I understand electric power steering is coming (actually, I have it on my Prius), because it is supposed to be more energy efficient. Is a generator that much more efficient than a hydraulic pump, or is it that there is energy going into turning the belt and pump even when not pumping against pressure?
Here in England, your "Drive by Wire" is prohibited. In the event of a total power failure all controls have to be manually usable, so both steering and brakes have to be usable without the engine's assistance.
The Prius system is legal under those requirements, as much as a hydraulic assist is. You _can_ drive a car with hydraulically-assisted power steering that has failed. It's no fun, but you can control it enough to get it off the road. The same is the case with the electrically assisted system.
That said.... with a car as small as the Prius, the real solution would seem to be rear wheel drive and manual steering. No reason to make things more complicated than they have to be, and I commute every day in a car with manual steering and it's just fine.
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AFAIK, drive-by-wire steering (i.e. no mechanical link) is not in use in any current cars, at least not in models in production. I don't know whether it would be too expensive or just impossible to implement it to be sufficiently reliable to be road legal.
Electric power steering means what you call electric assist, just like hydraulic power steering means hydraulic assist. Both power steering methods have the mechanical link, so when either of them stops working (e.g. due to failure in hydraulic pump or electricity) there's still steering, it's just harder because it's no longer assisted by power steering.
There has been much written about drive by wire steering and brakes, but I have not seen either. Drive by wire throttle is common in the United States for emission control. The throttle is completely controlled by a computer and there is no mechanical link between the gas pedal and the throttle body.
In general, electrical conversion (alternator/motor/wiring) is significantly more efficient than a hydraulic system of the same size/weight/cost. You can make hydraulics a bit more efficient by very precise machining, removing as much compliance from the tubing as possible, electronic controls, etc., but it hits a point of diminishing returns and there is always the frictional loss of pumping fluid around when its not being used for assist.
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