In a hydraulically assisted rack-and-pinion steering set-up, an engine-driven hydraulic pump provides the required pressure to supply the necessary assistance. The driver turns the steering wheel, changing the position of a valve which lets more pressurised fluid flow into the appropriate side of the steering rack's hydraulic cylinder. This pushes the rack in the desired direction, applying more force to the wheels and causing them to turn.
An electric power steering set-up, however, does away with the engine-driven hydraulic pump. Instead, in most configurations, a powerful electric motor is connected to the steering gear to provide the required assistance. Several EPS set-ups exist, however, each of which have specific advantages that make it ideal for certain installations.
Different types of electric power steering systems
Column-assist EPS: These set-ups have the assistance hardware inside of the cabin, with the motor acting on the steering column. This is ideal for installations that have tight engine bays and also means that the steering motor and ECU do not need to be protected against the weather, reducing cost and complexity.
Rack-assist EPS: The assistance motor is integrated into - or mounted to - the steering rack itself. A larger and more powerful motor can be used, improving steering performance, which is why this configuration tends to be used in bigger cars.
Pinion-assist EPS: This features a lightweight motor, integrated into the input shaft section of the rack, which drives the steering rack's pinion gear. This is a set-up is compact, making it suitable for vehicles with challenging packaging requirements.
Dual-pinion EPS: An electric motor drives a separate pinion gear on the rack, providing assistance, while the column connects to the rack's conventional pinion. Like other configurations, this can be easier to fit in a car - but it also reputedly offers good steering feel and improved rigidity.
Hydraulic EPS: Also known as electro-hydraulic power steering. In this configuration, a conventional hydraulically assisted rack is used but - instead of an engine-driven pump - hydraulic pressure is provided by an electric pump. This allows for more flexibility, in terms of packaging, while retaining steering feel and improving efficiency by stopping or slowing the pump as and when required.
The benefits of electric power steering
Improved efficiency is the primary advantage of using EPS, as energy is only consumed when steering assistance is required; in a hydraulic system, the engine-driven pump runs continually which results in increased fuel consumption. To put things into perspective, Honda claims a 3-5 per cent improvement in efficiency when using EPS.
The ability to operate without an engine is also essential for applications such as hybrid or electric cars, while electronic hardware allows for the steering to be automatically operated by emergency or driver assistance systems.
Similarly, if an engine stalls then assistance will continue - making the system slightly safer - while inbuilt sensors can automatically report on any faults. EPS systems require far fewer components, too. There's no need for heavy hydraulic pumps, fluid lines, belt and pulley assemblies or reservoirs, which cuts down on assembly complexity, maintenance requirements and so on.
Using EPS also grants engineers more freedom as the systems are typically compact and easily packaged. This can make them ideal for cars where space is a priority or where specific design criteria make it more difficult to use a conventional power-assisted set-up.
The disadvantages of electric power steering
EPS is often criticised for a lack of steering feel, compared to a conventional hydraulic set-up. A suitably large and powerful steering motor has a lot of inertia, which means that minor vibrations - say, a tyre picking up on a subtle crease in the road - are absorbed by the motor itself and never make it to the steering wheel. Lighter, more powerful motors can alleviate this issue.
There can also be problems with the way the system is calibrated, resulting in an odd steering feel or overly sharp responses. On the flip side, unexceptional performance is sometimes the result of improper suspension and chassis set-ups, which degrade the steering performance of the vehicle.
If a manufacturer puts in the time, and it's appropriate for the car in question, then there's no reason an EPS system shouldn't deliver what an enthusiast expects. For example, the Toyota GT86 - a car frequently praised for its steering - uses column-mounted EPS.
While EPS systems require fewer components and are mechanically more straightforward, they are electronically far more complicated and can require a substantial amount of power to operate in heavier cars; this consequence must be considered by the engineers and can increase cost if the electrical systems of the vehicle require upgrading to suit.
A brief history of electric power steering
The first production car to utilise an electric power steering set-up was the 1988 Suzuki Cervo; the column-assist system provided a way to fulfil the increasing customer demand for power-assisted steering without overly compromising the efficiency, weight and complexity of the vehicle.
Honda then moved the concept of electric power assistance into a very different domain, by fitting a co-axially assisted rack set-up to the NSX. Electric power assistance systems then began being widely adopted in the late 1990s, making their ways into cars such as the Mk2 Fiat Punto and - shortly after - the Toyota Prius, Honda S2000, BMW Z4 and Mazda RX-8.
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