Power steering pump power consumption comparison 1

[durablack2]

I am trying to determine the differences in power consumption for several non-variable hydraulic power steering pumps along a given route. I have the maps for each pump (speed, flow, pressure, power) - If I have pressure out of the pump, speed of the pump, I should be able to lookup the flow rate of the pump given those two conditions, and calculate power consumption correct? I could then substitute the pumps into my model and determine the differences in power consumption based off the maps as long as I keep pressure and speed constant, correct? Is there more to this or am I missing something?

A few other general questions about power steering systems -
Are power steering systems typically designed to work with a specific steering gear? Is the pump sized to the steering gear?

Does pressure out of pump remain constant with the steering wheel turned a constant amount?

Is pressure force to steering gear from pump the same at low and high vehicle velocities with constant steering angle?

 

[GregLocock]

Are power steering systems typically designed to work with a specific steering gear?

Not really

Is the pump sized to the steering gear?

Definitely

Does pressure out of pump remain constant with the steering wheel turned a constant amount?

No

Is pressure force to steering gear from pump the same at low and high vehicle velocities with constant steering angle?

No

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[GregLocock]

I think you are under a misapprehension concerning how it works, these diagrams seem correct

http://www.angelfire.com/retro2/kalyz_elysyrus/steeringpowersteering.html

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[BrianPetersen]

Fixed displacement pump connected to what amounts to an open-center valve. The pressure varies depending on the force balance inside the valve and that depends on how much torque it takes to operate the steering shaft, and that depends on what the driver is doing and about a million factors related to the tires, road surface, steering geometry and ratio, etc. There will be a back-pressure through the spool valve (depending on flow restriction) even if the driver isn't doing anything with the steering wheel. In other words, the situation is a whole lot more complicated than you think it is.

I don't know what your objective is with your testing, but if you want to minimize that power consumption, just pick the smallest pump you can get away with and the most efficient design that satisfies that criteria. You can always fine-tune the amount of fluid delivered (and the amount of power consumed) by playing with the pulley ratio from the engine.

The pump needs to deliver enough flow to handle operation at engine idling speed, and when negotiating something like a slalom course or quick lane change (or if the driver encounters an oversteer situation and has to make a fast correction) it needs to deliver enough flow. If it doesn't, the driver will encounter something called "pump catch" - turning the steering fast enough to demand more flow than the pump can demand - and when this happens, essentially the steering becomes completely manual for a moment. With the amount of steering effort required to manually steer a large vehicle you do NOT want this happening. It will always be possible for the driver to spin the steering wheel fast enough to overwhelm the pump ... but you do not want it happening under foreseeable emergency maneuvering circumstances.

In other words ... it's fine to pay attention to the power consumption, but it also has to work properly.

 

[cibachrome]

With hydraulic power assisted steering, the assist piston can become a pump when steering velocity is very high such that the return line delivers oil to the reservoir faster than the plumbing can accept it.
The "pump catch" situation occurs when the power piston drives the return fluid pressure way up AND the assist side of the power piston can not fill fast enough because of pump flow rate limits. If you are concerned about fuel consumption and/or horsepower loss, consider changing to a closed center steering system. Oil only flows when the assist is needed, not like open center systems where oil flows all the time in the hose and valve circuit and is diverted by a steering valve rotation to the assist chamber. Less heat, less power consumption, but response time issues. There is also EHPS, where the pump is driven by an electric motor that is engaged (usually) at low speed when assisted steering is needed. Some serious high speed operation issues with these two types because the overall steering compliance is kept low by the (lack of) hydraulic reaction. Now you have a valve flopping around looking for boost force and not getting any. Coupled with high tierod loads and friction, you get nightmare levels of driving control feelings. But hey, you saved 1 - 1.5 HP.

 

[BrianPetersen]

Fully electric-servo steering has been the auto manufacturers' response to wanting to cut out the power taken to drive the hydraulic pump. There have been a lot of complaints about lack of steering feel with those, but to me, this seems like a calibration issue, not something inherent in the design.

Electric-servo still consumes power, but it's easier for the electronics to not demand any more than what is really needed to operate the load.

 

[NormPeterson]

Is it not possible for the inertia of the motor to be effectively damping small vibrations originating at the contact patches?


Norm

 

[GregLocock]

The inertia of the motor is multiplied n^2 times the gear ratio in the rack (if it is a rack mounted system) That adds a reasonable inertia to the system. The gear is actually a recirculating ball screw device, where you have a nasty compromise between friction and backlash. It is possible to calibrate around inertia and friction, but you are in danger of the steering system feeling as if it is motoring in some circumstances, that is, the steering assist is driving the steering wheel.

My impression is that an EPAS does not have to be bad steering system, if the calibrators are wide awake, and have made the right system decisions, but some companies just don't seem to care that much, and unfortunately an EPAS needs a lot more tuning than HPAS. Mainly it is a lot easier to build speed sensitivity in to an EPAS than an HPAS, so that's a big plus to offset the minusses.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[NormPeterson]

I'm guessing that the drift-pull and active nibble compensations that some manufacturers are adding on top of a straight electric-power assist serve to further complicate that tuning effort.


Norm

 

[GregLocock]

Yes, and they are other areas where EPAS results in fewer customer complaints than HPAS.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[durablack2]

Good stuff everyone, thanks a lot for the input. After reading up on them a bit more the past month and reading the above I have a much better understanding. I never really thought they were simple but can honestly say that I never would have thought about much of the above information on my own or by reading tech documents.

After re-reading my question I am realizing that I left out some key information. If I have test data - pressure, flow, speed from a drive test and the map for a specific pump which includes efficiency wrt flow, pressure, speed(say Pump A), I can then calculate the power used by the pump during the test.

Using the pressure and speed data acquired from testing, I can then use that data, plug it into the map for a different pump (say pump B), and calculate the power that that pump would use. Because the two pumps do not have the same flow rates at a given pressure and speed, power will be different. A few assumptions would be necessary to make which are - the two pumps would not result in the system having the same steering feel, the operating temp of the fluid is controlled, and the vehicle can drive the route / make all turns with pump B.

If my calculator were good enough, I could do this with any type of power steering system - EPAS, EHPS, variable hydraulic, etc.

On a second note, does anyone know what kind of power or fuel savings you would get from switching to an EPAS system from a conventional hydraulic system at low steering input conditions? Such as city / highway driving in a straight line? I don't think there is any, and could possibly be more power used due to poor mechanical efficiency of the electronic system at high torque steering inputs like track or stand still turning.

 

[GregLocock]

"On a second note, does anyone know what kind of power or fuel savings you would get from switching to an EPAS system from a conventional hydraulic system at low steering input conditions? "

Some. The EPAS will be essentially quiescent whereas the PS pump will still be pumping at the usual volumetric rate, although the pressure will be low.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?