Power steering is a system for reducing the steering effort on cars by using an external power source to assist in turning the wheels. Power steering was invented in the 1920s by Francis W. Davis and George Jessup in Waltham, Massachusetts. Chrysler Corporation introduced the first commercially available power steering system on the 1951 Chrysler Imperial under the name Hydraguide.MGR
Most power steering systems work by using a belt driven pump to provide hydraulic pressure to the system. This hydraulic pressure is generated by a rotary-vane pump which is driven by the vehicle's engine. As the speed of the engine increases, the pressure in the hydraulic fluid also increases, hence a relief valve is incorporated into the system to allow excess pressure to be bled away.
While the power steering is not being used, i.e. driving in a straight line, twin hydraulic lines provide equal pressure to both sides of the steering wheel gear. When torque is applied to the steering wheel, the hydraulic lines provide unequal pressures and hence assist in turning the wheels in the intended direction.
Some more modern implementations of hydraulic systems also include an electronic pressure valve which can reduce the hydraulic pressure of the power steering lines as the vehicle's speed increases (Variable assist power steering)
Electric Power Steering, such as those found on the Acura NSX, and on most FIAT, Lancia, Alfa Romeo cars, uses completely electric components. Sensors detect the motion and torque of the steering column and a computer applies assistive power via electric motors. This allows varying amounts of assistance to be applied depending on driving conditions. Most notably on FIAT group cars the amount of assistance can be regulated using a button named "CITY", while on Volkswagen/Audi group cars, the amount of assistance is automatically regulated depending on vehicle speed.
In contrast to hydraulic systems, with an electric system the "communication" is only one way. The steering wheel transmits forces to the front wheels assisted by the motors, but the forces on the front wheels are not transmitted back through the motors to the steering wheel. Electric-only systems thus do not provide as much feedback to the driver, a criticism commonly expressed as a lack of steering "feel".
So called "hybrid" systems use the same hydraulic assist technology as standard systems, with the hydraulic pressure being provided by an electric motor instead of a belt driven one. Those systems can be found in Volkswagen, Audi, SEAT and Skoda cars.