Many larger cars are fitted with power-assisted steering (PAS) as standard equipment. It reduces the effort needed to turn the steering wheel and it makes manoeuvring at low speed, when parking in a confined space, for instance, much easier than with conventional steering. Power assistance also contributes to safety. With conventional steering, the steering wheel may be wrenched out of a driver’s grip if a tyre bursts or if the car hits a large bump. Power-assisted steering stops the steering wheel ‘kicking back’ and helps the driver to correct a sudden swerve. PAS is essentially a power-assisted, not a power-operated, system. The driver is always left with some work to do, to retain the ‘feel’ of the steering.
Apart from reducing steering effort, one of the advantages of PAS is that it enables steering characteristics to be adapted more readily to suit different consumer requirements. In particular, the gearing of the system can be varied almost at will. PAS can also be made to offer varying degrees of assistance, from a great deal, to a more moderate amount that retains a greater impression of ‘feel’ for enthusiastic drivers. This technique now extends to speed-sensitive assistance, in which the degree of assistance is high at low speed — for parking and general manoeuvring — but becomes less as speed increases, to decrease the risk of driver over-control.
Hydraulic Power Steering
Most systems use hydraulic fluid or a light oil, supplied under pressure by an engine-driven pump from a separate hydraulic tank. If the system fails, the car can still be steered manually. When the steering is ‘at rest’, that is, not being moved in either direction, the fluid passes through two equal-sized ports, giving equal pressure to both sides of a piston in a ram connected to the linkage.
Movement of the steering wheel first takes up a small amount of slack, which is used to open one port and close the other.
The fluid then applies pressure to one side of the piston, which moves the steering linkage in the required direction. The amount of fluid pressure applied to the piston depends on the force applied to the steering wheel by the driver. The main components of the system are the pump which supplies the fluid, driven either by an extension on the generator or by a V-belt, sensing valves operated by movement of the steering wheel or by deflection of the road wheels, the ram and piston assembly, and connecting pipes.
Electric Power Steering System
Modern cars are now also available with electric power assistance that works through an electric motor or an electro-hydraulic system. In the second case, the system retains hydraulic assistance but pressure is provided via an electric pump rather than an engine-driven pump.
The alternative is to do away with hydraulics altogether and adopt a system in which the power assistance (or even fully-powered operation) is provided by an electric motor. In either case, the system is generally known as EPAS — Electric Power-Assisted Steering. Both variations to EPAS consume power only when steering assistance is needed, and their installation is easier because there is no need to align a mechanically driven pump with a belt drive from the crankshaft.
The pure electric system, found in the Maruti range of vehicles such as the Zen and Wagon R, consists essentially of an electric motor geared to the steering linkage, a control module and suitable sensors, the most important being a steering torque sensor.
The electro-hydraulic approach still requires a reservoir, the electrically-driven pump, an hydraulic output jack or motor and associated piping, although given flexibility of installation, these can be engineered into an extremely compact module.