Diesel Engine Performance

tgl EnginePerformance (1) BrakeTorque,ShaftHorsepower,SpecificFuelConsumption 1) Brake torque (Turning force) Torque is also calledtumingforce. It is the force neededto turn an arm of a speciliclength. For example,the tighteningtorque,neededto tightena bolt with a spanneris e4pressedas the , force that turns the bolt due to a force applied to one end of the spanner. If the spanner is short in length, a large force must be applied. But if the same force is applied to a long spanner, the brake torque (turning force) incteases. Therefore, brake torque can be found using the following formula: Brake torque : Force x Arm length [rt's apply tbis to tn engine. The force (f) is the explosionpressluegeneratedin the combustion chamber that is applied to the piston, and arm length (r) is e4pressedas the radius of Lhecrankshaft. This means that the brake torque of an engine is the

Force x Lengih

Brake Torque

force that turns the crank-

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EnglneBrake Torque

shaft,the output shaft.

The unit of brake torque is kgm (kilogrammeter). 2) Shafthorrepower(Brake horsepower)(Brakepower) When a stationaryobjectis moved6 ge1faindistanceby applyinga certainforce, the product of the two is e4pressedas work. Usrng the last illustrationis as an example,work can be expressedby the followingformula: Work : Force (F)

Distanceof movement(Arc length/) " As this formulaindicates, work hasnothi'g to do with the amountof time it takesto movcthe object. When tie time it takesto do the work is addedto the concrptof work,the resultis expressed by the unit calledhorsepower.Horsepowerindicateshow long it takesto perlorm the work or how much work an engineperformswirhin a fixed period of time. Horsepoweris expressedin PS (Pferde-Starke, which meanshorsepowerin German)in countrieswhere the metric system is adopted. 1 Ps is the force ndededto movea 75 kg object 1 m in L second. L PS : 75 kg.m/sec

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tfiP, 76 q " H/+

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Therefore, horsepower, work and torque are related and can be expressedby the following [ormulas:

BraketorqueT : Forcex Arm length = F xR - F =i R Work I|z : Force x Moved distance(Arc length) : F x I

(1)

{Z) If a span.terturns 1 timss,the arc lengthI (moveddistance)will be 2 rrRn. Therefore,the total work W wrll be as follows: ! (3) F x Zrftn The relationbetweenwork and torquecanbe obtainedby substitutingformula (3) by formula (1). (4)

Wo rkW=T xZ rn

Work canbe convertedinto horsepowerby formula(5) by applyingthe followingfwo concepts. "L horsepower(PS) meansto do 75kg.mof.workin L second.""Correctthe numberof rotations per minuls to the numberof rotationsper second." r*n (ps) shaft horsepowe s p =Txztn = 60x75 7t6.2

(5)

If r and I in formula (5) are the speed(tp*) of the engtneand the brake torque (kgm) of the engine,respectively,the shafthorsepoweris equalto the horsepowerof the engins. This formula showsthat the shafthorsepoweris proportionalto the enginespeedif the brake torque of the engineis fixed. In other words,the shafthorsepowerwill be doubledif the engine speedis doubled. . 3) Speciticfuel eonsumption The quantityof fuel consumedfor operationof an engrnedependson the enginssizeand the operating hours. Therefore,fuel consumptionper horsepowerper hour is used to compare engiqes. This is e4pressedin units of grl"S-hr (gra- PS hour).

(2) ThermalEtficiencyand MechanicalEfficiency The ratio of the caloriesof suppliedfuel to the calorieschangedinto work is called the themtal eflicienq of a combustionengine. Various types of thermal efficiencyare used to expressperformance.

Thermalefliclency I

f I

Theorellcalthermal efllclency

r'-

Actualthermalefflciency

f I

Indicated thermalefficlency Brakethermalefficlency

-

1) Theoretical thermal eftrciency Theoretical thermal efEciencyis the ratio of the calorics that can be changed into work by a theoretical cycle to the calories supplied to this cycle. 2) Indicated thermal e{Iiciency Indicated thermal efficiency is the ratio of the work (calories) that the working gas mixture

givesto thepistonin thecylinder to thesupplied calories.

