Regenerative systems (KERS)

[xpensive]

The pump drawing does not do it for me WB.
Cant see how the pistons return or how they recharge with fluid.
The system also requires an hydraulic motor.

Most hydraulic pumps can also operate as motors with minor modification. If you need the pump to operate as a motor with the same direction of rotation you only need a servovalve to swap inlet with outlet.

The power to weight ratio of these pumps (standard aerospace versions) are quite impressive.

I don't think you need to go to aerospace to find them power/weight-efficient, a maxed out Parker (X-Volvo) 9.8 cc piston pump/motor will take you close to 60 kW continously, 77 kW intermittent, at 7.5 kg and 10/11 kRpm.

You should avoid the servo valves though, too much losses, a simple system of directional valves should do fine.

[Edis]

The pump drawing does not do it for me WB.
Cant see how the pistons return or how they recharge with fluid.
The system also requires an hydraulic motor.

Most hydraulic pumps can also operate as motors with minor modification. If you need the pump to operate as a motor with the same direction of rotation you only need a servovalve to swap inlet with outlet.

The power to weight ratio of these pumps (standard aerospace versions) are quite impressive.

I don't think you need to go to aerospace to find them power/weight-efficient, a maxed out Parker (X-Volvo) 9.8 cc piston pump/motor will take you close to 60 kW continously, 77 kW intermittent, at 7.5 kg and 10/11 kRpm.

You should avoid the servo valves though, too much losses, a simple system of directional valves should do fine.

A similar displacement aerospace pump have a weight of 4.5 kg.

I don't think directional valves will be enough as you need some means of controlling the output and the braking torque. But regular servo valves perhaps have a bit high pressure loss.

[xpensive]

A variable displacement pump/motor could help you with that, adds to the weight though.