor, one, two, or three batteries and an electrochemical capacitor, which is sometimes called a supercapacitor or ultracapacitor. Circuit models of each component were developed and used to examine system performance. Results demonstrate that adding a capacitor to the starting system can increase the peak rotation rate of the engine and in addition, reduce starting system mass and volume. Significantly, system effectiveness is often greater if the capacitor is rated for and operated at a voltage higher than the battery. This permits use of a smaller, lower-cost capacitor. The voltage seen by the starter in such mixed-voltage systems is below its rating.