Challenge!

It appears that the upper limit of accumulator pressure experience is in the range of 4 to 5k psi. This establishes the upper boundry on energy density. This in turn, seems to make HHV regerative braking technologies less applicable for small to mid size cars.

It has been suggested by some HHV specialists that 7 kpsi accumulator system might be the ENABLING point for mid/small HH vehicle regerative braking.

Does anyone have any ideas/suggetions for accomplishing a 6+ kpsi (say, a 10 gallon) accumulator subsystem?

What in your experience is the upper pressure limit?

Possibly "Dieselling?"

BudT, please clarify/expand your response.

Anytime Mineral Oil is rapidly compressed the vapor in any voids can get so hot it explodes.

I have heard it called Dieselling and have seen it happen in circuits with trapped air. The seals and/or metal in the area wher the explosions occur are balckened by the explosions or rubber is cooked from tthe heat. There is usually no other part of the circuit that shows over heating symptons.

Do a Google search to see if any of the old threads on this might come up.

Excellant point!

Then the puzzle becomes maintaining constant liguid pressure with varying volume and the volume must be devoid of any oxidizers.

This does not appear to be an easy one!!

How about using a Weight Loade Accumulator and use the vehicle as the weight.

It would mean some portion of the vehicle would hve to change height position but multiple short stroke units could keep the travel short.

Weight Loaded accumulatrs maintain constant dizcharge prssures

I would hate to be sitting on top of a 7kpsi accumulator if it got ruptured in an accident.

1. We use an inert gas, typically dry nitrogen, to charge hydraulic accumulators to prevent dieseling.

2. A Google search for “hydraulic accumulators” turned up many hits. One Lorimer, touts “Accumulators are available with pressures to 30,000 psi, temperatures to 500° F, and swept volumes up to 10 gallons”.

Lewey, what levels of pressure, volumes, and charge/discharge rates have your accumulator systems used?

Sorry, I try to limit my input on these BB’s to the original thread’s question/response(s) as many if not most people seem to only read and respond to the last post, regardless of its relevance.

You posted a good question. It got off-track.

In this case, as I have not applied accumulators in systems with working pressures above the 3 to 5,000 psi you asked about, I saw no real benefit in going over standard applications.

If you are unsure of accumulators, most distributors and all accumulator manufacturers will be happy to supply you with the “cookie-cutter” formulae and basic rules that should get you close enough so you can then dial-it-in.

Does the EPA already have a >>7kpsi accumulator?? In another thread s houston posted a link to a Charles Gray (director of the EPA’s Advanced Technology Division) view graph presentation, http://files.harc.edu/Projects/Transportation/HydraulicHybridsGray.pdf. On page 15 the hydraulic hybrid accumulators are listed as having an energy density >50kw-sec/gallon. If I plugged in the right numbers, a 50kw-sec/gallon energy density would require the adiabatic compression of an ideal gas to >6200psi. Using the specific heats for nitrogen and its compressibility factor would probably result in a required pressure of nearly 7kpsi. Then if the volume of the bladder and shell are considered, the required pressure may well be approaching 8kpsi. Furthermore, such a storage system requires a reservoir of nearly equal volume so the EPA may already have a 16kpsi accumulator to achieve the 50kw-sec/gallon energy density!! Perhaps I’ve misinterpreted, miscalculated or misestimated so check with the EPA.

I question the applicability of HHV technology in small cars even with 7kpsi accumulators. You should be able to buy a 12volt lead acid battery off-the-shelf for less than $35 that has a Reserve Capacity of 100minutes. That equates to about 500Watt-hours of energy, or 1800kw-secs. Such a battery will have a volume of (I guess) 1.5-2gallons. That gives it an energy density of 900-1200kw-secs/gallon. That includes the case and the electron reservoir! Since most trips made by small and mid-sized cars are less than 20miles, and electricity costs about $0.06/kw-hr, the plug-in EHVs would seem to have a major advantage, even if the batteries aren’t highly efficient? If the EHV uses 0.25kw-hr/mile and gas is at $2.25/gallon, the HHV will need to get 150mpg to match the fuel cost. Prices and efficiencies may vary but it seems the HHV has a lot of ground to make up on small cars so perhaps the “HHV specialists” should concentrate on doing more than just a marginal job on the larger vehicles.

