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Some information on a subject that has been mentioned here before:
Hydraulic Powertrains Propel These Hybrid Trucks
Not all hybrid vehicles rely on electric motors, batteries and wires. Some propel the vehicle with a combination of hydraulic pump-motors, fluid lines and accumulators -snip-
You do have to read the entire article even if it is a long read. I can't copy the entire article into a post, I just copied the first couple lines. Nothing of interest or emphasis was meant by the first lines. Copying the first few lines is a common practice used by most news services.
"Beam me up Scotty. There's no intelligent life down here..."
I know. It's just interesting the way the article is written.
Looks like a good idea. I've some small experience with hydraulic drive systems. They work well and have many advantages.
There are some issues that have to be specifically delt with in the design. If the system is not properly designed, the drive motor can cavitate during an overdriven situation (IE compression braking when the vehicle wants to go faster than the motor is driving). If that happens, a runaway condition can be created. There are valves that will prevent this and I'm sure it's been considered. The engineering aspects of this design are interesting.
The inefficiencies are mainly from fluid leakage (internally) in the pumps and motors. Any clearance in the pump or motor is an internal leakage path. Then, there are the flow losses through the hoses/lines from pump to motor and back. Increasing pressure helps in some areas and hurts in others. Back in the day when I was doing hydraulic powered mowing equipment, we were often not much more than 50% efficiency for the system, as compared to 85+% for a mechanical driveline, for example. That's why I was suprised that the efficiencies had improved so much that its even a candidate, but apparently it must be the case.
It was mostly gear motors/pumps or gerotor motors/pumps that we used. As I recall, the bent-axis piston stuff was more efficient, but much more expensive. Maybe the prices have become more in line for the 'value' equation now.
Just as a side note: Most high performance electric motors have efficiencies above 93% and electric motor drives have efficiencies in the mid to high 90's. The big advantage of hydraulic systems, from what I understand, is that they are more power dense, not energy dense nor more efficient, than electric hybrid systems.
Whether a hydraulic system is more power dense or not is really irrelevant in case of a PS. With a hydraulic system, it always uses some power because it's constantly driven by the belt, but with an electric system, it only uses power when the steering wheel is actually moved.
That is correct. I was mainly aiming my comment at the hydraulic hybrid powertrain versus electric hybrid powertrain, not at power steering. However speaking of electric power steering, does anyone use pure electric power steering or are they all just hydraulic pumps driven by electric motors instead of belt driven?
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