What is power steering?
It is a hydraulic system which assists the driver in
reducing the steering effort on vehicles.
It utilizes hydraulic pressure in turning the front
wheels of the automobile(car).
The system usually consists of a hydraulic pump and
fluid reservoir, a power-actuating mechanism such as a
power cylinder, a control valve arrangement, and a series
of flexible hydraulic hoses and couplings to route the
hydraulic fluid under pressure.
Types of power steering systems:
The five main types of power steering
system in cars are:
Rack and pinion
Linkage or booster
In integral system, the power cylinder and the control valve
functions are combined into one unit located in the steering
gearbox at the steering column’s end.
Here, a pressure flow director valve senses steering wheel
movement and directs the fluid pressure to either side of a rack
piston, which is directly geared to the pitman arm shaft.
As the rack piston travels up or down under fluid pressure,
the pitman shaft rotates, applying the needed boost to turn the
The spent fluid is then re-circulated back to the pump
The pressure line must withstand pressures ranging from
800 to 1300 psi. The return line normally kept less than 50 psi.
Rack and pinion system
In rack and pinion type, the gear on the steering column’s
end is similar to the pinion gear in the differential: cut on an
angle, and meshed with a steel bar (the rack) toothed on one
The rack is mounted parallel to the front axle and as the
steering wheel turns, it operates directly on the tie rods without
the use of a pitman arm, idler or intermediate (or relay) rod.
Adding a power assist to this type of steering is quite
simple. The power piston is actually part of the rack, and the
rack housing acts as the cylinder.
The control valve is located in the pinion housing.
Rotation of the steering shaft and pinion turns the valve to
direct hydraulic pressure to either end of the rack piston.
The Linkage (booster) system
In the linkage system, the pump supplies hydraulic pressure
to a control valve assembly attached to the pitman arm and
intermediate rod. This control valve senses, by mechanical
means, the steering wheel movement in either direction.
The valve then directs the proper amount of hydraulic
pressure to the power cylinder attached to the car’s frame on
one end and to the intermediate rod on the other.
As fluid pressure is applied to either end of the power
cylinder, it pushes or pulls the intermediate rod to affect the
The fluid is then re-directed back to the pump reservoir via
the return hose line.
The pressure and cylinder lines must withstand pressure
ranging from 800 to 1,100 psi. The return line normally carries
up to 40 psi.
The torque converter in an automatic transmission serves
the same purpose as the clutch in a manual transmission.
A torque converter is a fluid coupling that is used to transfer
rotating power from a prime mover, such as an IC engine or
electric motor, to a rotating driven load.
As a more advanced form of fluid coupling, however, a
torque converter is able to multiply torque when there is a
substantial difference between input and output rotational
speed, thus providing the equivalent of a reduction gear.
Parts of a torque converter:
There are four components inside the very
strong housing of the torque converter:
The housing of the torque converter is bolted to the
flywheel of the engine, so it turns at whatever speed the engine
is running at.
The fins that make up the pump of the torque converter are
attached to the housing, so they also turn at the same speed as
The pump inside a torque converter is a type of centrifugal
pump. As it spins, fluid is flung to the outside, much as the spin
cycle of a washing machine flings water and clothes to the
outside of the wash tub.
As fluid is flung to the outside, a vacuum is created that
draws more fluid in at the center.
The fluid then enters the blades of the turbine, which is
connected to the transmission. The turbine causes the
transmission to spin, which basically moves your car. The blades
of the turbine are curved. This means that the fluid, which
enters the turbine from the outside, has to change direction
before it exits the center of the turbine. It is this directional
change that causes the turbine to spin. So as the turbine causes
the fluid to change direction, the fluid causes the turbine to spin.
The fluid exits the turbine at the center, moving in a
different direction than when it entered. It is seen that the fluid
exits the turbine moving opposite the direction that the pump
(and engine) are turning. If the fluid were allowed to hit the
pump, it would slow the engine down, wasting power. This is
why a torque converter has a stator.
The stator resides in the very center of the torque
converter. Its job is to redirect the fluid returning from the
turbine before it hits the pump again. This dramatically increases
the efficiency of the torque converter.
The stator has a very aggressive blade design that almost
completely reverses the direction of the fluid. A one-way clutch
(inside the stator) connects the stator to a fixed shaft in the
transmission (the direction that the clutch allows the stator to
Because of this arrangement, the stator cannot spin with
the fluid -- it can spin only in the opposite direction, forcing the
fluid to change direction as it hits the stator blades.
Pump powered by engine fills and pressurizes
the accumulator-150 to 180 bars
Powers the front brakes first
Next the bottom of the spheres are filled with oil
Suspension works by means of compacting the
nitrogen in the spheres by means of the rods that
push the LHM into the spheres
• Ride height controlled by means of height
• Spheres may need recharging after 60000 plus
• Hydropneumatic is naturally a progressive
• Better control then suspension with steel
• less endogenous friction than steel
• Inexpensive in mass production
• Service sometimes requires a specifically trained
• Hydropneumatic suspension systems can be expensive
to repair or replace, if poorly maintained or
contaminated with incompatible fluids.
• Failure of the hydraulic system will cause a drop in ride
height and braking power will decrease. However, an
acute failure will not lead to acute brake failure as the
accumulator sphere holds enough reserve pressure to
ensure safe braking far beyond that needed to bring a
vehicle with a failed system to a standstill.
Hydrostatic Regenerative Braking
• Energy from braking is used to power the vehicle.
• Kinetic energy is converted to potential energy
• Hydraulic hybrid producers:
Design types: Series and Parallel
Bosch Rexroth’s design
• Parallel system
• A hydraulic system serves as an auxiliary
drivetrain and supplements a traditional one.
This design typically uses the hydraulic drive
when frequent stops and starts occur.
• Basic parts are: axial piston unit with gearbox,
accumulator, block control valve, ECU,PRV.
Storing braking energy
The hydraulic axial piston unit is coupled to the
drive train through a gearbox.
When braking ,the axial piston unit uses the
kinetic energy to charge the accumulator
Conventional brakes kick in after the
accumulator is filled.
Releasing the stored energy
• Pressurized fluid in the accumulator now
drives the axial piston unit that behaves like a
• The motor in turn drives the mechanical drive
• Hrb unit is engaged until the accumulator
• Valve control block controls the filling of the
Parker’s series system
• Runwise advanced series hybrid drive system
• Series design
• The system is comprised of an engine, primary
pump, secondary drive pump and/or motors,
accumulators, and Parker’s Power Drive Unit
(PDU). A two-speed hydrostatic drive (low speed
for 0-25 mph driving and high speed at 26-45
mph) for urban driving is combined with a
mechanical direct drive for efficient operation at
high speeds (45+ mph)
• Braking mechanism which uses brake fluid to
transfer pressure from the controlling unit
• Fluid used is mostly ethylene glycol
• Brake pedal
• Push rod (actuator rod).
• A master cylinder assembly containing
a piston assembly (made up of either one or
two pistons, a return spring, a series
of gaskets/ O rings and a fluid reservoir)
• Reinforced hydraulic lines
• Brake caliper assembly
Brake pedal is pressed, a pushrod exerts force
on the piston
causing fluid from the brake fluid reservoir to
flow into a pressure chamber through a
This forces fluid through the hydraulic lines
toward one or more calipers