ABSTRACTA hydraulic assisted power steering system on a vehicle has asteering pump which is directly driven from the enginecontinuously. In real world, the assistance from the steeringpump is useful only while manoeuvring. During a typicalhighway drive, assistance from this power steering pumpremains unused for majority (76%) of the time; although thecontinuously rotating power steering pump keeps consumingenergy from the engine. An electronic controller has beenprovided for the electromagnetic pairing device of the powersteering pump in order to provide assistance for steeringbased on driver demand only. The electromagnetic pairingdevice integrated on the steering pump can be made toengage / disengage based on the driver demand through theelectronic controller. The benefit with such a system includesreduced energy consumption from the engine (lower CO2emission) & consequent improvement in fuel economy. (4 to6% improvement in highway fuel efficiency) coupled with ondemand power assistance to steering

INTRODUCTIONConventional hydraulic power assisted steering system[HPAS] of a passenger car has a continuously driven pumpby an IC engine through a belt and pulley arrangement.Power assistance to steer the road wheels is typically requiredin parking manoeuvers and slow speed operation due to highefforts required to steer the road wheels. In dynamicconditions there is relatively lesser requirement of powerassistance to steering for example while moving on straightroads or on highways. In conventional hydraulic powerassisted steering system, in all of such situations powersteering pump is continuously driven that keeps oncirculating the oil through system & thus keeps on consumingenergy from IC engine. A study was conducted to determine

actual steering demand pattern & actual energy consumed bypower steering pump while driving on highway and in city.Based on this study a system was designed which will enablesaving of energy whenever possible by having a mechanismto disconnect the drive to the power steering pump when notrequired.

1.0. DUTY CYCLE STUDYFor assessing steering demand pattern, car was instrumentedto acquire

Steering wheel angle,

steering wheel torque,

discharge pressure from the steering pump

vehicle speed

engine rpm & power steering pump rpmThe data collection was done while driving on highways & incity.

1.1. Actual Duty CyclesDuring these trials on public roads, it was observed thatsteering wheel movement is not above 10 in eitherdirection from straight ahead condition on typical highwaysfor 90% of the time & in city for 58% of time. Chart 1 belowgraphically represents the findings.

Energy Efficient Hydraulic Power Assisted SteeringSystem (E2HPAS)

2012-01-0976Published

04/16/2012

Chirag Padmakar Sonchal, Jaypal gajankush, Anand Vijay Kulkarni and Sandeep pawarTata Motors, Ltd.

Copyright 2012 SAE International

doi:10.4271/2012-01-0976

http://dx.doi.org/10.4271/2012-01-0976

Chart 1. % distribution against time of steering wheelangle on highway & in city driving condition.

1.2. Power Consumption Calculations ofHPAS pumpChart 3 & 4 below represent the generic characteristics of thepower steering pump for a hatchback. Using pump pulley tocrank shaft pulley ratio, pump rpm was calculated fromengine rpm data log. Refer following Chart 5 (a, b, c) & 6 (a,b, c) for actual pump rpm, flow & pressure characteristics inthe operational band of 25 from straight ahead condition onhighway & in city driving conditions.

Chart 3. specific Discharge Vs pump rpm.

Hydraulic power developed by steering pump is given by:

Chart 4. typical pump power consumption curve.

Chart 5 a. Steering Pump rpm data in highway drivingcondition for 25 to +25 Degrees Of steering wheel

rotation.

Chart 5 b. Steering Pump flow data in highway drivingcondition for 25 to +25 Degrees Of steering wheel

rotation.

Findings of data collection on highways Usually the power steering pump rpm is in 2500 to 3500band refer chart 5a

power steering pump discharge is in the band of 15 to 25LPM (litres per minute) refer chart 5 b

pump output pressure remains in the band of 1.7 bar to 5 barrefer chart 5 c

Steering pump hydraulic power output most of the timeremains in the band of 50 W to 100 W & on an averagepower output is 88.3W refer chart 5 d

Chart 5 c. Steering Pump output pressure data inHighway driving condition for 25 to +25 of steering

wheel rotation.