I believe the UPS delivery trucks are marginal because I once read that an Eaton engineer stated the 44gallons of high pressure accumulators on the delivery truck held 2000hp-secs of energy at 5000psi, (that’s about 45hp-secs/gallon or 34kw-secs/gallon.) Also, page 14 of the above mentioned presentation states the data is typical for a Class 6 delivery truck so I’m assuming the UPS trucks are Class 6 which can weigh up to 26,000lbs. The example on page 14 includes a rather mild energy recover cycle that includes braking at 0.1g from 35mph, which would take nearly 16secs to come to a full stop. The presentation also indicates they are using 110cc motors and the pictures (pp 34,36,37,42) look like there are two motors in the normal configuration plus a third auxiliary motor for steep hills. At 2000rpm the three motors and 44gallons of accumulators are barely adequate for the mild braking cycle. Braking any harder or from a higher speed would result in a steep drop in the recovery efficiency. I don’t know what the gearing ratio is but the motor plot on page 18 only goes up to 2500rpm so the midrange speed may be less than 2000rpm, further reducing the braking capacity of the motors. A good hybrid drive should have the storage capacity to brake at higher speeds while maintaining adequate reserve energy to moderate the engine load during acceleration and hills.

Piston accumulators rated for much higher pressures are available but my experience with them is that they are heavy and they do not maintain their gas charge. Also the piston can add unwanted dynamics. The thermal characteristics of the bladder in the lightweight carbon fiber accumulators should not be a problem because to optimize the energy density you should have the same compression ratio at 7kpsi as at 5kpsi and thus the same temperature rise. So the limitation must be due to the strength limitation of the carbon fiber shell. To answer s houston’s question, yes, hydraulic storage units that can operate at pressures >>7kpsi and with energy densities much greater than what the EPA is working with can be built. However, the design is innovative and the EPA is not into innovation, they’re more into reviving 30year old technology.

In my duscussions with EPA I was told that they had not built a system above 5 kpsi. The 7 kpsi was a suggested "optimum" power density level for a 10 gallon system in the opinion of the people that I talked with. Thinking was a 2+ cubic foot accumulator and a 2 cubic foot reservoir. And you are correct, that is a large volume for a small unibody vehicle.

The impression I got was that the advantage of hydraulics over electric is the rate at which power could be transfered.

The other point they seem to consider important was the 80 to 88 cc volume per revolution variable displacement pump. Unfortunately I did not clarify if that was max or min ... a mistake on my part!

To update the 7kpsi accumulator, I did contact EPA/OTAQ and was informed that the accumulators and pump/motors used in the UPS truck are rated for 7kpsi service. The energy density figures in the referenced view graph presentation are for 7kpsi and is calculated using the volume of the precharge gas only. The volumes of the bladder, container, and the fluid are excluded from the energy density calculation. So even with 7kpsi accumulators it doesn’t seem the car manufacturers are showing a lot of interest in this technology. I heard GM is going to sell an electric hybrid Suburban SUV that has acceleration equal to the standard Suburban, a slightly more powerful engine but a reduced towing capacity. I assume the electric drive components can’t take the continuous heat load so it would seem to be an ideal application for the hydraulic drive. I’m sure GM doesn’t like reducing the towing capacity so if hydraulic hybrids were considered viable for SUVs I think GM would give them serious consideration before selling electric hybrids with a reduced towing capacity. If hydraulic drives are not viable in SUVs they certainly can’t be considered viable in passenger cars. I suspect the sliding contact between heavily loaded parts and the flow restrictions of valve plates are limiting the use of hydraulic drives in passenger cars. Such designs are inefficient at lower power levels, such as cruising, and generally somewhat noisy. I think pump/motors that eliminate these losses and can operate efficiently at around 12kpsi would be an enabling technology. I think the cost of fuel is the enabling factor allowing the reintroduction of hydraulic drives, not innovation by the EPA. Without improvements in pump/motor designs I doubt that hydraulics will become a significant factor in transportation and hydraulic power will be replaced by electric power, as is happening in air craft, until hydraulics is reduced solely to material handling applications.