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The Citroën Guide Steering: Power Assisted Steering 38
Power Assisted Steering
The PAS steering (DIRASS, Direction Assistée)
used on Citroëns is not radically different from
similar systems on other cars. Naturally, having a
high pressure hydraulic system at disposal influ
-
ences the layout.
The fluid requirements of the various hydraulics subsystems
differ significantly: while the brakes require only a very little
amount of LHM and the suspension somewhat more, the
power steering cannot work without large amounts of min
-
eral fluid provided at a moment’s notice. A flow distribu
-
tor built into the first hydraulic circuit—that of the hydrau
-
lic pump, the main accumulator and the pressure regula
-
tor—controls the hydraulic pressure between the steering
circuit and the suspension-brake circuits on PAS cars.
The rest is rather simple. A hydraulic ram cylinder is
mounted on the rack of a traditional rack-and-pinion steer
-
ing gear unit. The pressure of the hydraulic fluid supplied to
assist the driver in turning the steering wheel is controlled
by the flow distributor and a control valve. The flow dis
-
tributor has the following components:
1
a slide valve to divide the amount of fluid;
2
another slide valve to limit the amount of fluid;
3
a pressure limiting valve to limit the pressure of the
LHM when the steering wheel is turned completely to
lock;
The steering control valve has three important elements:
4
a distributor mounted to the pinion;
5
a rotor fixed on the end of the steering rack;
6
a torsion bar between the distributor and the rotor.
On the main illustration, the power assisted steering system
is shown when it operates with the steering wheel in the
straight-ahead position and the pressure regulator is
switched on. The slide valve 1 inside the flow distributor di
-
vides the mineral fluid coming from the high pressure
pump between the main and the steering hydraulic circuits
(the main circuit having priority). Both the distributor 4 and
the rotor 5 are in neutral position—the torsion bar be
-
tween the two is not functioning). Both chambers of the
ram cylinder are fed without pressure. All the fluid arriving
through the distributor flows back to the LHM reservoir.
When the pressure regulator switches off while the steer
-
ing wheel still is in its straight-ahead position, the pressure
starts to rise until it reaches 170 bar again and disconnects
the feed to the main accumulator. The main slide valve of
the pressure regulator is connected to the second feeding
channel of the flow distributor. All the fluid supplied by the
HP pump now feeds the flow distributor where the slide
valve 2 is responsible for limiting the amount of fluid trans
-
ported by the control valve. The whole amount of fluid still
returns to the reservoir.
Now let’s assume the driver starts to steer to the right.
The rotor 5 starts to rotate with reference to the distri-
butor 4. The control valve closes the path of the fluid com-
ing from the flow distributor which no longer is allowed to
enter the valve. The pressure begins to rise in the circuit be
-
tween the control valve and the flow distributor, moving
the slide valve 1, which in turn modifies the ratio of fluid, fa
-
voring the PAS circuit. The fluid will enter the right chamber
of the ram cylinder while the left chamber can be emptied
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