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 influences 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 mineral fluid provided at a moment’s notice. A flow distributor built into the first hydraulic circuit—that of the hydraulic pump, the main accumulator and the pressure regulator—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 steering 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 distributor 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 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 divides 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 between 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 steering 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 transported 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 distributor 4. The control valve closes the path of the fluid coming from the flow distributor which no longer is allowed to enter the valve. The pressure begins to rise in the circuit between the control valve and the flow distributor, moving the slide valve 1, which in turn modifies the ratio of fluid, favoring the PAS circuit. The fluid will enter the right chamber of the ram cylinder while the left chamber can be emptied into the reservoir via the rotor of the control valve. This pressure difference moves the piston 7 to the left inside the cylinder, helping the car to make a right turn. If the steering wheel stays at the right lock, the pressure limiting valve 3 inside the flow distributor maintains a maximum pressure of 140 bar—when the pressure rises above this value, the fluid pushes the ball of the valve backwards, sending the excess fluid back to the reservoir..
When the steering wheel is turned to the left, the rotor 5 rotates in the opposite direction. It starts by cutting of the return of fluid to the main reservoir. The pressure will rise again in the circuit between the flow distributor and the control valve. The rotor allows the LHM to enter both chambers of the steering ram, however, the pressed area of the left chamber is twice as large as that of the right chamber, thus the piston will move to the right, helping the car turn to the left.
The hydraulic assistance is only needed while the driver is actually turning the steering wheel. When the rotating force on the steering wheel ceases—the driver has finished turning the wheel—, the angle difference between the distributor 4 and the rotor 5, made possible by the flexibility of the torsion bar 6 disappears, reverting the system to the neutral position, stopping the power assistance. When the driver releases the steering wheel back to the straight-ahead position, an opposite operation will start.
Later XMs and Xantiae omit this distributor and use a two-section high pressure pump with two independent outputs instead: six pistons provide LHM for the power steering, two pistons for the rest of the hydraulics.