tgl Engine Performance

(1) Brake Torque, Shaft Horsepower, Specific Fuel Consumption1) Brake torque (Turning force)Torque is also called tuming force. It is the force needed to turn an arm of a specilic length.

For example, the tightening torque, needed to tighten a bolt with a spanner is e4pressed as 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 span-

ner, 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 en-

gine. The force (f) is the ex-

plosion presslue generated in

the combustion chamber that

is applied to the piston, and

arm length (r) is e4pressed as

the radius of Lhe crankshaft.

This means that the brake

torque of an engine is the

force that turns the crank-

shaft, the output shaft.

Brake Torque

Force x Lengih

|IIlL\tl

dEnglne Brake

Torque

The unit of brake torque is kgm (kilogram meter).

2) Shaft horrepower (Brake horsepower) (Brake power)When a stationary object is moved 6 ge1fain distance by applying a certain force, the product

of the two is e4pressed as work. Usrng the last illustration is as an example, work can beexpressed by the following formula:

Work : Force (F) "

Distance of movement (Arc length /)

As this formula indicates, work has nothi'g to do with the amount of time it takes to movc theobject.

When tie time it takes to do the work is added to the concrpt of work, the result is expressedby the unit called horsepower. Horsepower indicates how long it takes to perlorm the work orhow much work an engine performs wirhin a fixed period of time. Horsepower is expressed inPS (Pferde-Starke, which means horsepower in German) in countries where the metric systemis adopted. 1 Ps is the force ndeded to move a 75 kg object 1 m in L second.

L PS : 75 kg.m/sec

\ f r = 096 t iP,t t t? " &, lb WENG

t f iP , 76 q " H/+

fT4

Therefore, horsepower, work and torque are related and can be expressed by the following

[ormulas:

Brake torque T : Force x Arm length = F xR - F =iR

(1)

Work I|z : Force x Moved distance (Arc length) : F x I {Z)If a span.ter turns 1 timss, the arc length I (moved distance) will be 2 rrRn. Therefore, the

total work W wrll be as follows: !

F x Zrftn (3)

The relation between work and torque can be obtained by substituting formula (3) by formula

(1).

WorkW=TxZrn (4)

Work can be converted into horsepower by formula (5) by applying the following fwo concepts.

"L horsepower (PS) means to do 75kg.m of.work in L second." "Correct the number of rotationsper minuls to the number of rotations per second."

shaft horsepowe s p =Txztn = r*n (ps)60x75 7t6.2

If r and I in formula (5) are the speed (tp*) of the engtne and the brake torque (kgm) ofthe engine, respectively, the shaft horsepower is equal to the horsepower of the engins.

This formula shows that the shaft horsepower is proportional to the engine speed if the braketorque of the engine is fixed. In other words, the shaft horsepower will be doubled if the enginespeed is doubled. .3) Specitic fuel eonsumptionThe quantity of fuel consumed for operation of an engrne depends on the engins size and the

operating hours. Therefore, fuel consumption per horsepower per hour is used to compareengiqes. This is e4pressed in units of grl"S-hr (gra- PS hour).

(2) Thermal Etficiency and Mechanical EfficiencyThe ratio of the calories of supplied fuel to the calories changed into work is called the

themtal eflicienq of a combustion engine. Various types of thermal efficiency are used toexpress performance.

f TheorellcalthermalI efllclency

Thermal efliclency I f Indicated thermal efficlencyr ' - Actualthermaleff l -

-ciency I Brake thermal efficlency

1) Theoretical thermal eftrciency

Theoretical thermal efEciency is 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 mixturegives to the piston in the cylinder to the supplied calories.

E N G .* g S

(5)

',1-II

l

Brake thsrmal efficiencY =Calories of fuel supPlied

to engine

Brake horsepower (Shaft hdrsepower) (PS)

Low calorific value of fuel (kcaVkg)

Fuel consumption (kglbr)

3) Brake thermal efliciencY

Brake thermal efficiency can be erpressed using the following formula:

Calories changed intopower by engine 75 x3,600 xP, 632 x P"

427 xHuxB HuxE

: Area of piston head (cmz)= Piston stroke (m)= Number of cylinders

E tgl"e speed per minute (rp-)

Indicated mean effective pressrue (kg/cm)

where, P,Hug

4) Mechanical elliciencyThe ratio of the brake horsepower to the indicated horsepower is called the mechanical

eJficienqt.It is expressed by the following formula:

Brake horsepowerMechanical efficiency : lndicated horsepower

(3) Mean Effective Pressure and Indicated HorsepowerW-hen the work that the gas pressure gives to a piston dunog 1 cycle (4 strokes) is divided by

the volt'me stroke by the piston (leveled out to 1 e4pansion stroke) the result is the meaneffective pressrue. This is equal to -4re14-: Ar93 i- shown in the indicator diagram on theprevious page. The mean effective n#",U1-*l;;Ht L*p** the performance of engines withdiffcrent piston displacements and speeds.

Brake mean effective pressure

- Indicated mean effective pressure x Mechanical efficiencyr Indicated mean effective pressue - Friction mear effective pressure

The relatiouships between &e indicated horsepower and the indicated mean effective pressurecan be e4pressed by the following formulas:

4-cycle engtne

Pr=AxLxnxNxP, where, Pr : tndicated horsepower

2x75x6O

}-cycle sngins

Pr=AxLxnxNxP,

75x60

ENG* I 6, LI ^-^-

AL

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N

Pt.

