Quote:
Originally Posted by Old Mechanic
I would think you would want to insulate the accumulator to conserve the heat energy for better overall efficiency.
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Not just the accumulator ... every part of the system ... Hydraulic motor , Pump, transfer pipes, etc ... every joule of energy lost ( heat , sound , whatever ) is lost energy and lowered efficiency of the system.
Quote:
Originally Posted by Old Mechanic
Ian made a good point about the efficiency of electric regeneration. My counterpoint is a worst case scenario where you are forced to make a panic stop. In 20 revolutions of the wheels you have to recover your several hundreds of horsepower-seconds of energy in a vehicle weighing over 1 ton. That basically means every revolution of each wheel needs to be able to recover about 10 horsepower seconds of energy in a total time of a few seconds. In this scenario I am not aware of any electric powered vehicle with this capability. Maybe the KERS system in the new Formula 1 designs, but at what cost?
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Kinetic Energy = 1/2 m V^2
In Joules , kilograms , m/sec
1 ton = 2,000 pounds = ~909 kg
60 mph = ~96 kmph = ~27 m/s
~331,330 Joules = ~92 wh of energy.
If you can't store ~92 wh of energy than your system can only try to recover as many kwh of energy as it can store.
For example the KERS systems are limited to 400kJ of energy ( ~111 wh )... in 5kg of flywheel weight ( ~22wh/kg )
I you want to take 1 hour to stop needs ~92 watts of braking power.
In 1 minute = ~5,520 w = ~5.5 kw of braking power
It's really hard to keep enough traction to accelerate or decelerate faster than gravity 9.8 m/s^2.
Or about ~2.8 seconds
To brake 1 ton from 60 mph to 0 mph in ~3 seconds would require ~110 kw of braking power over those ~3 seconds ( ~132 feet ) ... if you can brake slower it takes less power.
The Kinetic energy is linear with Mass ... so 2 Tons would be double the braking power ~220 kw to stop in the same ~3 Seconds from 60 MPH.
Braking might get wind resistance , rolling resistance, gravity as some 'free' braking power ... depending on context.
Getting a ~110 kw rated EV is not especially hard ... if that is what one wants ... Tesla is more than this ... or 12kg EMRAX motors can do peaks of 80kw for ~1min ... two would more than cover the braking power in all of ~25 kg ... I don't know of any hydraulic motors or pumps that can match or beat the power density of EMRAX Electric Motors ... thus .. unless you know of a hydraulic motor I don't ... the motor part would weigh more than what is available today for an equally powered electric motor.
While not common ... there are EVs with motors and batteries that can do better than this.
Formula 1 KERS are also limited to 60kw... 24kg... and 13L of space ... 2.5 kw/kg & 4.6 kw/L ... fairly good but not the best... It's the 12 kw/kg power density of the flywheel that is the hard part for beating KERS ... Maybe with some high end capacitors... while the energy density ~22wh/kg of the flywheel is not great ... that 12kw/kg power density of the flywheel is a real monster.
If we are comparing power density of the Hydraulic accumulator to that of the power density of flywheels , or capacitors ... I think the hydraulic will have a hard time ... against chemical batteries ... that Will be much easier ... Not many Chemical Batteries have better than about about ~2 kw/kg power densities ... but that much kw/kg power density is still no small feat for hydraulic storage either... and if you leave the power density discussion to talk about energy density ... that is VERY hard for the hydraulic to compete against modern chemical battery tech.
But ... there are still other considerations ... Mean Time Before Failure ... $ ... etc... I don't think it is accidental or foolishness that battery based HEVs are currently more dominate in the market ... if that changes because something else got better for whatever reason ... fine / great ... be it hydraulic , flywheel , or whatever.