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3) Brake thermal efliciencY Brake thermal efficiency can be erpressed using the following formula:

Brake thsrmal efficiencY =

where,P, Hu g

Calorieschangedinto powerby engine Caloriesof fuel supPlied to engine

7 5 x 3 , 6 0 0x P ,

632 x P"

4 2 7 x Hu x B

Hu x E

Brake horsepower(Shafthdrsepower)(PS) Low calorific value of fuel (kcaVkg) Fuel consumption(kglbr)

4) Mechanicalelliciency The ratio of the brake horsepowerto the indicatedhorsepoweris called the mechanical eJficienqt. It is expressedby the followingformula: Brakehorsepower Mechanical efficiency :

lndicated horsepower

(3) Mean EffectivePressureand IndicatedHorsepower W-henthe work that the gaspressuregivesto a pistondunog 1 cycle(4 strokes)is divided by the volt'me stroke by the piston (leveledout to 1 e4pansionstroke) the result is the mean effectivepressrue. This is equal to -4re14-: Ar93i- shownin the indicator diagramon the previouspage. The meaneffectiven#",U1-*l;;Ht L*p** the performanceof engineswith diffcrent piston displacementsand speeds. Brake meaneffectivepressure - Indicated mean effectivepressurex Mechanicalefficiency r Indicatedmeaneffectivepressue- Friction mear effectivepressure The relatioushipsbetween&e indicatedhorsepowerandthe indicatedmeaneffectivepressure can be e4pressedby the followingformulas: 4-cycleengtne where,Pr : tndicatedhorsepower AxLxnxNxP, Pr = A : Area of piston head (cmz) 2x75x6O L = Pistonstroke(m) }-cycle sngins tl = Numberof cylinders Pr=

AxLxnxNxP,

75x60

N

E tgl"e speedper minute (rp-)

Pt.

Indicated mean effective pressrue (kg/cm)

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(4) Lossesand HeatBalance f) Heat loss The caloriesthat the coolingwater,coolingair, etc.take awayfrom the burnt gasis calledthe heat loss.Thc heat lossof an engineconsistsmainlyof coolingloss(heat takenawayby cooling water throughthe combustionchamberwall), exhaustloss(heattakenawayby the exhaustgas), and radiation loss (heat dissipatedin the form of radiation). 2) Frictlonloss hntping loss , Pumprngloss consistsof the loss that resultsfrom dis'charF"gburnt gas and the loss tlat resultsfrom intake of fresh air. Mechothal loss Mechanicallossconsistsof the friction lossfrom pistons,pistonthgs, bearingsand otherparts, and the loss from driving auxiliary equipment,such as fan and generator. Friction loss is inlluencedthe most by the speedof an engine. It is alsoinfluencedby the temperatureof the cooling water aqd the viscosiry of the lubricant. Coollng loss 3) Heat balance

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Fadlation loss

Finding the heatbalanceof a combus- c"rorrn" ujll,l'

tion engine meansto systematicdlycalcu- offuel(100%)N

Mschanlcal loss

late how (in what ratio) the ehcrgyof the luel is distributed betwcen the effective work and tle other losses.

Effectlve work

"" I "1::---:---:---:---:-::--''r HeatBalance

The vaiue of the ratio depends on the en&inet1pe, and the speed and tlie load of the engine. An example of a diesel engtne under full load is given below:

. . . ..

Effectivework (Brakehorsepower) Exhaustand radiationloss Coolingloss Mechanicalloss(Friction"pumpinglossetc.)

38-30Vo T - 3A% 3L - 30Vo 7 - 5%

(5) Air lntakeand VolumetricEtficiency The amountof air intake into the cylindersof a dieselenginedirectly influencesits output. Therefore,air intake must be performedefficiently. 1) Yolumetrlc efliclencyand chargingelliciency Volumetric efficiencycan be usedto comparethe stateof air intake. The volnmetricefficiencyof a 4-cycleenginecanbe expiessedby the followingformulq if the atmosphericpressureis P and the inlet temperatureof air intakeis L Volrrrnetricelficiency =.