F 't-:t .*rrr*r- :'.-

(4) Losses and Heat Balancef) Heat lossThe calories that the cooling water, cooling air, etc. take away from the burnt gas is called the

heat loss. Thc heat loss of an engine consists mainly of cooling loss (heat taken away by cooling

water through the combustion chamber wall), exhaust loss (heat taken away by the exhaust gas),

and radiation loss (heat dissipated in the form of radiation).2) Frictlon losshntping loss ,Pumprng loss consists of the loss that results from dis'charF"g burnt gas and the loss tlat

results from intake of fresh air.Mechothal lossMechanical loss consists of the friction loss from pistons, piston thgs, bearings and other parts,

and the loss from driving auxiliary equipment, such as fan and generator. Friction loss is

inlluenced the most by the speed of an engine. It is also influenced by the temperature of the

cooling water aqd the viscosiry of thelubricant.

3) Heat balance

Finding the heat balance of a combus- c"rorrn" ujll,l'tion engine means to systematicdly calcu- of fuel(100%)N

\

38 -30VoT -3A%3L - 30Vo

Coollng loss

Fadlation loss

Mschanlcal loss

Effectlve worklate how (in what ratio) the ehcrgy of theluel is distributed betwcen the effective "1::---:---:---:---:-::--''r "" I

work and tle other losses. Heat Balance

The vaiue of the ratio depends on the en&ine t1pe, and the speed and tlie load of the engine.

An example of a diesel engtne under full load is given below:. Effective work (Brake horsepower). Exhaust and radiation loss. Cooling loss.. Mechanical loss (Friction" pumping loss etc.) 7 - 5%

(5) Air lntake and Volumetric EtficiencyThe amount of air intake into the cylinders of a diesel engine directly influences its output.

Therefore, air intake must be performed efficiently.

1) Yolumetrlc efliclency and charging elliciency

Volumetric efficiency can be used to compare the state of air intake.

The volnmetric efficiency of a 4-cycle engine can be expiessed by the following formulq if the

atmospheric pressure is P and the inlet temperature of air intake is L

Volrrrnetricelficiency =.Weight of actual air intake at P and I

Weigbt of air occupying cylinder volume at P and I

The volumetric efficiency of a diesel engine is about 0.8 - 0.9. This is because the intake

manifold has less choke and its speed is lower than the gasoline engins' The volumetric

efficiency of a gasoline ensine is about 0.65 - 0.8 because the air intake pressure is lowerecl by

the resistance of the intake manifold and the carburetor.ENG- ? 7

Z) Percentage of excess airIt goes without salng that what is needed to burn the fuel completely is enough orygen. Since

the orygen content of air is fixed, the maximum quantity of fuel that can be burnt completely can

be calculate{ from the weight of the air intake during the intake stroke of the ongins. The weight

of the air that is theoretically necessary to complete burn L kg of light oil is L4.2 kg.

Percontage of excess air :Weight of actual air intake

Theoretical 1ereht "t ffd::f#

for complete combustion

Weight of actual air intake

Weight of injected fuel x 14.2

In a gasoline engine, most of the orygen in the air can be chemically combined with the fuel

(burnt) because the fuel and air are fully mixed before ignition. In a diesel engins, it is hard to

ftrlly mix &e fuel and air because the injection of fuel into the air takes place at the sa-e ti"'e

as combustion. Therefore, a larger percentage of excess air is needed for a diesel engrne than

for a gasoline engins. GeneraLly, the percentage of excess air for a diesel englne is about 1.2 -

1-.4 under full load (maximum injection quantity), and over 2.5 when tle load is small (small

injection quantity) at a low speed.

(6) Engine Performance

The load test will be e4plained here because its results are usefirl at maintenance shops.The purpose of the load test is to test the performance of an engine under loads of. L00Vo,

75Vo,50Vo alid?SVo at various speeds. The load test is performed by connecting a dynamometerto the engins.'

Measure the following items during this test and enter the results in an engine test score table

like the table on page ?8.(a) Measure the following items at the beginning and the end of each load test:

. ! Weather conditions. Room temperature. flrrmidity. Atnospheric pressure. Time of begrnning. Time at end of test

(b) Measure the following items at each measurement point:

' Dynamometer load

' Speed. Fuel sonsrrmption. Lubricanttemperaturc. Cooling water temperature. Lubricant pressure. Injection timing. Exhaust temperature

Observe and record information about tho state of operation, such as exhaust color,

knocking, vibration, sounds, gas leakagc, oil leakage and state of charge. E N G * t II

(71 Englne Performance Curves1) Full load performance curve

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:

Brake torque

Lower part of right vertical axis:

Specific fuel consumption

This curve shows general engins perfor-

mance trends, 8@ rO@ 1200 14@ 16@ r@ ?c@ 22@

Englne speed (rpm)

Full Load Performance Curve

2) Equal specific fuel consumption curvesThis curve is used to show the specilic fuel

gss5umption of the engine in detail.Horizontal axis: Engine speedVertical axii: Brake torque

Take brake torque values at appropriate spc-cific fuel consumption points and plot them.

This cuwe connects points with the samespecific fuel consumption into contour lineslike those on a map. There is a circle showingthe minimum speci{ic fuel consumption of theenglne at the center of the equal specific fuel consrtmption curves. The outer curves correspondto larger levels of specific fuel 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

horsepower changes at a specilic speed.

This graph is called a lish ltook curvebecause it looks like a group of tish hooks.

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Flsh Hook Gurve

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Englno speed

Equal Speclflc Fuel Consumptlon Curve

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ENG- I9