DJA3032 CHAPTER 4

CHAPTER 4: ENGINE’S CRITERIONS AND COMPARISON

byMOHD SAHRIL BIN MOHD FOUZI, Grad. IEM (G 27763)

DEPARTMENT OF MECHANICAL ENGINEERING

© MSF @ POLITEKNIK UNGKU OMAR

(DJA3032) INTERNAL COMBUSTION ENGINE

INTRODUCTION :

This topic covers analysis of indicator power, brake power, torque specific fuel consumption, thermal efficiency, volumetric efficiency, mechanical efficiency as well as Morse test analysis.



THE INDICATOR POWER (I.P.)

This is defined as the rate of work done by the gas on the piston as evaluated from an indicator diagram obtained from the engine.

Indicator power represents the maximum power from the engine under ideal or perfect condition. I.p is calculated on the basis of engine size, displacement, operational speed and the pressure developed theoretically in the cylinder. Ip will always be more than b.p.

+ +

+ +

+ +

+ +

+ +

+ +

+ ++ +

+ ++ +

+ +

+ +

+ +

-

-

-

-

-

-

-

-

-

--

-

-

-

-

--

-

-

-

-

-

Ip at P-V Diagram

Power Loop

Pumping Loop



Formula & EquationPiALNnIp Indicator Power,

2PiALNnIp For 4 stroke engine,

For 2 stroke engine, PiALNnIp

4 stroke engine N = 1/2

2 stroke engine N = 1

Which is;

A = AreaL = Length of DisplacementPi = Indicate Mean Effective PressureN = Revolution per Minute (RPM)n = Number of cylinder



THE BRAKE POWER (B.P)

Brake power is known as the indicated power output, bp is the power developed inside the engine cylinder by the combustion of the charge. It is also called brake power because a brake is used to slow down the shaft inside a dynamometer.

Brake horsepower is often used to compare engines and their characteristics. Automotive manufacturers use brake horsepower to show the differences between engines.

Brake power, bp = 2NT

Which is;

N = rpm, T = torque = W x R, W = weight in Newton, R = radius from rotation point.



FRICTION POWER

Friction power (fp) is defined as the horsepower being used to overcome internal friction. Anytime two objects touch each other while moving, friction is produced. Friction has a tendency to slow down the engine.

f.p = i.p – b.p

Mechanical efficiency, ηm = , ηm = 80% - 90% pipb

..

Engine speed rpm

i.p

b.p

f.p

Power vs engine speed

Power



THERMAL EFFICIENCY,(ήbt )The power output of the engine is obtained from the chemical energy of the fuel supplied. The overall efficiency of the engine is given by the brake thermal efficiency, ήbt

energyfuelpowerbrake

bt ..

OR vQnetmfpb

t ..

(mf is the mass of fuel consumed per unit time, and Qnet, v is the lower calorific value of the fuel)



SPECIFIC FUEL CONSUMPTION, (s.f.c)Specific fuel consumption (s.f.c) is the mass of fuel consumed per kW develop per hour, and is a criterion of economical power production,

./,.

.. kWhkgpb

mfcfs

INDICATOR MIN EFFECTIVE PRESSURE, i.m.e.p. Pm

ipbp

m

m

b

bm

bm

PPi

PiPPiLANn

LANnP



Example 1:One engine petrol 4-cylinder has 57mm, L =90mm, speed =2800 rpm, the dynamometer radius is 0.356m, and brake load is 155 N. Calculate the brake power.

Solution:

Piston area = 222 00255.0057.044

md

Stroke length = 0.09m

Torque, T = W x R =155N x 0.356 = 55.2Nm

b.p. = kWskNm 2.16/1060

2.55280023



Example 2:

One engine petrol has b.p = 16.2kW and i.p = 19.2kW. Therefore, Mechanical efficiency, m

%3.84843.02.192.16

m

m

m

kWkW

ipbp

Solution:



Example 3:One engine petrol four stroke and four cylinder.1. Area of piston = 0.0025mm2

2. Lange of stroke = 0.09m3. Engine speed, = 2800 rpm4. b.p = 16.2 Nm

Solution:

2. PbALNnpb

bar717

10428000900025010602216

ALNn2pbbPorbmep 5

3

.