Weightof actualair intake at P and I Weigbt of air occupying cylinder volume at P and I

The volumetricefficiencyof a dieselengineis about 0.8 - 0.9. This is becausethe intake manifold has less choke and its speed is lower than the gasolineengins' The volumetric efficiencyof a gasolineensineis about0.65- 0.8 becausethe air intake pressureis lowereclby the resistanceof the intake manifoldand the carburetor.

EN G -? 7

Z) Percentageof excessair It goeswithoutsalng that whatis neededto burn the fuel completelyis enoughorygen. Since the orygencontentof air is fixed,the maximumquantityof fuel that canbe burnt completelycan be calculate{from the weightof the air intakeduringthe intakestrokeof the ongins. The weight to completeburn L kg of light oil is L4.2kg. of the air that is theoreticallynecessary Percontageof excessair :

Weightof actualair intake

Theoretical 1ereht"t ffd::f#

for complete combustion

Weightof actualair intake Weightof injectedfuel x 14.2 In a gasolineengine,most of the orygenin the air can be chemicallycombinedwith the fuel (burnt) becausethe fuel and air are fully mixedbeforeignition. In a dieselengins,it is hard to ftrlly mix &e fuel and air becausethe injectionof fuel into the air takesplace at the sa-e ti"'e as combustion.Therefore,a larger percentageof excessair is neededfor a dieselengrnethan the percentageof excessair for a dieselenglneis about 1.2for a gasolineengins. GeneraLly, 1-.4under full load (maximuminjectionquantity),and over 2.5 when tle load is small (small injection quantity)at a low speed.

(6) EnginePerformance The load test will be e4plainedhere becauseits resultsare usefirlat maintenanceshops. The purposeof the load test is to test the performanceof an engineunder loads of.L00Vo, 75Vo,50Voalid?SVoat variousspeeds.The load testis performedby connectinga dynamometer to the engins.' Measurethe followingitemsduringthis testand enterthe resultsin an enginetest scoretable like the table on page?8. (a) Measurethe followingitemsat the beginningand the end of eachload test: ! Weatherconditions . . Room temperature . flrrmidity . Atnospheric pressure . Time of begrnning . Time at end of test point: (b) Measurethe followingitems at eachmeasurement ' Dynamometerload ' Speed . Fuel sonsrrmption . Lubricanttemperaturc . Coolingwater temperature . Lubricant pressure . Injectiontiming . Exhausttemperature Observeand record information about tho state of operation, such as exhaustcolor, knocking,vibration,sounds,gasleakagc,oil leakageand stateof charge. * t I IE N G

(71 EnglnePerformanceCurves 1) Full load performancecurve

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This is the most typical curve used to show the performance of an enqine. The following 4 types of performance are showri: Horizontal axis: E"gre speed Left vertical axis: Brake power Upper part of right vertical axis:

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Brake torque Lower part of right vertical axis:

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Specific fuel consumption This curve shows general engins performance trends,

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Englnespeed(rpm) Full Load PerformanceCurve

2) Equal specificfuel consumption curves This curveis usedto showthe specilicfuel gss5umptionof the enginein detail. Horizontal axis: Enginespeed {1, g Vertical axii: Brake torque I o x Take brake torque valuesat appropriatespce cific fuel consumptionpoints and plot them. This cuwe connectspoints with the same specific fuel consumptioninto contour lines Englno speed like thoseon a map. There is a circleshowing EqualSpeclflcFuel ConsumptlonCurve the minimumspeci{icfuel consumption of the englneat the centerof the equalspecificfuel consrtmption curves.The outer curvescorrespond to larger levelsof specificfuel consrrmption. 3) Fish hook curve Performance curves at several speeds are drawn on one graph as shown on the riglrt. Horizontal axis: Brake horsepower Vertical axis: Specific fuel gonsrrmption The graph shows how the specilic fuel consumption will change as. the brake horsepowerchangesat a specilic speed.

This graphis calleda lish ltook curve it lookslikea groupof tishhooks. because grakc hoJ!.polvor tPsl

ENG -I9

Flsh Hook Gurve

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