How Electric Power Assisted Steering (EPAS) Works, And Why It's Better Than Hydraulic

In simpler times before any form of driving assistance, steering was as analogue as it gets, using a rack and pinion system to direct a car in the desired direction. Next came hydraulically-assisted steering, which dominated the automotive world from 1951 when pioneered by Chrysler.

This form of system uses a hydraulic pump which is powered from a belt attached to the engine. A power ram is moved by the hydraulic fluid which is pressurised through the movement of the belt. A control valve then dictates how much hydraulic pressure is needed to move the wheels in either direction depending on the steering input. The hydraulics amplify the amount of load being applied to the steering rack, thus reducing the levels of effort needed to change direction.

Although hydraulic systems are still very much used today and have been all but perfected, they do have their downsides. As the pump is technically driven by the engine, hydraulic assistance is deemed as a parasitic loss. This means that a small amount of power is sapped from the engine to run the pump, which reduces the overall efficiency of the powertrain. Performance cars these days also need to produce a set of modes for the driver to pick from and most of them include a steering adjustment. This is not convenient for hydraulics as the hydraulic fluid being pumped through the system will have a set viscosity (how easily a fluid flows) therefore some alternate form of restriction has to be used.

Engineers in the last decade then decided to replace the old-school hydraulics with electric motors, which isn’t surprising considering the general shift towards entirely electrically-powered cars. The motors are usually placed either at the base of the steering column or directly on the steering rack and have become a fairly simple solution to advancing power steering into the 21st Century. Electronic sensors pick up the amount of steering lock being applied and add in a proportional amount of additional force to the steering input. Electrical charge is used to rotate the motor and through energy transfer, a lateral force is produced that aids the movement along the steering rack.

The main argument against eletronics is steering feel. As hydraulics are tactile due to the presence of a viscous fluid, they are loved by purists due to the amount of feedback that can be transferred through the steering rack and back to the steering wheel. So, when electric steering systems first came into play, many road testers complained about a lack of feedback. Due to electricity effectively being a non-tactile commodity, it is a fair assumption to make that very little reaction force will make its way back through an electric motor.

As EPAS (electric power assisted steering) systems have been developed and refined however, manufacturers like Porsche have managed to create electronic systems that all but match the feel of a hydraulic system and then go on to surpass the mechanical method in many fields. It has done this by changing the direction of the feedback loop within the electronics; most manufacturers employ a system that inputs a steering assistance force calculated from a torque sensor from the wheels, while Porsche uses yaw sensors, steering angle and other values from the stability control to increase and decrease assistance accordingly and at a much more frequent rate. This means that the sense of ‘feel’ is reintroduced to the EPAS system, and apart from those who drive completely unassisted cars, no real differences can be found between these EPAS systems and a HPAS system.

Other advantages of EPAS come in the shape of efficiency, convenience and packaging. Chevrolet has seen a 2.5 per cent increase in fuel economy since switching to electronic steering due to the lack of parasitic wastage from the engine. Radar-guided parking also works in conjunction with an EPAS system and with the massive surge towards automation, electronic steering is definitely here to stay.

The Porsche 911 is a great case study for the packaging advantages of an EPAS system. Before the switch to electronic, a vast route of hydraulic piping had to be engineered into the car’s design to travel from the rear-mounted engine to the front wheels. On the 991 generation of cars, Porsche went with a full EPAS system so that the motors could be positioned at the front of the car with no lengthy piping required, in turn aiding the 911’s weight distribution - an important consideration for a rear-engined car.

In terms of race modes and sports buttons within performance cars of the last decade, EPAS systems allow adjustments to be made in the weight and speed of the steering input simply by changing the amount of charge created by the electronic motor, which in turn changes the amount of steering assistance. In an HPAS system, these changes may have to be implemented through physical changes within the steering system like a change of rack or fluid pressure to affect the amount of steering input required to manoeuvre the car.

It’s now getting difficult to tell the difference between a modern EPAS system and a refined HPAS system, and the benefits of electronics far outway those of the more traditional hydraulic setups. Although some companies produce hybrid systems that incorporate an electric motor to power a hydraulic ram system, the vast majority of manufacturers are now leaning towards fully electric assisted steering. Don’t expect to have any hydraulic pump issues on your future servicing bills…