....)..(.



Example 4:One engine used 6.74 l/hour, density of fuel, =0.735 kg/m3 and Qnet.v =44200 kJ/kg and brake power is 16.2 kw. From the b.m.e.p equation this engine has 7.55 bar and 84% of mechanical efficiency.

Calculate the brake thermal efficiency , (ηbt ) specific fuel consumption (s.f.c) and Indicator min effective pressure, (i.m.e.p.) or ( Pm).Solution:

Mass of fuel, mf = skg /001377.0735.03600

74.6

%6.26266.044200001377.0

2.16.

. orvQnetmf

pbbt

kWhkgpb

mfcfs /306.02.16

3600001377.0.

..



Solution: continue…..

barP

barPP

PP

LANnPLANnP

m

m

bm

m

bm

m

bm

98.884.0

55.7

.98.884.055.7.,...

........

........

barPpemi

pembpemi

pemipemb

m

m

m

OR



Example 5:

A four stroke engine has a bore of 72 mm and a stroke of 102 mm. Its rated speed is 3200 rev/min and it is tested at this speed against a brake which has a torque arm of 0.362 m. The net brake load is 165 N and indicator power is 24.5 kW and the fuel consumption is 6.78 l/h. The specific gravity of petrol used is 0.83 and it has a lower calorific value, Qnet, .v =44200 kJ/kg. Calculate the speed, for the following :

i. engine torque, Tii. brake power, b.piii. brake min effective pressure, b.m.e.piv. brake thermal efficiency, bt v. specific fuel consumption, s.f.cvi. mechanical efficiency, m vii. and indicator min effective pressure, i.m.e.p



Solution:

Piston area = 222 00255.0057.044

md

Stroke length = 0.09m

i. Torque, T = W x R = 165N x 0.362m = 59.73Nm

ii. b.p. = kWskNm 015.20/1060

73.59320023

bar348

1043200090002501060201520

ALNn2pbbPorbmep 5

3

.

....)..(.

iii.

iv. Mass of fuel, mf = skg /1056.183.03600

78.6 3



iv. %1.30301.0442001056.1

015.20.

.3 or

vQnetmfpb

bt

v. ./28.0015.20

36001056.1.

..3

kWhkgpb

mfcfs

%6.8181.05.24

015.20

m

m

m

kWkW

ipbp

vi.

.....,...

........

........

bar22108160348Ppemi

pembpemi

pemipemb

m

m

m

vii.



Morse Test

The Morse test is only applicable to multi-cylinder engines.

One cylinder is cut out, by shortening the plug if S.I engine is under test, or by disconnecting an injector if a C.I. engine is under test.

If the value of i.p. of the cylinders denoted by I1, I2, I3, and I4 (considering a four-cylinder engine), and the power losses in each cylinder are denoted by L1, L2, L3, and L4, then the value of b.p., B, at the test speed with all cylinders firing is given by :

B = (I1 – L1 ) + (I2 - L2) + ( I3 – L3 ) + (I4 – L4 )



If number 1 cylinder is cut out, then the contribution I1 is lost; and if the losses due to that cylinder remain the same as when it is fired, then the b.p., B1, now obtain at the same speed is :

B1 = ( 0 – L1 ) + (I2 - L2) + ( I3 – L3 ) + (I4 – L4 ) 1

Subtracting the second equation from the first gives

B – B1 = I1 2

By cutting out each cylinder in turn the values I2, I3, and I4 can be obtained from equations similar to B – B1 = I1 .

