Electric Power Steering

Electric power steering, or EPS, is a steering-assistance system that uses an electric motor rather than an engine-driven hydraulic pump to reduce the effort a driver needs to turn the wheel. It has become the standard arrangement on modern cars, displacing the hydraulic systems that dominated for decades. The change has been driven by demands for better fuel economy, simpler packaging and, above all, the ability to integrate steering into the electronic driver-assistance and automated-driving features that customers increasingly expect.

In an EPS system, sensors measure the torque the driver applies to the steering wheel and the steering angle, feeding this information to an electronic control unit. The unit calculates how much help is required and commands an electric motor to provide it. The motor's assistance is applied either to the steering column, in column-assist systems common on smaller cars, or directly to the steering rack via a pinion or a separate gear, in the more powerful rack-assist designs used on heavier and more performance-oriented vehicles. The familiar rack-and-pinion mechanism still translates that effort into the side-to-side movement that turns the wheels.

The efficiency advantage is significant. A hydraulic pump is belt-driven and therefore draws power from the engine continuously, even when the car is travelling dead ahead and no assistance is needed. An electric motor, by contrast, consumes energy only at the instants the driver is actually turning, which trims fuel consumption by a small but worthwhile margin and reduces parasitic load on the engine. It also removes the hydraulic circuit entirely, so there is no pump, no high-pressure hoses and no fluid to leak, top up or replace, which lowers maintenance and eliminates a category of failure and contamination.

Because assistance is governed by software, EPS can be tuned far more flexibly than a hydraulic system. The level of assistance can be varied with road speed, light and easy for parking, firmer and more reassuring at motorway pace, and many cars offer selectable steering modes. More importantly, the same electric motor that assists the driver can also apply steering inputs on its own command, which is what makes modern driver-assistance functions possible: lane-keeping assist that nudges the car back between the lines, lane centring, and automated parking systems that steer the car into a space while the driver controls the pedals or simply supervises.

The technology is not without trade-offs and nuances. Early EPS systems were sometimes criticised for a slightly artificial, numb feel, because the rich feedback of fluid under pressure had to be synthesised electronically, though calibration has improved markedly. EPS also depends on the car's electrical supply and electronics, so a fault can disable assistance, leaving the steering heavy but still mechanically connected and controllable. As a development of conventional power steering and an enabler of features such as all-wheel steering, EPS has become a foundational building block of the contemporary car's chassis and its move towards automation.