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Originally Posted by Old Mechanic
James seems to lack a lot of knowledge about hydraulic accumualtors and their application in a hybrid vehicles. Try studying the UPS hydraulic hybrid trucks which have been on the road for years.
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I have. They're trucks, which means large & heavy, which means the weight of the hydraulic accumulator is not much of an issue. They're also DELIVERY trucks, which means that they have a largely stop & go drive cycle, which is perfectly suited to the limited but efficient hydraulic energy storage.
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When you have acquired some knowledge and understanding the size of the UPS accumulators, in relation to the gross weight of the trucks (26,000 lbs), you will be better informed to make judgements about the amount of weight and space necessary, especially in a car weighing less than 10% of that amount. The weight penalty issue is non existant.
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On the contrary. The UPS & similar HH trucks have very limited energy storage, just enough for a few starts (because in stop & go operation, the energy for each start is mostly replaced by the next braking). Very few cars spend much time being driven in stop & go cycles. (I suppose some applications, like urban taxis, might be.) You aren't going to store enough energy for 10-40 miles of driving in any sort of portable hydraulic accumulator.
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Plenty of accumulators available today, with operating pressures of 3000 PSI which is what is used in many commercial applications right now...
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Sure, and if they store significant amounts of energy, they are large & heavy. It's just physics - which determines how much energy is stored in compressed gas - and strength of materials.
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The catastrophic failure issue has been resolved long ago.
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Sure, by making the accumulators sufficiently strong, which means heavy.
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As far as size and weight, to quote one of the EPA heads of the research in 2006, Charles Gray, "I can hold a 500 HP hydraulic motor in my hand."
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Now who's dodging issues? The weight of the motor isn't the issue: the weight of the accumulator is.
When you get down to the basic physics involved, hydraulic energy storage is really nothing but a spring. You just can't store the same amount of energy (per mass) mechanically as you can chemically. It's why we use gunpowder instead of catapults.