GreenHybrid - Hybrid Cars - View Single Post

martinjlm · #17 (**permalink**) 09-13-2007, 12:33 PM

Quote:

Originally Posted by WaltPA

You are correct that the mechanical steering system is always receiving "inputs" from you thru the steering wheel.

However, the purpose of electrically assisted power steering is to add assistance or "boost" to your "inputs". The need for "boost" varies from quite a bit when you are stopped (parking) to none when you are traveling on a highway.

When you need no assistance or "boost" from the electrically assisted power steering system, there is really no need to keep it active.

On a conventional car, with hydraulically assisted power steering, the hydraulic pump is connected to the engine via belt, and therefore is always running, even when no "boost" is needed. However, with electrically assisted power steering, it is possible to just turn 'off' the "boost" when not needed.

BTW, electrically assisted power steering is not the same as drive-by-wire. I believe you are confusing the two.

Walt,

I see now where you're coming from and we are actually not that far apart. You are absolutely correct in your "at park" scenario. You don't have belts and pumps running when you have electric power steering. I pretty much glossed over that in my explanations, but to your point, that is not trivial. Still doesn't come close to 8% F/E improvement.

I was addressing "vehicle in motion" operation of electric power steering. Here's where we differ. You state that at highway speeds, for example, the system should just go to sleep. In my experience, when the vehicle is in motion, EPS is constantly monitoring driver and road inputs to calculate degree of assist required. High speed lane changes will require far less assist from EPS than parking maneuvers, as you have pointed out. But there is always the potential that the EPS may need to retard the driver input to reduce the likelihood of a roll-over caused by a driver over-correcting (single most common cause of roll-overs). If the system is dormant it might not pick up the over-correcting input.

I can assure you that I am not confusing EPS with drive-by-wire. I'm familiar with both technologies. Now, there is a slight chance that the Lexus / Toyota I drove in Japan may have been equipped with drive-by-wire. I'm fairly certain, but not beyond all doubt, that it was EPS. The lane keeping technology that was being demonstrated could potentially be achieved with either.

Peace,

Martin