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The 2006 EPA work concluded that there was the need for "clean sheet of paper" design for a hydraulic pump motor that would replace the bent axis design used in their modeling and prototypes. They had a 3800 pound test mule that got 80 MPG average and the converted a Ford SUV as well as a group of UPS delivery vehicles to HH configuration.
The INNAS Hydrid shows a direct comparison between conventional and HH configuration. They also address the issue of lower efficiency in hydraulic drives compared to gears, which is offset by limiting engine operation to only the sweet spot of BSFC, since the drive is from the accumulator instead of the engine directly.
One of the problems with the EPA design that was addressed in the INNAS version was to put the drives in the wheels themselves. EPA's version drove the differential in the same manner as the propeller shaft. This meant that pump RPM was 3.5 times higher than direct in wheel drives.
Even with all the flaws the wheel to wheel efficiency was closing in on 80%, while electric regeneration involves many more steps and cumulative losses as well as the fact that you simply can not put current back in a battery at anything approaching the rate it can be applied.
Another point to understand is the configuration of an electric motor and battery for charging the accumulator. This eliminates the expensive controller from the equation since all you need is an on and off operational tactic for accumulator replenishment. This may be the final configuration since battery longevity can be enhanced by gradually discharging the battery instead of many cycles of draining and recharging when the battery attempts to become the capacitive storage of every system energy fluctuation.
Time will tell if the hydraulic option becomes a practical solution. Understanding that a single 60-0 stop wastes the energy that could drive the vehicle .7 mile makes it easier to comprehend that like the 1 st generation Insight, it's not a long range battery that you need, it's one of sufficient capacity to store and release one full braking or acceleration event.
regards
Mech
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