Chart 5d. Steering pump hydraulic power output data inhighway driving condition for 25 to +25 of steering

wheel rotation.

Findings of data collection in city Usually the power steering pump rpm is in 1200 to 2500band refer chart 6a

power steering pump discharge is in the band of 8 to 20LPM (litres per minute) refer chart 6b &

pump output pressure remains in the band of 3 bar to 6 barrefer chart 6c

Steering pump hydraulic power output most of the timeremains in the band of 50 W to 100 W & on an averagepower output is 84W refer chart 6d

Chart 6a. Steering Pump rpm data in City drivingcondition for 25 to +25 Degrees Of steering wheel

rotation.

Chart 6b. Steering Pump flow data in City drivingcondition for 25 to +25 Degrees Of steering wheel

rotation.

Chart 6c. Steering Pump output pressure data in Citydriving condition for 25 to +25 of steering wheel

rotation.

Chart 6 d. Steering pump hydraulic power output data incity driving condition for 25 to +25 of steering wheel

rotation.

From all the above real time data acquisition it is observedthat steering pump on an average consumes 84 to 88 Watts ofhydraulic power in idle operation zones. The equivalent Shaft

input power consumed from engine, comes out to be 95Watts (assuming 90%) of pump efficiency.

2. CALCULATION OF PROJECTEDFUEL SAVINGS POSSIBLEBasic power flow diagram for an IC engine to its auxiliarydrive is used for calculating the net thermal input powerrequired at the engine. Following assumptions are made inorder to consider worst case criteria.

1). The efficiency of hydraulic pump is 100%

2). There is no power loss in auxiliary belt drive train.

3). Thermal efficiency of an IC engine is 30%Based on the power flow diagram, equivalent thermal powerin terms of heat for 90 watts of shaft power is calculated asfollows:

Based on Higher calorific value of fuel, mass flow rate of fuelrequired to produce this thermal power is calculated asfollows:

The above values when multiplied by the time duration (sec)for which steering pump remains OFF will give the mass ofthe fuel saved. Thus,

Equivalent volume of fuel (litres) can be further calculatedusing density of the respective fuel This is the projectedamount of fuel that can be saved with this amount of power.

3.0. HOW THIS ENERGY CAN BESAVED?3.1. A device for selectively coupling / decoupling the drive to HPAS pumpIn existing drive train, energy is wasted due to continuousdrive given to the HPAS pump even in the situations whichdo not demand power assistance to steering at all. It ispossible to save this energy by introducing a mechanism bywhich drive to the steering pump can be disconnected. Forthis purpose, an electromagnetic clutch was accommodated inbelt drive to HPAS pump. Exploded view of the HPAS pumpdriven through electromagnetic clutch assembly is illustratedin figure 1 below.

After integrating this clutch in the present auxiliary drive,pump shaft (1) gets connected with clutch plate (6) and pulley(5) rotates on a stationary adaptor (3). After activatingelectromagnetic coil (4), due to magnetic field, clutch plate(6) gets attracted towards pulley face & drive is transferredfrom rotating pulley to pump shaft. Once the coil gets deenergized, the clutch plate gets back to its original positiondue to retracting springs and drive to the pump getsdisconnected. Once the pump shaft gets disconnected, onlypulley rotates on the adaptor & the power is not wasted inrotating HPAS pump & churning the oil.

3.2. Finding out maximum possible benefitwith this conceptA constant speed fuel consumption test was performed atvarious speeds & gear combination with & without assistanceto power steering pump. From the following chart illustratingthe test results it can be seen that upto 8% fuel savings ispossible if assistance to power steering pump is disabled.

4.0. DEVELOPMENT OFMETHODOLOGY FOR OPTIMIZINGPAS PUMP DUTY CYCLE4.1. Selection of a controllerThe next challenge was to automate the process of sensingthe demand for assistance to steering & to communicate toelectromagnetic clutch to activate/deactivate the drive to thePAS pump. Based on the type of application an 8 bitmicrocontroller from PIC family PIC 16F72 was chosen. Thischip can be programmed to function as per programmedlogic. Suitable hardware & circuit was built so that multipleinputs can be interfaced.