Then, for the engine,

I = I1 + I2 + I3 + I4 3



Example 6

A four-cylinder petrol engine has a bore of 57 mm and a stroke of 90 mm. Its rated speed is 2800 rev/min and it is tested at this speed against a brake which has a torque arm of 0.356 m. The net brake load is 155 N and fuel consumption is 6.74 l/h. The specific gravity of the petrol used is 0.735 and it has a lower calorific value, Q net, v, of 44 200 kJ/kg. A Morse test is carried out and the cylinders are cut out in the order 1.2.3.4. with corresponding brake loads of 111, 106.5, 104.2, and 111 N, respectively. Calculate for the speed of the following;

i. engine torque, Tii. brake power, bpiii. brake min effective pressure, b.m.e.piv. brake thermal efficiency, bt v. specific fuel consumption, s.f.cvi. mechanical efficiency, m

vii. and indicator min effective pressure, i.m.e.p



Solution

i. Engine torque, T

mNT

mNTmR

NWRWT

.2.55356.0155

356.0155

ii. Brake power, b.p

b.p = 2NT

kWpbs

mNpb

2.16.1060

.2.5528002. 3

iii. brake min effective pressure, b.m.e.p, Pb,

barP

P

LANnbpP

bpLANnP

b

b

b

b

56.7104280000255.009.0

106022.1610

10602

,2

5

3

5

3

222 00255.0057.0

44, mdAPistonArea



iv. brake thermal efficiency, bt

,,vnetf

bt Qmbp

%6.26266.04420010376.1

2.163

bt

bt

skg3103761fm

73503600

746fm

/.

..

v. specific fuel consumption ,s.f.c

,bpm

sfc fkW216

s3600skg3103761sfc

./.

kWhkg3060sfc /.



vi. mechanical efficiency, m

Using equation I1 = B – B1for each cylinder in turn, and substituting brake loads instead of the values of b.p since speed is constant, we have;I1 = B – B1 =155 – 111 = 44 NI2 = B = B2 = 155 – 106.5 = 48.5 NI3 = B – B3 = 155 – 104.2 = 50.8 NI4 = B – B4 = 155 – 111 = 44 N

Hence for the engine, the indicated load , I, is given by I = I1 + I2 +I3 +I4 = 44 + 48.5 + 50.8 +44 = 187.3 N.

Therefore from equation, pipb

..

828.03.187

155 or 82.8%

loadindicatorloadbreak

..



vii. indicator min effective pressure, i.m.e.p

i.m.e.p, Pi = barpemb

m

13.9828.056.7....

Volumetric efficiency, v

The power output of an I.C engine depends directly upon the amount of charge which can be induced into the cylinder.

This is referred to as the breathing capacity of the engine and is expressed quantitatively by the volumetric efficiency, which is defined as the ratio of the volume of air induced, measured at the free air conditions to the swept volume of the cylinder,

sv V

V



The volumetric efficiency of an engine is affected by many variables such as ;i. compression ratioii. valve timing iii. induction and port design iv. mixture strengthv. latent heat of evaporation of the fuelvi. heating of the induced chargevii. cylinder temperatureviii.atmospheric condition.

The power output of an engine depends on its capacity to breathe, and if a particular engine had a constant thermal efficiency then its output would be in proportion to the amount of air induced.

The volumetric efficiency with normal aspiration is seldom above 80%, and to improve on this figure, supercharging is used. Air is forced into the cylinder by a blower or fan which is driven by the engine.



