The Ingocar Valentin approach
The Ingocar Valentin approach is commendable indeed. He has motor-in-wheel motor/pumps. These act a motor and pump (kinetic energy braking reclaim) and the brakes. All simple, light, small and all in one unit. Brilliant.
My reservation is efficiency of the driving motors. I do not actually know it. On part load many hydraulic motors lose efficiency. A British company has produced the Digital Displacement hydraulic motor pump. This is electronically controlled and loses no efficiency at any speed range or loading. This motor/pump is obviously more complex than simple hydraulic motors. The company, Artemis, in 2008 converted a BMW 530i equipped with a Digital Displacement Hybrid transmission to prove a point. MPG improved by 100% in city driving. This in a far from optimized configuration using a lot of the existing BMW mechanicals. Transmission progression was seamless.
They used the standard 3 litre engine turning a hydraulic pump which charged a small accumulator. Two Digital Displacement hydraulic motor/pumps drove the cars two rear wheels. If the accumulator was bigger the engine need only be around 1000cc. Or sized for average hp down a highway, approx 20-30 hp. The web site has videos and documents of the testing, etc.
artemisip.com/appli_auto_transm.htm