SuperCaps for engine cranking
13 April 2011
Passive Components
For optimum cranking of a diesel/combustion engine, currents of more than 1000 A are needed. A middle sized car, for example, needs a starter performance of approximately 1000 A initial pulse to reach the maximum breakaway torque followed by a continuous current of around 100 A. The start cycle itself needs up to 2 seconds.
To supply this amount of power, a conventional battery needs a capacity of more than 70 Ah. By using a SuperCap Module eg, 200 F/14 V, consisting of six SuperCap C 1200 F in series, which will be connected in parallel to a small battery (eg, 12 V/12 Ah) it is possible to crank a combustion engine more reliably and with higher rmp even at temperatures down to less than -30°C. In comparison to a conventional battery which weighs about 18 kg, the combination of the smaller SuperCap battery weighs less than 5 kg and requires even less ground space. The SuperCap module is immediately recharged at start up by the board net and therefore makes it well suited for start and stop applications.
Large diesel engine starting applications differ greatly from starting small engines due to the heavier weight of the moving parts. A locomotive diesel engine may need an initial surge of approximately 2500 A to power up the hydraulic pumps, which in turn creates the pressure to suspend the engine shaft before starting. Once the engine changes from being in static friction to sliding friction, the current requirement drops back into the 800 A range for the rest of the start cycle. During cold weather, there is a so-called ‘pre-heat period’ of 30 to 60 seconds. This pre-starting preparation will consume most of the capacity of conventional battery packs. The high initial start current has a big effect on battery life, even though they are designed to meet it. By putting a SuperCap module in parallel with the battery, it is possible to get up to 75% more starting torque at a higher cranking rmp.
SuperCaps are maintenance free and do not degrade with time like lead-acid batteries. Life-time expectancy of >10 years and up to 1 million discharge/charge cycles can be reached. Unlike conventional batteries, the SuperCaps do not contain toxic materials and can be considered as safe components.
Further reading:
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