An analysis of the dry exhaust showed no oxygen and negligible carbon monoxide. The engine was tested in an atmosphere of 1.013 bar and 15° C. Estimate the volumetric efficiency of the engine.The condition of the exhaust implies a stoichiometric air/fuel ratio which petrol can be taken to be 14.5/1. Estimate the volumetric efficiency of the engine. Given: bore = 57 mm length of stroke = 90 mm mass of fuel, mf = 0.001377 kg/s number of cylinders = 4 rpm, N = 2800 rpm

Example 7

Solution

mRTpv p

mRTv Volume drawn in,



mass of fuel, mf = 0.001377 kg/s or 1.377 x 10-3 kg/s

Solution: …..continue

mass of air flow = 14.5 x 0.001377 = 0.01996 kg/s

smv /0163.010013.1

28810287.001996.0 35

3

volume drawn in,

swept volume of engine = ALn m3/cycle = min/2

3mALnN

881.00185.00163.0

/0185.06024

2800409.0053.0 32

sv

s

VV

smV

v = 88.1 %



Compression Ignition Engine (C.I.) as compared to Spark Ignition Engine (S.I.)

Advantages of Compression Ignition Engine (C.I.)

Low fuel usage especially low weight and low velocity handling. Low fuel consumption. Electric ignition is not needed. Reliable because every cylinder has their own injection system. Therefore, no

risk to all cut cylinders when there is failure ignition, for example, in S.I. engine. Well distribution of mixture because every cylinder has a nozzle injection. Better attribution of torsion during large range of handling velocity. Less maintenance, only need to be serviced after a longer period. Less heat period.



Disadvantages of Compressed Ignition Engine.

Large fix (weight ratio to power brake) because of larger ratio from maximum pressure to minimum pressure. Therefore, bigger and strong cylinder is needed.

Starting cost which is very expensive. Engine produces loud noise. Polluted exhaust gas.



Differences between petrol engine and diesel engine

Economic fuelBecause of high pressure ratio, C.I engine has higher heat competence than S.I. engine; higher heat competence means the usage of fix fuel is very low. With reference to both test engines which have the same loading cylinder, automotive engine usually works from ¼ and ½ weight load, whereby the difference of using fuel is very obvious.

Commercial vehicle engine

Although the price of petrol and diesel is the same, C.I engine will show a higher economical price of fuel especially in this country where there is a big difference between the price of petrol and diesel. There is also a higher cost of maintenance, road tax and miscellaneous cost.



Pressure limit

C.I. engine can work constantly with pressure ratio up to 23:1 and increase heat competence. Pressure ratio for ‘exploded equation’ is 8:1 for petrol and 10:1 for high octane fuel.

Work pressure

As for the same power output, maximum pressure of C.I.engine. is higher than S.I.engine.

Weight engine

Because of the effective maximum pressure is very high, C.I.engine. is very heavy for power output.

Differences between petrol engine and diesel engine….. continue



A four-cylinder petrol engine has a bore of 94 mm and a stroke of 150 mm. Its rated speed is 3300 rev/min and it is tested at this speed against a brake which has a torque arm of 0.475 m. The net brake load is 157 N and fuel consumption is 7.8 l/h. The specific gravity of the petrol used is 0.85 and it has a lower calorific value, Q net, v, of 44 200 kJ/kg. A Morse test is carried out and cylinders are cut out in the order of 1.2.3.4. with corresponding brake loads of 104, 108.5, 112.4, and 111 N, respectively. Calculate the speed, for the following;i. engine torque, Tii. brake power, bpiii. brake min effective pressure, b.m.e.piv. brake thermal efficiency, bt v. specific fuel consumption, s.f.cvi. Indicator load, Ivii. mechanical efficiency, m xi. indicator min effective pressure, i.m.e.p

Exercise 1



Exercise 2

A four-cylinder petrol engine has a bore of 60 mm and a stroke of 92 mm. Its rated speed is 2800 rev/min. The engine was tested in an atmosphere of 1.013 bar and 17 0C. The air/fuel ratio for petrol is 13/1 and mass of fuel consumed per unit time, mf is 0.001377 kg. Estimate the volumetric efficiency of the engine.

Define the meaning of Volumetric efficiency in engine.

Exercise 3

List 5 variables which can affect the volumetric efficiency.

Exercise 4

List five advantages of compression ignition engine and compare them with spark ignition engine.

Exercise 5



