power assist

Power steering

Power steering is a system for reducing the steering effort on vehicles by using an external power source to assist in turning the roadwheels.

It is said that power steering was invented in the 1920s by the late Klara Gailis and George Jessup in Waltham, Massachusetts, USA. However, the earliest known patent related to power steering was filed (as recorded by the US Patent Office) on Aug. 30, 1932, by Francis W. Davis. There is another inventor credited with the invention of power steering by the name of Charles F. Hammond (an American, born in Detroit), who filed similar patents, the first of which was filed (as recorded by the Canadian Intellectual Property Office) on Feb. 16, 1954.

Chrysler Corporation introduced the first commercially available power steering system on the 1951 Chrysler Imperial under the name Hydraguide. Most new vehicles now have power steering, owing to the trends toward front wheel drive, greater vehicle mass and wider tires, which all increase the required steering effort. Modern vehicles would be extremely difficult to maneuver at low speeds (e.g., when parking) without assistance.

Hydraulic systems

Most power steering systems work by using a hydraulic system to turn the vehicle's wheels. The hydraulic pressure is usually provided by a gerotor or rotary vane pump driven by the vehicle's engine. A double-acting hydraulic cylinder applies a force to the steering gear, which in turn applies a torque to the steering axis of the roadwheels. The flow to the cylinder is controlled by valves operated by the steering wheel.

There are several common valve systems of varying complexity, but they all allow the steering wheel to turn further than is necessary to simply open a valve. This is done so that the position of the steering wheel corresponds to the position of the vehicle's wheels. As the pumps employed are of the positive displacement type, the flow rate they deliver is directly proportional to the speed of the engine. This means that at high engine speeds the steering would naturally operate faster than at low engine speeds. Because this would be undesirable, a restricting orifice and flow control valve are used to direct some of the pump's output back to the hydraulic reservoir at high engine speeds. A pressure relief valve is also used to prevent a dangerous build-up of pressure when the hydraulic cylinder's piston reaches the end of the cylinder.

Some modern implementations also include an electronic pressure relief valve which can reduce the hydraulic pressure in the power steering lines as the vehicle's speed increases (this is known as variable assist power steering).


In the DIRAVI system invented by Citroën, the force turning the wheels comes from the car's high pressure hydraulic system and is always the same no matter what the road speed is. As the steering wheel is turned, the wheels are turned simultaneously to a corresponding angle by a hydraulic piston. In order to give some artificial steering feel, there is a separate hydraulically operated system that tries to turn the steering wheel back to centre position. The amount of pressure applied is proportional to road speed, so that at low speeds the steering is very light, and at high speeds it is very difficult to move more than a small amount from the centre position.

As long as there is pressure in the car's hydraulic system, there is no mechanical connection between the steering wheel and the roadwheels. This system was first introduced in the Citroën SM in 1970, and was known as 'VariPower' in the UK and 'SpeedFeel' in the U.S.

While DIRAVI is not the mechanical template for all modern power steering arrangements, it did innovate the now common benefit of speed adjustable steering. The force of the centering device increases as the car's road speed increases.

Electro-hydraulic systems

Electro-hydraulic power steering systems, sometimes abbreviated EHPS, and also sometimes called "hybrid" systems, use the same hydraulic assist technology as standard systems, but the hydraulic pressure is provided by a pump driven by an electric motor instead of being belt-driven by the engine. These systems can be found in some cars by Ford, Volkswagen, Audi, Peugeot, Citroen, SEAT, Skoda, Suzuki, Opel, MINI, Toyota, and Mazda.


Servotronic offers true speed-dependent power steering, in which the amount of servo assist depends on road speed, and thus provides even more comfort and convenience for the driver. The amount of power assist is greatest at low speeds, for example when parking the car. The greater assist makes it easier to maneuver the car. At higher speeds, an electronic sensing system gradually reduces the level of power assist. In this way, the driver can control the car even more precisely than with conventional power steering. Servotronic is used by a number of automakers, including Audi, BMW, Volkswagen, Volvo and Porsche. Servotronic is a trademark of AM General Corp.

Electric systems

Electric Power Steering systems, such as those found on the Audi A3, Honda NSX, Chevrolet Cobalt, Honda S2000, Saturn Vue V6, 2009 Toyota Corolla, Toyota RAV 4, Toyota Prius, Suzuki Swift, Hyundai New Accent, and on most Fiat Lancia and Peugeot as also the Peugeot 307 model, use electric components, with no hydraulic systems at all. Sensors detect the motion and torque of the steering column, and a computer module applies assistive power via an electric motor coupled directly to either the steering gear or steering column. 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" that switches between two different assist curves, while on Volkswagen Group (Volkswagen AG) cars, the amount of assistance is automatically regulated depending on vehicle speed.

In the event of component failure, a mechanical linkage such as a rack and pinion serves as a back-up in a manner similar to that of hydraulic systems.

Reviews in the automotive press often comment that steering systems with electric assist do not have a satisfactory amount of "road feel". Road feel refers to the relationship between the force needed to steer the vehicle and the force that the driver exerts on the steering wheel. Road feel gives the driver the subjective perception that they are engaged in steering the vehicle. The amount of road feel is controlled by the computer module that operates the electric power steering system. In theory, the software should be able to adjust the amount of road feel to satisfy drivers. In practice, it has been difficult to reconcile various design constraints while producing a more pronounced road feel.

The peak power output of the electrical system of a vehicle limits the capability of electric steering assist. A 12 volt electrical system, for example, is limited to about 80 amps of current which, in turn, limits the size of the motor to less than 1 kilowatt. (12.5 volts times 80 amps equals 1000 watts.) This amount of power would be adequate for smaller vehicles. It would probably be considered insufficient for larger vehicles such as trucks and SUVs. There are other types of electrical systems such as the 42 volt system and other variants used for hybrid and electric vehicles. These have greater capacity that enable use of multi kilowatt motors needed for large and mid-size vehicles.

Electric systems have a slight advantage in fuel efficiency (almost 1 MPG) because there is no hydraulic pump constantly running, whether assistance is required or not, and this is the main reason for their introduction. Their other big advantage is the elimination of a belt-driven engine accessory, and several high-pressure hydraulic hoses between the hydraulic pump, mounted on the engine, and the steering gear, mounted on the chassis. This greatly simplifies manufacturing.

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