Hydractive Sphere As Accumulator?

Hi all

Can you? i was thinking about putting a hydractive sphere on the pressure regulator to give more volume and thus a bigger pressure reserve for rapid steering at idle when the pump output is low, im currently not quite getting enough and the steering momentarily goes stiff after going lock to lock a couple of times when trying to get into my incredibly tight parking spot, does anyone know if the pressures are the same on a stock hydractive sphere as they are on an acc??

D

Hi Dean,

Your proposal has crossed my mind before and I have often wondered why Citroen have stayed with an accumulator sphere of 400cc volume at 62 bar for almost all their hydraulic suspension models (not sure about C5's and C6's which I think use different shaped 'spheres'). Commonality of spares maybe??

The front centre hydractive sphere would seem to be the one to try at 500cc and 70 bar to give a ~20% increase in usable capacity ie the volume of fluid that the sphere can supply to the hydraulics as the gas pressure falls from 170 to 145 bar (from pump cut out to cut in pressure). A larger sphere would mean that the pump would cut in less frequently but run for a longer period to raise the pressure from 145 to 170 bar. This might not be good for the HP pump??

The rear centre hydractive sphere could also be used but would not offer any advantage as it has the same 400cc volume as the accumulator and is charged to a lower initial pressure at 50 bar.

Ken

PS I can see another advantage in fitting a front hydractive centre sphere to the accumulator if it has the 'Multicouche' diaphragm (sphere marked with three dimples) because these spheres lose nitrogen pressure at a much slower rate than spheres fitted with the more common 'Desmopan' or 'Urepan' diaphragms (polyurethane).

This post has been edited by kenhall1202 on January 13, 2009 12:16 am

Thanks Ken, i may try this temporarily, see what results i get and post the findings but you predictions sound right. Don't think it would hurt the pump, its no different than running a partially flat acc sphere, so long as there is pressure there to absorb the pulses from the pump it should be ok?

D

When Robert and I removed my front centre sphere on the S1, we found it had an accumulator sphere fitted... I must say, the ride has improved since the correct sphere with increased volume went back on.
I know that's the opposite to what you're suggesting Dean, but as the reduced volume caused worse centre sphere performance, it stands to reason that fitting a hugher volume one in place of the accumulator may yield a few gains. Though, I don't know enough to say whether deviating from the originally specified sphere will have other unpredictable results, perhaps some of our resident hydraulic gurus could advise.

Ciarán

There's some figures in this thread that George worked out -

http://club-xm.com/forum/index.php?showtopic=2571&hl

Seems OK for a least a test run or two.

Steve.

I think George's previous calculation for a standard accumulator sphere has overestimated the LHM volume available between 170 and 145 bar. Using Boyle's Law and absolute pressures the calculation is:

(400 x 63 /146) - (400 x 63 /171) = 25.2 cc

For a 500 cc sphere at 70 bar the calculation is:

(500 x 71 /146) - (500 x 71 /171) = 35.6 cc

The increased 'reserve' of HP LHM is thus 35.6 x 100 / 25.2 = 141% quite significant!

Ken

Thats nearly half as much again Ken! yep will give this a go then, not soon but in a month or so, and will report back

Thanks gents

D

One potential annoyance comes to mind with a bigger accumulator in that it will take a bit of extra time for everything to come up to pressure after the car has stood for a while (assuming that anti-sink is not fitted). Maybe this is one reason why Citroen have kept to the 400 cc/ 62 bar accumulator as the best compromise between pressure reserve volume and the time taken to get to 170 bar.

ken

This is a really good idea! I'd be keen on testing it out too.

The pump probably does the same work overall for both spheres: one tops up more but shorter cycles and the other tops up less frequently but for longer cycles of the pump. It should follow the first law of thermodynamics and the pump doing the same overall work.

It might even be better for the pump since although the work done is similar or the same, it is subject to less 'spikes' in load, or at least less frequent spikes in load each time the regulator closes.

Hope someone can do this and give an update on the result.

Curious this subject has come up.I replaced my PR+ACC and brake control valve about a month ago with exchange items from pleiades.After fitting,replenishing and bleeding the PR,including through to the brakes, the PR was clicking continuously.Tried bleeding again,no difference.'Phoned pleiades and they agreed fitting a hydractive sphere in lieu of an ACC sphere would be ok.So i have a rear hyractive sphere fitted as an ACC sphere as a temporary measure and a credit note for the defective ACC sphere.I really have not noticed any change in the suspension characteristics at all, and the PR cycle time is approx 25s.

using a rear hydractive sphere wouldn't make any difference as it is the same volume as the acc but lower charge pressure, you would need to use the front hydractive sphere to get the benefit.
I cant see having to wait a little longer will be a problem, for joe public it may be an issue but if you know the mod you have made it will be fine.
Will have to wait for this month at least because im owed money and after xmas things are a little tight so come feb.....................

D