What exactly is power steering? Why is it that there is nowhere in my car's engine compartment for the top-up of power-steering fluid? In my old car, there was a filler cap for steering fluid and I recall having to top up when it began to leak. The term is actually a shortened version of "power-assisted steering". It is a supplementary sub-system which reduces the driver's effort at the steering wheel.
Hence the vehicle's directional control is still a mechanically linked set-up from steering wheel to front wheels. Only when the driver moves the steering does the assistance intervene.
Until about 30 years ago, all the steering power-assistance systems were of the hydraulic type. These used a hydraulic pump driven by the V-belt at the front of the engine - similar to how an air-conditioning compressor is driven. There were some later versions which employed electrically driven pumps which were more efficient.
Either way, the hydraulic power-assisted steering was a rather elaborate assembly requiring, in addition to the pump, an oil reservoir, hydraulic hoses and control valves. It nonetheless worked well.
With age, the oil seals in the system would deteriorate, allowing fluid to leak. It meant you had to top up the hydraulic fluid until repairs were done.
Current steering system designs no longer rely on hydraulics. Instead, power assistance is provided solely by electric motors and control electronics.
An electric motor provides the assistance as a torque boost at the steering column, or as a linear actuator at the steering rack, and functions only when motion is detected at the steering wheel.
Electric power-assistance systems require less space, need no hydraulic fluid, consume less energy and do not suffer from leaks.