How Power Steering Works: Hydraulic, Electric and Beyond
Power steering adds force to the driver's effort. See how the hydraulic pump and valve work, what electric systems replace, and when each arrived.
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Power steering adds force to what the driver applies to the steering wheel, so a heavy car can be turned with light effort. The first widely used type pushed pressurised fluid against a piston. Most newer cars use an electric motor instead. Here is how each system works and when they arrived.
The mechanical core
In most passenger cars the steering wheel turns a small gear, the pinion, that meshes with a toothed bar called the rack. Wikipedia describes the idea simply: turning the pinion moves the rack in a straight line. The rack's ends connect to the front wheels. Power steering is added on top of this mechanism and is not a separate steering system.
Hydraulic power steering
1A Auto's guide describes the hydraulic system in four parts. A reservoir holds the fluid. A pump driven by the engine's belt moves the fluid, and release valves manage the high pressure at high engine speed. A torsion bar in the steering shaft twists when you turn the wheel, and that twist rotates a spool inside a rotary valve. The spool lines up ports so that fluid flows to one side of a piston in the rack, which helps push the rack along.
Wikipedia adds that the flow of fluid grows with engine speed, so a flow-control valve returns the excess to the reservoir, and a relief valve limits the pressure. If the assist fails, the car can still be steered manually, although the wheel feels heavier.
Electro-hydraulic and electric systems
- Electro-hydraulic: the same hydraulic assist, but an electric motor drives the pump instead of the engine belt, so assist can work with the engine off
- Electric (EPS): an electric motor and a control unit replace the hydraulics; sensors measure the driver's torque on the steering column, and the motor applies the assist at the column or at the steering gear
Wikipedia says electric systems allow the amount of assist to vary with driving conditions and can be linked to features such as lane assist. Compared with hydraulics, they use fewer parts and no fluid. They also avoid a pump running all the time, which the page says harms fuel economy in hydraulic designs. The listed drawback is that when an electric system fails, the steering becomes heavy.
A short history
- 1903: a Columbia 5-ton truck used a separate electric motor to help turn the front wheels
- 1926: Francis W. Davis of Pierce-Arrow demonstrated what Wikipedia calls the first practical power steering system; General Motors judged it too expensive to produce
- 1951: Chrysler's Hydraguide appeared on the Imperial, and Cadillac followed in 1952
- 1970: the Citroën SM introduced assist that varied with speed
- 1988: the Suzuki Cervo became the first mass-produced passenger car with electric power steering, according to Wikipedia
- 1990: the Honda NSX offered electric rack assist
Wikipedia says military demand for easier steering on heavy vehicles during the Second World War pushed the technology forward. It also mentions an early claim that a man named Fitts fitted a system in 1876, but little else is known about it.
Steer-by-wire
A further step removes the mechanical link between the wheel and the road wheels altogether, leaving sensors, a controller and motors. Wikipedia cites the Lexus RZ 450e of 2023 as an example of a car with such a system.
Support parts in hydraulic cars
1A Auto notes that hydraulic systems may include an oil cooler to stop the fluid overheating, and that pressure sensors can tell the engine computer to raise idle speed during low-speed parking so the engine does not stall under the pump's load. In some heavy trucks and SUVs the assist acts on a recirculating-ball steering gear rather than a rack, though the rack is more common.
Summary
Hydraulic assist uses a belt-driven pump and a valve controlled by a twisting torsion bar. Electric assist replaces them with a motor, sensors and software. Both build on the same rack-and-pinion core, and the shift from one to the other is mainly about efficiency, simplicity and the ability to tune the feel.
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