This week’s technical article is about power steering. The concept is simple: help the driver steer the car with the minimal of fuss. The implementation is a little trickier. The key to good power steering is the weight of the steering – too soft and you risk having very light steering and possibly a lack of feel – too heavy and it defeats the purpose of having the powered help.
There is also a difference in weight between different regions. Many US cars feel much lighter than their European or Asian counterparts and this is probably due to a mix of law and market preferences.
The first commercially available power steering was fitted to the sixth generation Chrysler Imperial in 1951. That year Chrysler introduced two models: the Imperial and the Custom Imperial. Power steering was fitted as standard equipment on the Custom but was a costed option on the base model. Like many features from the 1950s, it had a great name: Hydraguide, to go with GM’s Hydramatic gearboxes found on Cadillacs and Oldsmobiles and Harley Davidson’s Hydra Glide from the same period!
Power steering works by using a hydraulic ram (or linear hydraulic motor) to turn the steering gear. The hydraulic pump that operates the ram is connected to the engine and as such receives higher power when the engine is revving higher. A flow control valve is used to smooth out the delivery of power to the pump and a pressure relief valve is used to ensure that there is no build up of pressure that could blow any linkages. The pump uses a rotary vane design that increases the size of the vanes as the speed increases. With the increased vane size comes the ability to pump more hydraulic fluid.
In addition to this, there is a rotary valve with a torsion bar to separate the pump from the steering. The rotary valve is used to sense when the steering wheel is turned and then enables the rest of the power steering unit to function. It is pointless having power assist if it is not needed!
Some cars have variable power steering and this is the most common system with an electronic relief valve that regulates and reduces the pressure as the cars speed (and therefore the engine speed) increases. This seems a little odd – you apply a pump to the engine to use some of its power to run the hydraulic system but then as the speed increases, you waste that power because it becomes too much. At the same time you are taking power away from the driving wheels to run the pump, so the power is not directed efficiently. Surely there must be a better way to use the engine’s power such that the driving wheels get the maximum amount and the pump only uses what it needs.
Indeed there is, and some manufacturers realised this in the late 1960s. Citroen introduced a new more efficient hydraulic system on its luxury SM model in the early 1970s. This maintained a constant power output to the hydraulic system and actually used two of them: one to turn the front wheels and another to turn the steering wheel, so in effect, the driver was not mechanically connected to the steering! The system used a powered castor angle – Citroen had bought Panhard and it was Major Krebs of Panhard who invented the castor angle in the steering system 80 years earlier.
Citroen’s system was connected to their hydro-pneumatic suspension and braking systems to achieve a safer overall system – safer that is, until the whole hydraulic system fails and then you have no steering, suspension or braking! The system known as DIRAVI (Direction à rappel asservi or in English: “steering with a controlled return”) was also fitted to some Maserati models as well, thanks to Citroen’s ownership of the Italian marque at that time.
Some manufacturers removed the hydraulic pump completely from the engine, thus directing more power to the driven wheels. The steering was powered by an electric pump and were known as electro-hydraulic systems – not to be confused with the pure electric steering systems that some European manufacturers also used.
The electric version uses sensors and other electronics to figure out the speed and steering input. This is then converted to an input for an electric motor that is either connected to the steering column or works directly with the steering gears.
We have all got used to using power steering, as it has become standard equipment on nearly all new cars these days and this function is one that will never be removed although better systems will need to be designed as cars move towards electric and other power sources – we’ll be back to the same issues as there were in the 1960s where the power steering function takes too much from the electric or bio-fuel motor and will need to be redesigned, but that is the great thing about automotive technology, it’s constantly moving and getting better!