Combining gas and diesel engines could yield best of both worlds
When you eliminate throttle controls or fuel delivery controls and let the engine run at its best efficiency then you can get close to 60% thermal efficiency.
That is one of the key reasons I like the hydraulic hybrid. The engine is not the controlling factor in vehicle speed. The hydraulic accumulator allows this separation of function and acts like an energy damper, both for acceleration and regeneration.
The engine can now cycle on and off, running with no throttle control whatsoever. It either produces absolutely best bsfc, or it is not running. The cycle percentage depends only on sustained energy requirements as an average. Sitting in crawling traffic the cycle could be lower than 5% while 100% would be climbing a long steep grade. The only control mechanism is an on or off switch.
Another design that seems promising is the Chiron from INNAS.
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Look at their floating cup in wheel drives. This is a fixed displacement design with a transformer to control pressure. Fluid is constantly flowing through a fixed displacement motor. My design eliminates the transformer and controls the displacement directly by changing the stroke of the pistons, It is also a rotary instead of axial configuration.
In the EPA documents from 2006 they were looking for a "clean sheet of paper" design for a drive motor to get regeneration efficiency (something Moody never even considered) beyond 80%.Their major issue was using a bent axis pump-motor connected to the differential at prop shaft speed, where the efficiency of a hydraulic motor is poor compared to driving the wheel directly at less than 30% of the RPM of a prop shaft speed drive.
regards
Mech