4.2. Determination of Steering Angle valueA suite of evasive manoeuvers involving steering wereconducted acquiring necessary parameters. It was observedthat at the speeds 80 kmph and above the whole manoeurvreof a lane change is completed within steering hand wheelangle rotation of 7 in either direction. This meant thatcontroller should turn ON assistance to steering beforesteering handwheel rotation exceeds 7 from its centre ineither direction.

Fig 1. Exploded view of Clutched Hydraulic P.S. Pump

Fail-Safe mode: What if controller or any of the circuit fails?It was decided to provide a manual override (user interrupt)which when activated can put the system in the normal PASmode. This override bypasses all the prevailing inputconditions of vehicle speed & steering wheel position.

5. ACTUAL VEHICLE LEVELIMPLEMENTATION OF STEERINGANGLE BASED LOGICThe vehicle model selected for this experiment did not haveany steering wheel angle signal; hence an additionaltransducer (high resolution endless rotary potentiometer) wasfitted for sensing steering wheel angle movement. Due topackaging constraints on vehicle & closed end constructionof this potentiometer (usually steering wheel sensors are

hollow & can be accommodated on steering column itself) itwas necessary to transmit motion of steering column topotentiometer using a positive drive through pair of gearshaving fine teeth. Refer fig 2 for positioning of steering anglesensor.

An algorithm program was created to activate/deactivate theassistance to steering. Following flowchart contains the logicof the program.

Logic explainedThe controller continuously keeps on sensing the steeringwheel rotation through signal received from steering wheelangle sensor fitted on the steering column as shown in fig 2.If the steering angle movement is above the threshold valueof 7 in either direction, the controller instantaneously

Chart 7. fig shows improvement in Fuel economy at various speeds due to disconnecting PAS pump in straight ahead condition.

Fig 2. Steering Angle sensor mounting scheme on the vehicle

energizes the electromagnetic clutch which activates the PASpump resulting in power assistance availability. As long as

the steering wheel angle is beyond 7 in either direction,power steering pump keeps working. Once the driver returns

the steering wheel to the centre after negotiating themanoeurvre, the controller waits for 4 seconds to judge anyintention of next steering manoeurvre. If for next 4 seconds,the controller does not sense any steering wheel movementbeyond 7 it de-energizes the clutch on the PAS pumpthereby disabling the drive to PAS pump.

Evaluation of System Response TimeTo demonstrate that there is no lag in response & that thepower assistance generation is instantaneous, a test wasconducted by acquiring:

1). Steering Wheel Angle

2). Potentiometer Voltage Output

3). P.S. Clutch Coil Supply voltage

chart 9 below illustrates the test results.

Evaluation of the benefitsFollowing tests were conducted for objectively determiningbenefits with this E2HPAS (Energy Efficiency HydraulicPower Assisted Steering) system

1. Emission test on Chassis dynamometer for assessing effecton emission.

2. Public road Fuel efficiency testsBenefits achieved with E2HPAS system are tabulated in thefollowing matrix:

Chart 9.

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6. SUMMARY/CONCLUSIONSE2HPAS is a promising low cost solution for saving fuel andreducing CO2 emission of passenger cars. This may notsubstitute the need of having electric power steering (EPS)system however, for companies which are starved off fundsfor investing in development/configuration of EPS for theirvehicles; this can be an intermediate solution. One of thesalient benefits of this system is that it can be designed &retrofitted on cars which are already in the field therebyproviding the opportunity of fuel savings to the existingcustomers of a company.

CONTACT INFORMATIONChirag Padmakar SonchalEngineering Research CentreTata Motors, Ltd., Pimpri, Pune, Indiachirag@tatamotors.comPhone: 0091-20-6613 5336

Anand Vijay Kulkarni, Jaypal Gajankush, and Sandeep pawarEngineering Research CentreTata Motors, Ltd., Pimpri, Pune, IndiaPhone: 0091-20-6613 5337

mailto:chirag@tatamotors.com