Engine Performance and Testing

8/6/2019 Engine Performance and Testing

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Engine Performance andEngine Performance and

TestingTesting


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Performance ParametersPerformance ParametersPowerPower

Indicated powerIndicated power

Brake powerBrake power

Friction powerFriction power

EfficiencyEfficiency

AirAir--standard efficiencystandard efficiency

Indicated thermal efficiencyIndicated thermal efficiency

Brake thermal efficiencyBrake thermal efficiency

Mechanical efficiencyMechanical efficiency

Volumetric efficiencyVolumetric efficiency

RelativeefficiencyRelativeefficiency

SpeedSpeed

Fuel consumptionFuel consumption


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Indicated powerIndicated power

Therateofwork done by the gason the piston

The indicated power is given by

ppmimi -- Indicated mean effective pressure (AverageIndicated mean effective pressure (Averagepressure acting on piston during powerpressure acting on piston during power

stroke)stroke)

ll -- stroke lengthstroke length

aa -- crosssectional area ofcylindercrosssectional area ofcylinder

nn -- noofworking strokes : n = N for 2noofworking strokes : n = N for 2--strokeenginesstrokeengines

= N/2 for 4= N/2 for 4--strokeenginesstrokeengines

lankpIP mi!


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Measurement ofIndicatedMeasurement ofIndicatedpowerpower -- Engine indicatorsEngine indicators

In the nineteenth and early twentieth centuries amechanical device known as an engine indicatorwas usedto produce indicator cardsor diagramsforslow-runningsteam and gasreciprocating engines.

Theworkofhigh speed engines isstill evaluated from

tracesofpressureobtained with electronic sensors and

displayed on electronic monitors and through digital

techniques.

h-average height ofthe diagram = Net area/length

S-spring constant

Shv


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Measurement ofIndicatedMeasurement ofIndicatedpowerpower Morse TestMorse Test

Assume a 4 cylinderengineAssume a 4 cylinderengine

All cylindersworkingAll cylindersworking

IP=IP1+IP2+IP3+IP4 = BP(1IP=IP1+IP2+IP3+IP4 = BP(1--4 )+FP4 )+FP----------------(1)(1)I Cylinder is cutI Cylinder is cutIP2+IP3+IP4 = BP(2IP2+IP3+IP4 = BP(2--33--4)+FP4)+FP ----------------(2)(2)

(1)(1) (2) = IP1 = BP(1(2) = IP1 = BP(1--4 )4 )-- BP(2BP(2--33--4)4)

Similarly IP2 = BP(1Similarly IP2 = BP(1--4 )4 )-- BP(1BP(1--33--4)4)IP3 = BP(1IP3 = BP(1--4 )4 )-- BP(1BP(1--22--4)4)

IP4 = BP(1IP4 = BP(1--4 )4 )-- BP(1BP(1--22--3)3)

Now IP = IP1+IP2+IP3+IP4Now IP = IP1+IP2+IP3+IP4


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Brake powerBrake power

Availablework at crankshaftAvailablework at crankshaft

NN Engine Speed in r.p.mEngine Speed in r.p.m TT-- TorqueTorque

60NT2B

T


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Measurement of TorqueMeasurement of Torque --Prony brake dynamometerProny brake dynamometer

W dT !


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Measurement of TorqueMeasurement of Torque --Hydraulic dynamometerHydraulic dynamometer


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Friction PowerFriction Power

The brake power differsfrom theindicated power in that it accountsfor

theeffect ofall oftheenergy losses(friction losses and energy required topump gas into and out oftheengine)

T

he difference between the two isreferred to as thefriction power, FP

ThusFP = IP BP


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Measurement of frictionMeasurement of frictionpowerpower -- Willans line methodWillans line method


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Mechanical efficiencyMechanical efficiency

Mechanical efficiency dependson pumping lossesMechanical efficiency dependson pumping losses(throttle position) and frictional losses (engine(throttle position) and frictional losses (enginedesign and enginespeed)design and enginespeed)

Typical valuesfor automobileengines are:Typical valuesfor automobileengines are:90% @2000 RPM and 75% @ max speed90% @2000 RPM and 75% @ max speed

Throttling increases pumping power and thus theThrottling increases pumping power and thus themechanical efficiency decreases, at idle themechanical efficiency decreases, at idle the

mechanical efficiency approaches zeromechanical efficiency approaches zero

IP

BP=m


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Thermal efficienciesThermal efficienciesThe thermal efficiency is defined as:

Thermal efficiencies can be given in termsofThermal efficiencies can be given in termsof

brakeor indicated valuesbrakeor indicated values

Indicated thermal efficiencies are typicallyIndicated thermal efficiencies are typically50% to 60% and brake thermal efficiencies are50% to 60% and brake thermal efficiencies are

usually about 30%usually about 30%

Relativeefficiency compares thermal efficiencyRelativeefficiency compares thermal efficiency

with airstandard efficiencywith airstandard efficiency

HVfin

th

Qm

W

Q

W=

inputheatofrate

outputpower= =


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Volumetric efficiencyVolumetric efficiency

Due to theshort cycle time at high enginespeeds andflowrestrictions airflow through the intake valveless than ideal amount ofairenters the cylinder

Theeffectivenessofan engine to induct air into thecylinders ismeasured by the volumetric efficiency

nV

m=airltheoretica

inductedairactual=

da

av


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Measurement of Air flow rateMeasurement of Air flow rate

a

.

a

a

wd

ad

AVm

2gHC

2gHCV

!

!

!


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Specific Fuel Consumption

For transportation vehiclesfuel economy isgenerally given as kmpl

In engine testing thefuel consumption is

measured in termsof thefuel massflowrate

Thespecific fuel consumption,sfc, is a measure

ofhowefficiently thefuel supplied to theengine

is used to produce power

BP

m=BSFC f

hrkW

g:units

IP

m=ISFC f


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Useful expressionsUseful expressions

Documents

Engine Performance and Testing