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Great question.
The UPS delivery vehicles use 82 gallons of hydraulic fluid and weigh 26,000 pounds gross.
That works out to 8.2 gallons for a 2600 pound car.
Now, your reference to the standard off the shelf pressure rating at 3000 PSI is another great point.
In the Americas cup racers (cost is no object) they use 12,000 PSI accumulators.
Using a GM type hydro formed frame like on the Pontiac Solstice, you could probably go to 5000 PSI, which is the figure I like, using the frame as the body of the accumulator.
Of course the 8.2 gals at 3k would be 4.9 gals at 5k or just over 2 gals at 12 k.
The EPA quote for energy density of accumulators was 50 kilowatts per kilogram. I can't remember if that was at 3 k or 5 k (I think 5). That's about 65horsepower seconds for every 2.2 pounds, and it is available virtually instantly and the life expectancy is longer than the vehicle itself.
That's 650 horsepower seconds of instant energy without any engine power added.
Basically you should size the accumulator to allow a single acceleration to about 80 MPH with no help from the engine. 10 gallons in a 2500 pound car would easily do that depending on the pressure and the full stroke displacement of your pumps.
The rate of acceleration would be astonishing, as long as you sized the accumulator and the max stroke displacement of the pumps to match the ideal traction of the tires with best friction pavement type. In other words you determine the torque required to overwhelm the traction of each tire and match that value with stroke, piston diameter, and accumulator capacity.
The weight of the components is offset by the elimination of all the drive train components you no longer need, everything from the flywheel to the wheels themselves. In may vehicles that's 25 % of the total weight of the vehicle. In some cases more.
How about a car that can beat anything from a light, since you can apply power to all 4 wheels to the limit of their traction with the ground.
In essence it takes the same amount of energy to accelerate a car at any rate of acceleration as long as the target speed is the same.
The average car stops from 60-0 in about twenty revolutions of the wheels when you stop as fast as possible. 80-0 would probably double the revolutions.
In the electric drag racer thread in this forum they did 0-110 in 10 seconds.
It took almost 20 minutes to recharge the batteries for a single pass on the drag strip.
Hydraulic hybrids can recover and reapply that energy at the same rate, and at efficiencies 3 times as good as electric regeneration.
Even if you powered a hydraulic hybrid with batteries and an electric motor, you would still be able to regenerate 3 times the energy in the same time period.
Green Car Congress had a post that stated that the electric car would still require a form of transmission to overcome the size of the electric motor issues. In other words a pure electric vehicle could use a much smaller motor with an efficient power train.
Make the electric motor replenish the accumulator and you have an electric-hydraulic hybrid, which would either extend the range or allow for a smaller battery and motor with the same range.
regards
Mech
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