CitroJim wrote:Mandrake wrote:I have to admit that I can't quite see how the new unit can function with one of the small pipes deleted - on the older regulators one of the small pipes is from the output of the height corrector (via the anti-sink valve) which controls the suspension pressure and the second one is full system pressure to provide a feed for the electrovalve. I can't see how deleting either of these would result in a working system...
Same here Simon, I always believed that the electrovalve needed both full system pressure and an electrical signal to operate it...
Furthermore I always believed that low system pressure could cause the valve not to operate...
I just took a closer look at the diagrams and have worked out how the one pipe system works, and it's actually a rather ingenious design tweak.
For the original design Hydractive 2 control block with two small pipes you're right Jim.
The basis of operation is that there is a floating "shuttle valve" which via side ports opens and closes the flow both between left and right suspension sides, and also the flow from both sides to the sphere.
As well as one of the ports, one end of the shuttle valve is also connected to the pressure to the sphere, and the other end is connected to the output of the electrovalve, which can be switched to either full system pressure or overflow. Both ends of the shuttle valve are the same diameter so have the same hydraulic ratio. So the pressure provided by the electrovalve (either full pressure or no pressure) works against the pressure in the centre sphere - which is at a partial pressure - on an unloaded car about 80 bars at the rear and 120 bars at the front, but increasing proportionally with suspension load.
Using the front as an example, full pressure (145-170 bars) applied by the electrovalve to one end of the piston overcomes the 120 bars provided at the sphere and and thus pushes the shuttle valve into the "soft" position, (with the compliance of the sphere giving somewhere for that oil displaced by the shuttle valve movement somewhere to go) whilst zero pressure allows the pressure from the sphere to push it back the other way.
Notice some potential problems in the design already though - front suspension pressure is already 120 bars even without passenger load, add four heavy people and that might go up to 130 bars or more. Meanwhile the pressure regulator allows the pressure to drop as low as 145 bars before cutting in again - and that's assuming that the pressure regulator isn't old and tired with weak springs. As you've found they sometimes get sick and struggle to produce enough pressure either due to weakening reaction springs, or due to leakage.
If the available pressure is not reliably and significantly higher than the active suspension pressure, it will fail to lock the valve firmly into the soft position. Furthermore, if you hit a large bump, that sudden pressure spike will also be applied to the sphere end of the shuttle valve trying to push it back against the incoming supply pressure - if that pressure pulse exceeds a weak system supply pressure it could cause the valve to momentarily move into the hard position for the duration of the bump causing harshness. (Since you have abruptly gone into hard mode on the leading edge of a bump)
This is one of the theories of harsh ride I had years ago, and suggested the idea of introducing a one-way ball valve in the small feed pipe to the electrovalve to prevent any such back flow through the electrovalve when the suspension hits a bump, but I was never able to test the theory in practice.
Another issue that most owners will have observed is a propensity for the suspension to stick in the hard mode for a while after you have fully raised the suspension, left the engine off for a minute and then restarted the engine and lowered the suspension again - the reason for this is when the suspension is fully raised there is no longer a pressure differential between the electrovalve and sphere ends of the shuttle valve. When you start the engine again with the suspension still in high there is not enough pressure differential to push the shuttle valve back into the soft mode position, (towards the sphere) also because the gas in the sphere is heavily compressed it doesn't have much "give" to allow the displacement of the valve, as the oil displaced by the valve can only go into the sphere when the unit is in hard mode.
So it ends up in a kind of "limbo" where the electrovalve is on (soft mode) but the shuttle valve is stuck in the hard mode position, unable to move into the soft position, but after a while the gradual pressure leakage through the valve allows it to move enough and suddenly it will break free - then the car pops up like a cork.
The new valve design solves all these problems.

If you look closely at the valve diagrams you'll see on the new type there is an additional spring (item 11) on the shuttle valve that doesn't exist in the old design. I'm going to call this a "bias" spring. This is pushing the shuttle valve into the soft mode position. The electrovalve still controls the pressure on the opposite end of the shuttle valve but instead of getting a high pressure supply, it supplies pressure from the other end of the valve - eg the centre sphere.
So now when the electrovalve is on, it connects both ends of the shuttle valve together so that they are both at equal pressure - eg typically 120 bars at the front. Because of this there is no hydraulic force to push the valve one way or the other and it would otherwise flop loose, however the "bias" spring will firmly push the valve into the soft position. When the electrovalve goes off, it will connect its end of the valve to overflow and thus the suspension pressure will overcome the strength of the spring and push the valve back to the hard position.
The first problem of insufficient system supply pressure is completely eliminated, as supply pressure is no longer used. In soft mode the pressure across the piston is equal and its only up to the spring to keep the valve in the soft mode. If the supply pressure drops to 120 bars, no harm done - it will stay perfectly in soft mode.
The second problem of instantaneous pressure increase hitting a bump trying to push the piston back the other way is also completely eliminated - because in soft mode both ends of the piston are connected together - any pressure rise from a bump will be equal on both sides and not try to move the valve, with the spring maintaining control of the valve in the soft position. If you hit a bump in hard mode then the pressure increase would only serve to keep the valve even more firmly in hard mode.
The third problem of the suspension sticking in hard mode temporarily after being fully raised is also solved - because previously the problem was that you were trying to force more oil into an already nearly full sphere to let the slide valve move into the soft position, now when the electovalve turns on it connects both ends of the piston together which means it can move to the soft position by letting a small amount of oil out of the sphere instead of trying to force more in.

Also, no matter what the suspension pressure is, there is no difference in the operating force applied to the shuttle valve, because in soft mode it is the force of the spring alone.
Yet another benefit of the new design is that in the old design, when the car was parked with the engine off, as the system supply pressure bled away from the main accumulator as soon as it dropped below suspension pressure the suspension would not be able to go back into soft mode when a door was opened as there was not sufficient pressure differential across the valve - again, solved by the new system. Now you could open a door hours after the engine had last been running and it would still switch to soft mode, because all it is doing is connecting both ends of the shuttle valve together, allowing a small amount of oil to flow out of the suspension spheres to push the valve back towards the soft position.
Brilliant!

Not only does it solve these problems, it also means an entire pipe can be deleted from the design and a simpler anti-sink valve with one less port can be used. (I bet the bean counters loved that) In fact now that I've seen it I have to wonder why it was not originally designed this way, it seems almost like the original design was in error and has some significant shortcomings.
I'd almost be tempted to retrofit the later type unit onto the earlier car except for the fact that the unions on the large pipes have changed!
