Speed-Sensitive Power Steering (SPS), Function - GF46.50-P-0001RDA
Model 172.4 with engine 274, 276.8
Model 172.4 with engine 651 as of model year 2016
Block diagram (up to model year 2017)
Block diagram (as of model year 2017)
Model 172.4 with engine 274, 276.8 with code 213 (Direct-Steer system)
Model 172.4 with engine 651 as of model year 2016 with code 213 (Direct-Steer system)
Compared to vehicles with speed-sensitive Power steering, vehicles with direct steering have a more agile turning behavior, especially in urban traffic, when maneuvering or on twisty roads. This has been achieved by modification of the Toothed rack and as a result of the associated increase in gear ratio from the center position as of approx. 6° steering wheel angle.
For the driver this means improved comfort by reducing the steering-wheel turns lock to lock from 2.63 for the standard steering to 2.17.
Function requirements, general
- Drive train operational
Speed-sensitive Power steering, general
The steering wheel's rotary motion is converted into horizontal movement through the steering gear of the rack-and-pinion steering. The steering gear of the rack-and-pinion steering has a variable gear ratio, i.e. the tooth pitch is greater in the steering wheel angle range ± 6° to ± 90° from the center position.
The speed-sensitive Power steering system consists of the following subfunctions:
- Speed-sensitive Power steering turn operation function sequence
- Function sequence for electrically controlled speed-sensitive Power steering reaction
- Rack-and-pinion steering straight-ahead driving function sequence
Speed-sensitive Power steering turn operation function sequence
If the steering wheel is turned, then the drive pinion is supported on the Toothed rack. The rotary valve is rotated against the stabilizer bar for the pilot bushing surrounding it, thereby changing the position of the control grooves in relation to one another.
The pressurized oil flow is now fed into the right working cylinder or into the left working cylinder and assists axial movement to the right or left.
The oil is pushed out of the unpressurized working cylinder and flows back into the hydraulic fluid reservoir of the Power steering pump via the open return grooves of the pilot bushing.
The required steering wheel torque is regulated to match the vehicle
The Power steering is provided by the electrohydraulic Power steering system.
To cool the Power steering fluid, an oil cooler in the form of a coiled pipe is fitted immediately upstream of the radiator. The oil cooler version depends on the engine mounted in each particular vehicle. The Power steering is regulated by the rack-and-pinion steering gear itself. The oil Pressure required for the Power steering is produced by the electrically driven Power steering pump of the electrohydraulic Power system. The Power steering pump is regulated by the electrohydraulic Power steering depending on requirements (vehicle at standstill, vehicle speed) to match a specified characteristic.
To recognize the vehicle status at any given time, the electrohydraulic Power steering reads in the following data:
- From the instrument cluster - the current vehicle speed
- From the front SAM control unit with fuse and relay module - the onboard electrical system status
- From the electronic ignition lock control unit - the circuit status
- From the steering wheel angle sensor integrated in the steering column tube module - the steering wheel angle speed. Control is effected by the speed-sensitive Power steering solenoid valve, which is actuated by the electrohydraulic Power steering according to vehicle speed.
A differentiation is made here between the following conditions:
- Function with vehicle speed v = 0 km/h
- Function with vehicle speed 0 km/h < v ≤ 200 km/h
- Function with vehicle speed v > 200 km/h
Shown on steering lock to left
Shown on steering lock to left and vehicle speed v = 0 km/h
Function with vehicle speed v = 0 km/h
The speed-sensitive Power steering solenoid valve receives maximum energy and as a result it is closed against the force of the integrated compression spring. The Pressure ratios in the reaction chambers are identical since the supply of oil is stopped. No force is applied to the reaction balls and no additional torque acts on the surfaces of the rotary valve.
The steering is light as the steering torque applied by the driver (i.e. the force required to overcome the force of the stabilizer bar) is very low. The oil flows through the constant constriction orifice to the Power steering pump Expansion reservoir almost depressurized.
Shown on steering lock to left and vehicle speed 0 km/h < v ≤ 200 km/h
Function with vehicle speed 0 km/h < v ≤ 200 km/h
As the vehicle speed increases, energization of the speed-sensitive Power steering solenoid valve decreases continuously; the valve is opened by the force of the integrated compression spring. The controlled Pressure is led to the reaction balls via the regulating valve, causing the Pressure applied to the surfaces of the rotary valve to increase.
The force on the rotary valve is increased further as a result, and the driver has to apply greater steering torque to operate the steering wheel. The constant constriction orifice reduces the regulated Pressure from the speed-sensitive Power steering solenoid valve. This is then routed practically depressurized as a backflow to the Power steering pump's hydraulic fluid reservoir.
Shown on steering lock to left and vehicle speed v > 200 km/h
Function with vehicle speed v > 200 km/h
The speed-sensitive Power steering solenoid valve receives minimum energy. As a result, the highest possible Pressure is routed from the speed-sensitive Power steering solenoid valve to the reaction balls via the regulating valve, thereby increasing the Pressure applied to the surfaces of the rotary valve to a maximum.
The constant constriction orifice reduces the regulated Pressure from the speed-sensitive Power steering solenoid valve. This is then routed in an almost depressurized state as a return flow to the Expansion reservoir of the Power steering pump.
Function sequence for electrically controlled speed-sensitive Power steering reaction
The electronic control system of the Power steering is integrated in the electrohydraulic Power steering. As the vehicle speed increases, the manual effort required to turn the steering wheel increases at the steering wheel (up to v = 200 km/h).
In the electrohydraulic Power steering, the determined vehicle speed is assigned a specified current by way of a characteristics map. This is then set in the speed-sensitive Power steering solenoid valve and influences the oil flow (hydraulic reaction assembly) within the rack-and-pinion steering gear and thus the manual force required to turn the steering wheel.
Actuation of the speed-sensitive Power steering solenoid valve is pulsed. The current is regulated by pulse-width modulation of these pulses.
Shown in straight-ahead driving
Rack-and-pinion steering straight-ahead driving function sequence
When driving straight ahead (steering wheel in center position) the rotary slide of the rotary slide valve is in the neutral position.
The control grooves of the rotary valve and the pilot bushing are aligned such that the oil flow from the Power steering pump is routed straight back via the return flow to the Expansion reservoir. The working cylinders are therefore unpressurized.
The stabilizer bar is pinned on one side with the rotary valve and on the other side with the drive pinion.
| Electrical function schematic for speed-sensitive Power steering (PML) | PE46.50-P-2050-97TAB | ||
| Overview of system components for speed-sensitive Power steering (SPS (PML)) | GF46.50-P-9999RDA |